PRIMARY OR PALÆOZOIC SERIES
CHAPTER XXII.
PERMIAN OR MAGNESIAN LIMESTONE GROUP.
Line of Separation between Mesozoic and Palæozoic Rocks. — Distinctness of Triassic and Permian Fossils. — Term Permian. — Thickness of calcareous and sedimentary Rocks in North of England. — Upper, Middle, and Lower Permian. — Marine Shells and Corals of the English Magnesian Limestone. — Reptiles and Fish of Permian Marl-slate. — Foot-prints of Reptiles. — Angular Breccias in Lower Permian. — Permian Rocks of the Continent. — Zechstein and Rothliegendes of Thuringia. — Permian Flora. — Its generic Affinity to the Carboniferous.
In pursuing our examination of the strata in descending order, we have next to pass from the base of the Secondary or Mesozoic to the uppermost or newest of the Primary or Palæozoic formations. As this point has been selected as a line of demarkation for one of the three great divisions of the fossiliferous series, the student might naturally expect that by aid of lithological and palæontological characters he would be able to recognise without difficulty a distinct break between the newer and older group. But so far is this from being the case in Great Britain, that nowhere have geologists found more difficulty in drawing the line of separation than between the Secondary and Primary series. The obscurity has arisen from the great resemblance in colour and mineral character of the Triassic and Permian red marls and sandstones, and the scarcity and often total absence in them of organic remains. The thickness of the strata belonging to each group amounts in some places to several thousand feet; and by dint of a careful examination of their geological position, and of those fossil, animal, and vegetable forms which are occasionally met with in some members of each series, it has at length been made clear that the older or Permian rocks are more connected with the Primary or Palæozoic than with the Secondary or Mesozoic strata already described.
The term Permian has been proposed for this group by Sir R. Murchison, from Perm, a Russian province, where it occupies an area twice the size of France, and contains a great abundance and variety of fossils, both vertebrate and invertebrate. Professor Sedgwick in 1832[[1]] described what is now recognised as the central member of this group, the Magnesian limestone, showing that it attained a thickness of 600 feet along the north-east of England, in the counties of Durham, Yorkshire, and Nottinghamshire, its lower part often passing into a fossiliferous marl-slate and resting on an inferior Red Sandstone, the equivalent of the Rothliegendes of Germany. It has since been shown that some of the Red Sandstones of newer date also belong to the Permian group; and it appears from the observations of Mr. Binney, Sir R. Murchison, Mr. Harkness, and others, that it is in the region where the limestone is most largely developed, as, for example, in the county of Durham, that the associated red sandstones or sedimentary rocks are thinnest, whereas in the country where the latter are thickest the calcareous member is reduced to thirty, or even sometimes to ten feet. It is clear, therefore, says Mr. Hull, that the sedimentary region in the north of England area has been to the westward, and the calcareous area to the eastward; and that in this group there has been a development from opposite directions of the two types of strata.
In illustration of this he has given us the following table:
THICKNESS OF PERMIAN STRATA IN NORTH OF ENGLAND.
| N.W. of England | N.E. of England | |
| Feet | Feet | |
| Upper Permian (Sedimentary) | 600 | 50–100 |
| Middle Permian (Calcareous) | 10–30 | 600 |
| Lower Permian (Sedimentary) | 3000 | 100–250[[2]] |
Upper Permian.—What is called in this table the Upper Permian will be seen to attain its chief thickness in the north-west, or on the coast of Cumberland, as at St. Bee’s Head, where it is described by Sir Roderick Murchison as consisting of massive red sandstones with gypsum resting on a thin course of Magnesian Limestone with fossils, which again is connected with the Lower Red Sandstone, resembling the upper one in such a manner that the whole forms a continuous series. No fossil footprints have been found in this Upper as in the Lower Red Sandstone.
Middle Permian—Magnesian Limestone and Marl-slate.—This formation is seen upon the coast of Durham and Yorkshire, between the Wear and the Tees. Among its characteristic fossils are Schizodus Schlotheimi (Fig. 410) and Mytilus septifer (Fig. 412). These shells occur at Hartlepool and Sunderland, where the rock assumes an oolitic and botryoidal character. Some of the beds in this division are ripple-marked. In some parts of the coast of Durham, where the rock is not crystalline, it contains as much as 44 per cent of carbonate of magnesia, mixed with carbonate of lime. In other places—for it is extremely variable in structure—it consists chiefly of carbonate of lime, and has concreted into globular and hemispherical masses, varying from the size of a marble to that of a cannon-ball, and radiating from the centre. Occasionally earthy and pulverulent beds pass into compact limestone or hard granular dolomite. Sometimes the limestone appears in a brecciated form, the fragments which are united together not consisting of foreign rocks but seemingly composed of the breaking-up of the Permian limestone itself, about the time of its consolidation. Some of the angular masses in Tynemouth cliff are two feet in diameter.
The magnesian limestone sometimes becomes very fossiliferous and includes in it delicate bryozoa, one of which, Fenestella retiformis (Fig. 413), is a very variable species, and has received many different names. It sometimes attains a large size, single specimens measuring eight inches in width. The same bryozoan, with several other British species, is also found abundantly in the Permian of Germany.
The total known fauna of the Permian series of Great Britain at present numbers 147 species, of which 77, or more than half, are mollusca. Not one of these is common to rocks newer than the Palæozoic, and the brachiopods are the only group which have furnished species common to the more ancient or Carboniferous rocks. Of these Lingula Crednerii (Fig. 415) is an example. There are 25 Gasteropods and only one cephalopod, Nautilus Freieslebeni, which is also found in the German Zechstein.
Fig. 413: Magnesian Limestone, Humbleton Hill, near Sunderland.[[3]]
Shells of the genera Productus (Fig. 414) and Strophalosia (the latter of allied form with hinge teeth), which do not occur in strata newer than the Permian, are abundant in the ordinary yellow magnesian limestone, as will be seen in the valuable memoirs of Messrs. King and Howse. They are accompanied by certain species of Spirifera (Fig. 416), Lingula Crednerii (Fig. 415), and other brachiopoda of the true primary or palæozoic type. Some of this same tribe of shells, such as Camarophoria, allied to Rhynchonella, Spiriferina, and two species of Lingula, are specifically the same as fossils of the carboniferous rocks. Avicula, Arca, and Schizodus (Fig. 410), and other lamellibranchiate bivalves, are abundant, but spiral univalves are very rare.
Beneath the limestone lies a formation termed the marl-stone, which consists of hard calcareous shales, marl-slate, and thin-bedded limestones. At East Thickley, in Durham, where it is thirty feet thick, this slate has yielded many fine specimens of fossil fish—of the genera Palæoniscus ten species, Pygopterus two species, Coelacanthus two species, and Platysomus two species, which as genera are common to the older Carboniferous formation, but the Permian species are peculiar, and, for the most part, identical with those found in the marl-slate or copper-slate of Thuringia.
The Palæoniscus above mentioned belongs to that division of fishes which M. Agassiz has called “Heterocercal,” which have their tails unequally bilobate, like the recent shark and sturgeon, and the vertebral column running along the upper caudal lobe. (See Fig. 418.) The “Homocercal” fish, which comprise almost all the 9000 species at present known in the living creation, have the tail-fin either single or equally divided; and the vertebral column stops short, and is not prolonged into either lobe. (See Fig. 419.) Now it is a singular fact, first pointed out by Agassiz, that the heterocercal form, which is confined to a small number of genera in the existing creation, is universal in the magnesian limestone, and all the more ancient formations. It characterises the earlier periods of the earth’s history, whereas in the secondary strata, or those newer than the Permian, the homocercal tail predominates.
A full description has been given by Sir Philip Egerton of the species of fish characteristic of the marl-slate, in Professor King’s monograph before referred to, where figures of the ichthyolites, which are very entire and well preserved, will be found. Even a single scale is usually so characteristically marked as to indicate the genus, and sometimes even the particular species. They are often scattered through the beds singly, and may be useful to a geologist in determining the age of the rock.
We are indebted to Messrs. Hancock and Howse for the discovery in this marl-slate at Midderidge, Durham, of two species of Protosaurus, a genus of reptiles, one representative of which, P. Speneri, has been celebrated ever since the year 1810 as characteristic of the Kupfer-schiefer or Permian of Thuringia. Professor Huxley informs us that the agreement of the Durham fossil with Hermann von Meyer’s figure of the German specimen is most striking. Although the head is wanting in all the examples yet found, they clearly belong to the Lacertian order, and are therefore of a higher grade than any other vertebrate animal hitherto found fossil in a Palæozoic rock. Remains of Labyrinthodont reptiles have also been met with in the same slate near Durham.
Lower Permian.—The inferior sandstones which lie beneath the marl-slate consist of sandstone and sand, separating the Magnesian Limestone from the coal, in Yorkshire and Durham. In some instances, red marl and gypsum have been found associated with these beds. They have been classed with the Magnesian Limestone by Professor Sedgwick, as being nearly co-extensive with it in geographical range, though their relations are very obscure. But the principal development of Lower Permian is, as we have seen by Mr. Hull’s table [p. 386], in the northwest, where the Penrith sandstone, as it has been called, and the associated breccias and purple shales are estimated by Professor Harkness to attain a thickness of 3000 feet. Organic remains are generally wanting, but the leaves and wood of coniferous plants, and in one case a cone, have been found. Also in the purple marls of Corncockle Muir near Dumfries, very distinct footprints of reptiles occur, originally referred to the Trias, but shown by Mr. Binney in 1856 to be Permian. No bones of the animals which they represent have yet been discovered.
Angular Breccias in Lower Permian.—A striking feature in these beds is the occasional occurrence, especially at the base of the formation, of angular and sometimes rounded fragments of Carboniferous and older rocks of the adjoining districts being included in a paste of red marl. Some of the angular masses are of huge size.
In the central and southern counties, where the Middle Permian or Magnesian Limestone is wanting, it is difficult to separate the upper and lower sandstones, and Mr. Hull is of opinion that the patches of this formation found here and there in Worcestershire, Shropshire, and other counties may have been deposited in a sea separated from the northern basin by a barrier of Carboniferous rocks running east and west, and now concealed under the Triassic strata of Cheshire. Similar breccias to those before described are found in the more southern counties last mentioned, where their appearance is rendered more striking by the marked contrast they present to the beds of well-rolled and rounded pebbles of the Trias occupying a large area in the same region.
Professor Ramsay refers the angular form and large size of the fragments composing these breccias to the action of floating ice in the sea. These masses of angular rock, some of them weighing more than half a ton, and lying confusedly in a red, unstratified marl, like stones in boulder-drift, are in some cases polished, striated, and furrowed like erratic blocks in the moraine of a glacier. They can be shown in some cases to have travelled from the parent rocks, thirty or more miles distant, and yet not to have lost their angular shape.[[4]]
Permian Rocks of the Continent.—Germany is the classic ground of the Magnesian Limestone now called Permian. The formation was well studied by the miners of that country a century ago as containing a thin band of dark-coloured cupriferous shale, characterised at Mansfield in Thuringia by numerous fossil fish. Beneath some variegated sandstones (not belonging to the Trias, though often confounded with it) they came down first upon a dolomitic limestone corresponding to the upper part of our Middle Permian, and then upon a marl-slate richly impregnated with copper pyrites, and containing fish and reptiles (Protosaurus) identical in species with those of the corresponding marl-slate of Durham. To the limestone they gave the name of Zechstein, and to the marl-slate that of Mergel-schiefer or Kupfer-schiefer. Beneath the fossiliferous group lies the Rothliegendes or Rothtodt-liegendes, meaning the red-lyer or red-dead-lyer, so-called by the German miners from its colour, and because the copper had died out when they reached this underlying non-metalliferous member of the series. This red under-lyer is, in fact, a great deposit of red sandstone, breccia, and conglomerate with associated porphyry, basalt, and amygdaloid.
According to Sir R. Murchison, the Permian rocks are composed, in Russia, of white limestone, with gypsum and white salt; and of red and green grits, occasionally with copper ore; also magnesian limestones, marl-stones, and conglomerates.
Permian Flora.—About 18 or 20 species of plants are known in the Permian rocks of England. None of them pass down into the Carboniferous series, but several genera, such as Alethopteris, Neuropteris, Walchia, and Ullmania, are common to the two groups. The Permian flora on the Continent appears, from the researches of MM. Murchison and de Verneuil in Russia, and of MM. Geinitz and von Gutbier in Saxony, to be, with a few exceptions, distinct from that of the coal.
In the Permian rocks of Saxony no less than 60 species of fossil plants have been met with. Two or three of these, as Calamites gigas, Sphenopteris erosa, and S. lobata, are also met with in the government of Perm in Russia. Seven others, and among them Neuropteris Loshii, Pecopteris arborescens, and P. similis, and several species of Walchia (see Fig. 426), a genus of Conifers, called Lycopodites by some authors, are said by Geinitz to be common to the coal-measures.
Fig. 428: Noeggerathia cuneifolia.
Brongniart.[[5]]
Among the genera also enumerated by Colonel Gutbier are the fruit called Cardiocarpon (see Fig. 427), Asterophyllites, and Annularia, so characteristic of the Carboniferous period; also Lepidodendron, which is common to the Permian of Saxony, Thuringia, and Russia, although not abundant. Neoggerathia (see Fig. 428), the leaves of which have parallel veins without a midrib, and to which various generic synonyms, such as Cordaites, Flabellaria, and Poacites, have been given, is another link between the Permian and Carboniferous vegetation. Coniferæ, of the Araucarian division, also occur; but these are likewise met with both in older and newer rocks. The plants called Sigillaria and Stigmaria, so marked a feature in the Carboniferous period, are as yet wanting in the true Permian.
Among the remarkable fossils of the Rothliegendes, or lowest part of the Permian in Saxony and Bohemia, are the silicified trunks of tree-ferns called generically Psaronius. Their bark was surrounded by a dense mass of air-roots, which often constituted a great addition to the original stem, so as to double or quadruple its diameter. The same remark holds good in regard to certain living extra-tropical arborescent ferns, particularly those of New Zealand.
Upon the whole, it is evident that the Permian plants approach much nearer to the Carboniferous flora than to the Triassic; and the same may be said of the Permian fauna.
[1] Trans. Geol. Soc. Lond., Second Series, vol. iii, p. 37.
[2] Edward Hull, Ternary Classification, Quart. Journ. Science, No. xxiii, 1869.
[3] King’s Monograph, pl. 2.
[4] Ramsay, Quart. Geol. Journ., 1855; and Lyell, Principles of Geology, vol. i, p. 223, 10th edit.
[5] Murchison’s Russia, vol. ii, pl. A, fig. 3.
CHAPTER XXIII.
THE COAL OR CARBONIFEROUS GROUP.
Principal Subdivisions of the Carboniferous Group. — Different Thickness of the sedimentary and calcareous Members in Scotland and the South of England. — Coal-measures. — Terrestrial Nature of the Growth of Coal. — Erect fossil Trees. — Uniting of many Coal-seams into one thick Bed. — Purity of the Coal explained. — Conversion of Coal into Anthracite. — Origin of Clay-ironstone. — Marine and brackish-water Strata in Coal. — Fossil Insects. — Batrachian Reptiles. — Labyrinthodont Foot-prints in Coal-measures. — Nova Scotia Coal-measures with successive Growths of erect fossil Trees. — Similarity of American and European Coal. — Air-breathers of the American Coal. — Changes of Condition of Land and Sea indicated by the Carboniferous Strata of Nova Scotia.
Principal Subdivisions of the Carboniferous Group.—The next group which we meet with in the descending order is the Carboniferous, commonly called “The Coal,” because it contains many beds of that mineral, in a more or less pure state, interstratified with sandstones, shales, and limestones. The coal itself, even in Great Britain and Belgium, where it is most abundant, constitutes but an insignificant portion of the whole mass. In South Wales, for example, the thickness of the coal-bearing strata has been estimated at between 11,000 and 12,000 feet, while the various coal seams, about 80 in number, do not, according to Professor Phillips, exceed in the aggregate 120 feet.
The Carboniferous formation assumes various characters in different parts even of the British Islands. It usually comprises two very distinct members: first, the sedimentary beds, usually called the Coal-measures, of mixed fresh-water, terrestrial, and marine origin, often including seams of coal; second, that named in England the Mountain or Carboniferous Limestone, of purely marine origin, and made up chiefly of corals, shells, and encrinites, and resting on shales called the shales of the Mountain Limestone.
In the south-western part of our island, in Somersetshire and South Wales, the three divisions usually spoken of are:
- Coal-measures: Strata of shale, sandstone, and grit, from 600 to 12,000 feet thick, with occasional seams of coal.
- Millstone grit: A coarse quartzose sandstone passing into a conglomerate, sometimes used for millstones, with beds of shale; usually devoid of coal; occasionally above 600 feet thick.
- Mountain or Carboniferous Limestone: A calcareous rock containing marine shells, corals, and encrinites; devoid of coal; thickness variable, sometimes more than 1500 feet.
If the reader will refer to the section in [Fig. 85,] he will see that the Upper and Lower Coal-measures of the coal-field near Bristol are divided by a micaceous flaggy sandstone called the Pennant Rock. The Lower Coal-measures of the same section rest sometimes, especially in the north part of the basin, on a base of coarse grit called the Millstone Grit (No. 2 on the previous page).
![[Fig. 85,]](#5745294564006620949_fig85.jpg.id-6396197169030525103.wrap-0.html.html){kind=link}
In the South Welsh coal-field Millstone Grit occurs in like manner at the base of the productive coal. It is called by the miners the “Farewell Rock,” as when they reach it they have no longer any hopes of obtaining coal at a greater depth in the same district. In the central and northern coal-fields of England this same grit, including quartz pebbles, with some accompanying sandstones and shales containing coal plants, acquires a thickness of several thousand feet, lying beneath the productive coal-measures, which are nearly 10,000 feet thick.
Below the Millstone Grit is a continuation of similar sandstones and shales called by Professor Phillips the Yoredale series, from Yoredale, in Yorkshire, where they attain a thickness of from 800 to 1000 feet. At several intervals bands of limestone divide this part of the series, one of which, called the Main Limestone or Upper Scar Limestone, composed in great part of encrinites, is 70 feet thick. Thin seams of coal also occur in these lower Yoredale beds in Yorkshire, showing that in the same region there were great alternations in the state of the surface. For at successive periods in the same area there prevailed first terrestrial conditions favourable to the growth of pure coal, secondly, a sea of some depth suited to the formation of Carboniferous Limestone, and, thirdly, a supply of muddy sediment and sand, furnishing the materials for sandstone and shale. There is no clear line of demarkation between the Coal-measures and the Millstone Grit, nor between the Millstone Grit and underlying Yoredale rocks.
On comparing a series of vertical sections in a north-westerly direction from Leicestershire and Warwickshire into North Lancashire, we find, says Mr. Hull, within a distance of 120 miles an augmentation of the sedimentary materials to the extent of 16,000 feet.
| Leicestershire and Warwickshire | 2,600 feet |
| North Staffordshire | 9,000 feet |
| South Lancashire | 12,130 feet |
| North Lancashire | 18,700 feet |
In central England, where the sedimentary beds are reduced to about 3000 feet in all, the Carboniferous Limestone attains an enormous thickness, as much as 4000 feet at Ashbourne, near Derby, according to Mr. Hull’s estimate. To a certain extent, therefore, we may consider the calcareous member of the formation as having originated simultaneously with the accumulation of the materials of grit, sandstone, and shale, with seams of coal; just as strata of mud, sand, and pebbles, several thousand feet thick, with layers of vegetable matter, are now in the process of formation in the cypress swamps and delta of the Mississippi, while coral reefs are forming on the coast of Florida and in the sea of the Bermuda islands. For we may safely conclude that in the ancient Carboniferous ocean those marine animals which were limestone builders were never freely developed in areas where the rivers poured in fresh water charged with sand or clay; and the limestone could only become several thousand feet thick in parts of the ocean which remained perfectly clear for ages.
The calcareous strata of the Scotch coal-fields, those of Lanarkshire, the Lothians, and Fife, for example, are very insignificant in thickness when compared to those of England. They consist of a few beds intercalated between the sandstones and shales containing coal and ironstone, the combined thickness of all the limestones amounting to no more than 150 feet. The vegetation of some of these northern sedimentary beds containing coal may be older than any of the coal-measures of central and southern England, as being coeval with the Mountain Limestone of the south. In Ireland the limestone predominates over the coal-bearing sands and shales. We may infer the former continuity of several of the coal-fields in northern and central England, not only from the abrupt manner in which they are cut off at their outcrop, but from their remarkable correspondence in the succession and character of particular beds. But the limited extent to which these strata are exposed at the surface is not merely owing to their former denudation, but even in a still greater degree to their having been largely covered by the New Red Sandstone, as in Cheshire, and here and there by the Permian strata, as in Durham.
It has long been the opinion of the most eminent geologists that the coal-fields of Yorkshire and Lancashire were once united, the upper Coal-measures and the overlying Millstone Grit and Yoredale rocks having been subsequently removed; but what is remarkable, is the ancient date now assigned to this denudation, for it seems that a thickness of no less than 10,000 feet of the coal-measures had been carried away before the deposition even of the lower Permian rocks which were thrown down upon the already disturbed truncated edges of the coal-strata.[[1]] The carboniferous strata most productive of workable coal have so often a basin-shaped arrangement that these troughs have sometimes been supposed to be connected with the original conformation of the surface upon which the beds were deposited. But it is now admitted that this structure has been owing to movements of the earth’s crust of upheaval and subsidence, and that the flexure and inclination of the beds has no connection with the original geographical configuration of the district.
COAL-MEASURES.
I shall now treat more particularly of the productive coal-measures, and their mode of origin and organic remains.
Coal formed on Land.—In South Wales, already alluded to, where the coal-measures attain a thickness of 12,000 feet, the beds throughout appear to have been formed in water of moderate depth, during a slow, but perhaps intermittent, depression of the ground, in a region to which rivers were bringing a never-failing supply of muddy sediment and sand. The same area was sometimes covered with vast forests, such as we see in the deltas of great rivers in warm climates, which are liable to be submerged beneath fresh or salt water should the ground sink vertically a few feet.
In one section near Swansea, in South Wales, where the total thickness of strata is 3246 feet, we learn from Sir H. De la Beche that there are ten principal masses of sandstone. One of these is 500 feet thick, and the whole of them make together a thickness of 2125 feet. They are separated by masses of shale, varying in thickness from 10 to 50 feet. The intercalated coal-beds, sixteen in number, are generally from one to five feet thick, one of them, which has two or three layers of clay interposed, attaining nine feet. At other points in the same coal-field the shales predominate over the sandstones. Great as is the diversity in the horizontal extent of individual coal-seams, they all present one characteristic feature, in having, each of them, what is called its underclay. These underclays, co-extensive with every layer of coal, consist of arenaceous shale, sometimes called fire-stone, because it can be made into bricks which stand the fire of a furnace. They vary in thickness from six inches to more than ten feet; and Sir William Logan first announced to the scientific world in 1841 that they were regarded by the colliers in South Wales as an essential accompaniment of each of the eighty or more seams of coal met with in their coal-field. They are said to form the floor on which the coal rests; and some of them have a slight admixture of carbonaceous matter, while others are quite blackened by it.
All of them, as Sir William Logan pointed out, are characterised by inclosing a peculiar species of fossil vegetable called Stigmaria, to the exclusion of other plants. It was also observed that, while in the overlying shales, or “roof” of the coal, ferns and trunks of trees abound without any Stigmariæ, and are flattened and compressed, those singular plants of the underclay most commonly retain their natural forms, unflattened and branching freely, and sending out their slender rootlets, formerly thought to be leaves, through the mud in all directions. Several species of Stigmaria had long been known to botanists, and described by them, before their position under each seam of coal was pointed out, and before their true nature as the roots of trees (some having been actually found attached to the base of Sigillaria stumps) was recognised. It was conjectured that they might be aquatic, perhaps floating plants, which sometimes extended their branches and leaves freely in fluid mud, in which they were finally enveloped.
Now that all agree that these underclays are ancient soils, it follows that in every instance where we find them they attest the terrestrial nature of the plants which formed the overlying coal, which consists of the trunks, branches, and leaves of the same plants. The trunks have generally fallen prostrate in the coal, but some of them still remain at right angles to the ancient soils (see Fig. 440). Professor Goppert, after examining the fossil vegetables of the coal-fields of Germany, has detected, in beds of pure coal, remains of plants of every family hitherto known to occur fossil in the carboniferous rocks. Many seams, he remarks, are rich in Sigillariæ, Lepidodendra, and Stigmariæ, the latter in such abundance as to appear to form the bulk of the coal. In some places, almost all the plants were calamites, in others ferns.[[2]]
Between the years 1837 and 1840, six fossil trees were discovered in the coal-fields of Lancashire, where it is intersected by the Bolton railway. They were all at right angles to the plane of the bed, which dips about 15 degrees to the south. The distance between the first and the last was more than 100 feet, and the roots of all were imbedded in a soft argillaceous shale. In the same plane with the roots is a bed of coal, eight or ten inches thick, which has been found to extend across the railway, or to the distance of at least ten yards. Just above the covering of the roots, yet beneath the coal-seam, so large a quantity of the Lepidostrobus variabilis was discovered inclosed in nodules of hard clay, that more than a bushel was collected from the small openings around the base of some of the trees (see [Fig. 457] of this genus). The exterior trunk of each was marked by a coating of friable coal, varying from one-quarter to three-quarters of an inch in thickness; but it crumbled away on removing the matrix. The dimensions of one of the trees is 15½ feet in circumference at the base, 7½ feet at the top, its height being eleven feet. All the trees have large spreading roots, solid and strong, sometimes branching, and traced to a distance of several feet, and presumed to extend much farther.
![[Fig. 457]](#5745294564006620949_fig457.jpg.id-3045222930299263174.wrap-0.html.html){kind=link}
In a colliery near Newcastle a great number of Sigillariæ occur in the rock as if they had retained the position in which they grew. No less than thirty, some of them four or five feet in diameter, were visible within an area of 50 yards square, the interior being sandstone, and the bark having been converted into coal. Such vertical stems are familiar to our coal-miners, under the name of coal-pipes. They are much dreaded, for almost every year in the Bristol, Newcastle, and other coal-fields, they are the cause of fatal accidents. Each cylindrical cast of a tree, formed of solid sandstone, and increasing gradually in size towards the base, and being without branches, has its whole weight thrown downward, and receives no support from the coating of friable coal which has replaced the bark. As soon, therefore, as the cohesion of this external layer is overcome, the heavy column falls suddenly in a perpendicular or oblique direction from the roof of the gallery whence coal has been extracted, wounding or killing the workman who stands below. It is strange to reflect how many thousands of these trees fell originally in their native forests in obedience to the law of gravity; and how the few which continued to stand erect, obeying, after myriads of ages, the same force, are cast down to immolate their human victims.
It has been remarked that if, instead of working in the dark, the miner was accustomed to remove the upper covering of rock from each seam of coal, and to expose to the day the soils on which ancient forests grew, the evidence of their former growth would be obvious. Thus in South Staffordshire a seam of coal was laid bare in the year 1844, in what is called an open work at Parkfield colliery, near Wolverhampton. In the space of about a quarter of an acre the stumps of no less than 73 trees with their roots attached appeared, as shown in Fig. 429, some of them more than eight feet in circumference. The trunks, broken off close to the root, were lying prostrate in every direction, often crossing each other. One of them measured 15, another 30 feet in length, and others less. They were invariably flattened to the thickness of one or two inches, and converted into coal. Their roots formed part of a stratum of coal ten inches thick, which rested on a layer of clay two inches thick, below which was a second forest resting on a two-foot seam of coal. Five feet below this, again, was a third forest with large stumps of Lepidodendra, Calamites, and other trees.
Blending of Coal-seams.—Both in England and North America seams of coal are occasionally observed to be parted from each other by layers of clay and sand, and, after they have been persistent for miles, to come together and blend in one single bed, which is then found to be equal in the aggregate to the thickness of the several seams. I was shown by Mr. H. D. Rogers a remarkable example of this in Pennsylvania. In the Shark Mountain, near Pottsville, in that State, there are thirteen seams of anthracite coal, some of them more than six feet thick, separated by beds of white quartzose grit and a conglomerate of quartz pebbles, often of the size of a hen’s egg. Between Pottsville and the Lehigh Summit Mine, seven of these seams of coal, at first widely separated, are, in the course of several miles, brought nearer and nearer together by the gradual thinning out of the intervening coarse-grained strata and their accompanying shales, until at length they successively unite and form one mass of coal between forty and fifty feet thick, very pure on the whole, though with a few thin partings of clay. This mass of coal I saw quarried in the open air at Mauch Chunk, on the Bear Mountain. The origin of such a vast thickness of vegetable remains, so unmixed, on the whole, with earthy ingredients, can be accounted for in no other way than by the growth, during thousands of years, of trees and ferns in the manner of peat—a theory which the presence of the Stigmaria in situ under each of the seven layers of anthracite fully bears out. The rival hypothesis, of the drifting of plants into a sea or estuary, leaves the non-intermixture of sediment, or of clay, sand, and pebbles, with the pure coal wholly unexplained.
The late Mr. Bowman was the first who gave a satisfactory explanation of the manner in which distinct coal-seams, after maintaining their independence for miles, may at length unite, and then persist throughout another wide area with a thickness equal to that which the separate seams had previously maintained.
Let A C (Fig. 430) be a three-foot seam of coal originally laid down as a mass of vegetable matter on the level area of an extensive swamp, having an under-clay, f g, through which the Stigmariæ or roots of the trees penetrate as usual. One portion, B C, of this seam of coal is now inclined; the area of the swamp having subsided as much as 25 feet at E C, and become for a time submerged under salt, fresh, or brackish water. Some of the trees of the original forest A B C fell down, others continued to stand erect in the new lagoon, their stumps and part of their trunks becoming gradually enveloped in layers of sand and mud, which at length filled up the new piece of water C E.
When this lagoon has been entirely silted up and converted into land, the forest-covered surface A B will extend once more over the whole area A B E, and a second mass of vegetable matter, D E, forming three feet more of coal, will accumulate. We then find in the region E C two seams of coals, each three feet thick, with their respective under-clays, with erect buried trees based upon the surface of the lower coal, the two seams being separated by 25 feet of intervening shale and sandstone. Whereas in the region A B, where the growth of the forest has never been interrupted by submergence, there will simply be one seam, two yards thick, corresponding to the united thickness of the beds B E and B C. It may be objected that the uninterrupted growth of plants during the interval of time required for the filling up of the lagoon will have caused the vegetable matter in the region D A B to be thicker than the two distinct seams E and C, and no doubt there would actually be a slight excess representing one or more generation of trees and plants forming the undergrowth; but this excess of vegetable matter, when compressed into coal, would be so insignificant in thickness that the miner might still affirm that the seam D A throughout the area D A B was equal to the two seams C and E.
Cause of the Purity of Coal.—The purity of the coal itself, or the absence in it of earthy particles and sand, throughout areas of vast extent, is a fact which appears very difficult to explain when we attribute each coal-seam to a vegetation growing in swamps. It has been asked how, during river inundations capable of sweeping away the leaves of ferns and the stems and roots of Sigillariæ and other trees, could the waters fail to transport some fine mud into the swamps? One generation after another of tall trees grew with their roots in mud, and their leaves and prostrate trunks formed layers of vegetable matter, which was afterwards covered with mud since turned to shale. Yet the coal itself, or altered vegetable matter, remained all the while unsoiled by earthy particles. This enigma, however perplexing at first sight, may, I think, be solved by attending to what is now taking place in deltas. The dense growth of reeds and herbage which encompasses the margins of forest-covered swamps in the valley and delta of the Mississippi is such that the fluviatile waters, in passing through them, are filtered and made to clear themselves entirely before they reach the areas in which vegetable matter may accumulate for centuries, forming coal if the climate be favourable. There is no possibility of the least intermixture of earthy matter in such cases. Thus in the large submerged tract called the “Sunk Country,” near New Madrid, forming part of the western side of the valley of the Mississippi, erect trees have been standing ever since the year 1811-12, killed by the great earthquake of that date; lacustrine and swamp plants have been growing there in the shallows, and several rivers have annually inundated the whole space, and yet have been unable to carry in any sediment within the outer boundaries of the morass, so dense is the marginal belt of reeds and brush-wood. It may be affirmed that generally, in the “cypress swamps” of the Mississippi, no sediment mingles with the vegetable matter accumulated there from the decay of trees and semi-aquatic plants. As a singular proof of this fact, I may mention that whenever any part of a swamp in Louisiana is dried up, during an unusually hot season, and the wood set on fire, pits are burnt into the ground many feet deep, or as far down as the fire can descend without meeting with water, and it is then found that scarcely any residuum or earthy matter is left. At the bottom of all these “cypress swamps” a bed of clay is found, with roots of the tall cypress (Taxodium distichum), just as the under-clays of the coal are filled with Stigmaria.
Conversion of Coal into Anthracite.—It appears from the researches of Liebig and other eminent chemists, that when wood and vegetable matter are buried in the earth exposed to moisture, and partially or entirely excluded from the air, they decompose slowly and evolve carbonic acid gas, thus parting with a portion of their original oxygen. By this means they become gradually converted into lignite or wood-coal, which contains a larger proportion of hydrogen than wood does. A continuance of decomposition changes this lignite into common or bituminous coal, chiefly by the discharge of carbureted hydrogen, or the gas by which we illuminate our streets and houses. According to Bischoff, the inflammable gases which are always escaping from mineral coal, and are so often the cause of fatal accidents in mines, always contain carbonic acid, carbureted hydrogen, nitrogen, and olefiant gas. The disengagement of all these gradually transforms ordinary or bituminous coal into anthracite, to which the various names of glance-coal, coke, hard-coal, culm, and many others, have been given.
There is an intimate connection between the extent to which the coal has in different regions parted with its gaseous contents, and the amount of disturbance which the strata have undergone. The coincidence of these phenomena may be attributed partly to the greater facility afforded for the escape of volatile matter, when the fracturing of the rocks has produced an infinite number of cracks and crevices. The gases and water which are made to penetrate these cracks are probably rendered the more effective as metamorphic agents by increased temperature derived from the interior. It is well known that, at the present period, thermal waters and hot vapours burst out from the earth during earthquakes, and these would not fail to promote the disengagement of volatile matter from the Carboniferous rocks.
In Pennsylvania the strata of coal are horizontal to the westward of the Alleghany Mountains, where the late Professor H. D. Rogers pointed out that they were most bituminous; but as we travel south-eastward, where they no longer remain level and unbroken, the same seams become progressively debitumenized in proportion as the rocks become more bent and distorted. At first, on the Ohio River, the proportion of hydrogen, oxygen, and other volatile matters ranges from forty to fifty per cent. Eastward of this line, on the Monongahela, it still approaches forty per cent, where the strata begin to experience some gentle flexures. On entering the Alleghany Mountains, where the distinct anticlinal axes begin to show themselves, but before the dislocations are considerable, the volatile matter is generally in the proportion of eighteen or twenty per cent. At length, when we arrive at some insulated coal-fields associated with the boldest flexures of the Appalachian chain, where the strata have been actually turned over, as near Pottsville, we find the coal to contain only from six per cent of volatile matter, thus becoming a genuine anthracite.
Clay-ironstone.—Bands and nodules of clay-ironstone are common in coal-measures, and are formed, says Sir H. De la Beche, of carbonate of iron mingled mechanically with earthy matter, like that constituting the shales. Mr. Hunt, of the Museum of Practical Geology, instituted a series of experiments to illustrate the production of this substance, and found that decomposing vegetable matter, such as would be distributed through all coal strata, prevented the further oxidation of the proto-salts of iron, and converted the peroxide into protoxide by taking a portion of its oxygen to form carbonic acid. Such carbonic acid, meeting with the protoxide of iron in solution, would unite with it and form a carbonate of iron; and this mingling with fine mud, when the excess of carbonic acid was removed, might form beds or nodules of argillaceous ironstone.[[3]]
Intercalated Marine Beds in Coal.—Both in the coal-fields of Europe and America the association of fresh, brackish-water, and marine strata with coal-seams of terrestrial origin is frequently recognised. Thus, for example, a deposit near Shrewsbury, probably formed in brackish water, has been described by Sir R. Murchison as the youngest member of the coal-measures of that district, at the point where they are in contact with the overlying Permian group. It consists of shales and sandstones about 150 feet thick, with coal and traces of plants; including a bed of limestone varying from two to nine feet in thickness, which is cellular, and resembles some lacustrine limestones of France and Germany. It has been traced for 30 miles in a straight line, and can be recognised at still more distant points. The characteristic fossils are a small bivalve, having the form of a Cyclas or Cyrena, also a small entomostracan, Cythere inflata (Fig. 432), and the microscopic shell of an annelid of an extinct genus called Microconchus (Fig. 431), allied to Spirorbis. In the coal-field of Yorkshire there are fresh-water strata, some of which contain shells referred to the family Unionidæ; but in the midst of the series there is one thin but very widely-spread stratum, abounding in fishes and marine shells, such as Goniatites Listeri (Fig. 433), Orthoceras, and Aviculopecten papyraceus, Goldf. (Fig. 434).
Insects in European Coal.—Articulate animals of the genus Scorpion were found by Count Sternberg in 1835 in the coal-measures of Bohemia, and about the same time in those of Coalbrook Dale by Mr. Prestwich, were also true insects, such as beetles of the family Curculionidæ, a neuropterous insect of the genus Corydalis, and another related to the Phasmidæ, have been found.
From the coal of Wetting, in Westphalia, several specimens of the cockroach or Blatta family, and the wing of a cricket (Acridites) have been described by Germar. Professor Goldenberg published, in 1854, descriptions of no less than twelve species of insects from the nodular clay-ironstone of Saarbrück, near Trèves.[[4]] Among them are several Blattinæ, three species of Neuroptera, one beetle of the Scarabæus family, a grasshopper or locust, Gryllacris (see Fig. 435), and several white ants or Termites. Professor Goldenberg showed me, in 1864, the wing of a white ant, found low down in the productive coal-measures of Saarbrück, in the interior of a flattened Lepidodendron. It is much larger than that of any known living species of the same genus.
Batrachian Reptiles in Coal.—No vertebrated animals more highly organised than fish were known in rocks of higher antiquity than the Permian until the year 1844, when the Apateon pedestris, Meyer, was discovered in the coal-measures of Munster-Appel in Rhenish Bavaria, and three years later, in 1847, Professor von Dechen found three other distinct species of the same family of Amphibia in the Saarbruck coal-field above alluded to. These were described by the late Professor Goldfuss under the generic name of Archegosaurus. The skulls, teeth, and the greater portions of the skeleton, nay, even a large part of the skin, of two of these reptiles have been faithfully preserved in the centre of spheroidal concretions of clay-ironstone. The largest of these, Archegosaurus Decheni, must have been three feet six inches long. Figure 436 represents the skull and neck bones of the smallest of the three, of the natural size. They were considered by Goldfuss as saurians, but by Herman von Meyer as most nearly allied to the Labyrinthodon before mentioned ([p. 371]), and the remains of the extremities leave no doubt they were quadrupeds, “provided,” says Von Meyer, “with hands and feet terminating in distinct toes; but these limbs were weak, serving only for swimming or creeping.” The same anatomist has pointed out certain points of analogy between their bones and those of the Proteus anguinus; and Professor Owen has observed that they make an approach to the Proteus in the shortness of their ribs. Two specimens of these ancient reptiles retain a large part of the outer skin, which consisted of long, narrow, wedge-shaped, tile-like, and horny scales, arranged in rows (see Fig. 437).
In 1865, several species belonging to three different genera of the same family of perennibranchiate Batrachians were found in the coal-field of Kilkenny in bituminous shale at the junction of the coal with the underlying Stigmaria-bearing clay. They were, probably, inhabitants of a marsh, and the large processes projecting from the vertebræ of their tail imply, according to Professor Huxley, great powers of swimming. They were of the Labyrinthodont family, and their association with the fish of the coal, of which so large a proportion are ganoids, reminds us that the living perennibranchiate amphibia of America frequent the same rivers as the ganoid Lepidostei or bony pikes.
Labyrinthodont footprints in coal-measures.—In 1844, the very year when the Apateon, before mentioned, of the coal was first met with in the country between the Moselle and the Rhine, Dr. King published an account of the footprints of a large reptile discovered by him in North America. These occur in the coal-strata of Greensburg, in Westmoreland County, Pennsylvania; and I had an opportunity of examining them when in that country in 1846. The footmarks were first observed standing out in relief from the lower surface of slabs of sandstone, resting on thin layers of fine unctuous clay. I brought away one of these masses, which is represented in Fig. 438. It displays, together with footprints, the casts of cracks (a, a′) of various sizes. The origin of such cracks in clay, and casts of the same, has before been explained, and referred to the drying and shrinking of mud, and the subsequent pouring of sand into open crevices. It will be seen that some of the cracks, as at b, c, traverse the footprints, and produce distortion in them, as might have been expected, for the mud must have been soft when the animal walked over it and left the impressions; whereas, when it afterwards dried up and shrank, it would be too hard to receive such indentations.
We may assume that the reptile which left these prints on the ancient sands of the coal-measures was an air-breather, because its weight would not have been sufficient under water to have made impressions so deep and distinct. The same conclusion is also borne out by the casts of the cracks above described, for they show that the clay had been exposed to the air and sun, so as to have dried and shrunk.
Nova Scotia Coal-measures.—The sedimentary strata in which thin seams of coal occur attain a thickness, as we have seen, of 18,000 feet in the north of England exclusive of the Mountain Limestone, and are estimated by Von Dechen at over 20,000 feet in Rhenish Prussia. But the finest example in the world of a natural exposure in a continuous section ten miles long, occurs in the sea-cliffs bordering a branch of the Bay of Fundy, in Nova Scotia. These cliffs, called the “South Joggins,” which I first examined in 1842, and afterwards with Dr. Dawson in 1845, have lately been admirably described by the last-mentioned geologist[[5]] in detail, and his evidence is most valuable as showing how large a portion of this dense mass was formed on land, or in swamps where terrestrial vegetation flourished, or in fresh-water lagoons. His computation of the thickness of the whole series of carboniferous strata as exceeding three miles, agrees with the measurement made independently by Sir William Logan in his survey of this coast.
There is no reason to believe that in this vast succession of strata, comprising some marine as well as many fresh-water and terrestrial formations, there is any repetition of the same beds. There are no faults to mislead the geologist, and cause him to count the same beds over more than once, while some of the same plants have been traced from the top to the bottom of the whole series, and are distinct from the flora of the antecedent Devonian formation of Canada. Eighty-one seams of coal, varying in thickness from an inch to about five feet, have been discovered, and no less than seventy-one of these have been actually exposed in the sea-cliffs.
In the section (Fig. 439), which I examined in 1842, the beds from c to i are seen all dipping the same way, their average inclination being at an angle of 24° S.S.W. The vertical height of the cliffs is from 150 to 200 feet; and between d and g—in which space I observed seventeen trees in an upright position, or, to speak more correctly, at right angles to the planes of stratification—I counted nineteen seams of coal, varying in thickness from two inches to four feet. At low tide a fine horizontal section of the same beds is exposed to view on the beach, which at low tide extends sometimes 200 yards from the base of the cliff. The thickness of the beds alluded to, between d and g, is about 2500 feet, the erect trees consisting chiefly of large Sigillariæ, occurring at ten distinct levels, one above the other. The usual height of the buried trees seen by me was from six to eight feet; but one trunk was about 25 feet high and four feet in diameter, with a considerable bulge at the base. In no instance could I detect any trunk intersecting a layer of coal, however thin; and most of the trees terminated downward in seams of coal. Some few only were based on clay and shale; none of them, except Calamites, on sandstone. The erect trees, therefore, appeared in general to have grown on beds of vegetable matter. In the underclays Stigmaria abounds.
These root-bearing beds have been found under all the coal-seams, and such old soils are at present the most destructible masses in the whole cliff, the sandstones and laminated shales being harder and more capable of resisting the action of the waves and the weather. Originally the reverse was doubtless true, for in the existing delta of the Mississippi those clays in which the innumerable roots of the deciduous cypress and other swamp trees ramify in all directions are seen to withstand far more effectually the undermining power of the river, or of the sea at the base of the delta, than do beds of loose sand or layers of mud not supporting trees. It is obvious that if this sand or mud be afterwards consolidated and turned to sandstone and hard shale, it would be the least destructible.
In regard to the plants, they belonged to the same genera, and most of them to the same species, as those met with in the distant coal-fields of Europe. Dr. Dawson has enumerated more than 150 species, two-thirds of which are European, a greater agreement than can be said to exist between the same Nova Scotia flora and that of the coal-fields of the United States. By referring to the section, Fig. 439, the position of the four-foot coal will be perceived, and in Fig. 440 (a section made by me in 1842 of a small portion) that from e to f of the same cliff is exhibited, in order to show the manner of occurrence of erect fossil trees at right angles to the planes of the inclined strata.
In the sandstone which filled their interiors, I frequently observed fern-leaves, and sometimes fragments of Stigmaria, which had evidently entered together with sediment after the trunk had decayed and become hollow, and while it was still standing under water. Thus the tree, a, Fig. 440, represented in the bed e in the section, Fig. 439, is a hollow trunk five feet eight inches in length, traversing various strata, and cut off at the top by a layer of clay two feet thick, on which rests a seam of coal (b, Fig. 440) one foot thick. On this coal again stood two large trees (c and d), while at a greater height the trees f and g rest upon a thin seam of coal (e), and above them is an underclay, supporting the four-foot coal.
Occasionally the layers of matter in the inside of the tree are more numerous than those without; but it is more common in the coal-measures of all countries to find a cylinder of pure sandstone—the cast of the interior of a tree—intersecting a great many alternating beds of shale and sandstone, which originally enveloped the trunk as it stood erect in the water. Such a want of correspondence in the materials outside and inside, is just what we might expect if we reflect on the difference of time at which the deposition of sediment will take place in the two cases; the imbedding of the tree having gone on for many years before its decay had made much progress. In many places distinct proof is seen that the enveloping strata took years to accumulate, for some of the sandstones surrounding erect sigillarian trunks support at different levels roots and stems of Calamites; the Calamites having begun to grow after the older Sigillariæ had been partially buried.
The general absence of structure in the interior of the large fossil trees of the Coal implies the very durable nature of their bark, as compared with their woody portion. The same difference of durability of bark and wood exists in modern trees, and was first pointed out to me by Dr. Dawson, in the forests of Nova Scotia, where the Canoe Birch (Betula papyracea) has such tough bark that it may sometimes be seen in the swamps looking externally sound and fresh, although consisting simply of a hollow cylinder with all the wood decayed and gone. When portions of such trunks have become submerged in the swamps they are sometimes found filled with mud. One of the erect fossil trees of the South Joggins fifteen feet in height, occurring at a higher level than the main coal, has been shown by Dr. Dawson to have a coniferous structure, so that some Coniferæ of the Coal period grew in the same swamps as Sigillariæ, just as now the deciduous Cypress (Taxodium distichum) abounds in the marshes of Louisiana even to the edge of the sea.
When the carboniferous forests sank below high-water mark, a species of Spirorbis or Serpula ([Fig. 431]), attached itself to the outside of the stumps and stems of the erect trees, adhering occasionally even to the interior of the bark—another proof that the process of envelopment was very gradual. These hollow upright trees, covered with innumerable marine annelids, reminded me of a “cane-brake,” as it is commonly called, consisting of tall reeds, Arundinaria macrosperma, which I saw in 1846, at the Balize, or extremity of the delta of the Mississippi. Although these reeds are fresh-water plants, they were covered with barnacles, having been killed by an incursion of salt-water over an extent of many acres, where the sea had for a season usurped a space previously gained from it by the river. Yet the dead reeds, in spite of this change, remained standing in the soft mud, enabling us to conceive how easily the larger Sigillariæ, hollow as they were but supported by strong roots, may have resisted an incursion of the sea.
![[Fig. 431]](#5745294564006620949_fig431.jpg.id-2446330455353441835.wrap-0.html.html){kind=link}
The high tides of the Bay of Fundy, rising more than 60 feet, are so destructive as to undermine and sweep away continually the whole face of the cliffs, and thus a new crop of erect fossil trees is brought into view every three or four years. They are known to extend over a space between two and three miles from north to south, and more than twice that distance from east to west, being seen in the banks of streams intersecting the coal-field.
Structure of Coal.—The bituminous coal of Nova Scotia is similar in composition and structure to that of Great Britain, being chiefly derived from sigillarioid trees mixed with leaves of ferns and of a Lycopodiaceous tree called Cordaites (Noeggerathia, etc., for genus, see [Fig. 428]), supposed by Dawson to have been deciduous, and which had broad parallel veined leaves without a mid-rib. On the surface of the seams of coal are large quantities of mineral charcoal, which doubtless consist, as Dr. Dawson suggests, of fragments of wood which decayed in the open air, as would naturally be expected in swamps where so many erect trees were preserved. Beds of cannel-coal display, says Dr. Dawson, such a microscopical structure and chemical composition as shows them to have been of the nature of fine vegetable mud such as accumulates in the shallow ponds of modern swamps. The underclays are loamy soils, which must have been sufficiently above water to admit of drainage, and the absence of sulphurets, and the occurrence of carbonate of iron in them, prove that when they existed as soils, rain-water, and not sea-water, percolated them. With the exception, perhaps, of Asterophyllites (see [Fig. 461]), there is a remarkable absence from the coal-measures of any form of vegetation properly aquatic, the true coal being a sub-aërial accumulation in soil that was wet and swampy but not permanently submerged.
![[Fig. 428]](#5745294564006620949_fig428.jpg.id-513908485208741710.wrap-0.html.html){kind=link}
![[Fig. 461]](#5745294564006620949_fig460.jpg.id-5012368810261221478.wrap-0.html.html){kind=link}
Air-breathers of the Coal.—If we have rightly interpreted the evidence of the former existence at more than eighty different levels of forests of trees, some of them of vast extent, and which lasted for ages, giving rise to a great accumulation of vegetable matter, it is natural to ask whether there were not many air-breathing inhabitants of these same regions. As yet no remains of mammalia or birds have been found, a negative character common at present to all the Palæozoic formations; but in 1852 the osseous remains of a reptile, the first ever met with in the carboniferous strata of the American continent, were found by Dr. Dawson and myself. We detected them in the interior of one of the erect Sigillariæ before alluded to as of such frequent occurrence in Nova Scotia. The tree was about two feet in diameter, and consisted of an external cylinder of bark, converted into coal, and an internal stony axis of black sandstone, or rather mud and sand stained black by carbonaceous matter, and cemented together with fragments of wood into a rock. These fragments were in the state of charcoal, and seem to have fallen to the bottom of the hollow tree while it was rotting away. The skull, jaws, and vertebræ of a reptile, probably about 2½ feet in length (Dendrerpeton Acadianum, Owen), were scattered through this stony matrix. The shell, also, of a Pupa (see [Fig. 442]), the first land-shell ever met with in the coal or in beds older than the tertiary, was observed in the same stony mass. Dr. Wyman of Boston pronounced the reptile to be allied in structure to Menobranchus and Menopoma, species of batrachians, now inhabiting the North American rivers. The same view was afterwards confirmed by Professor Owen, who also pointed out the resemblance of the cranial plates to those seen in the skull of Archegosaurus and Labyrinthodon.[[6]] Whether the creature had crept into the hollow tree while its top was still open to the air, or whether it was washed in with mud during a flood, or in whatever other manner it entered, must be matter of conjecture.
![[Fig. 442]](#5745294564006620949_fig442.jpg.id-6095667856343874262.wrap-0.html.html){kind=link}
Footprints of two reptiles of different sizes had previously been observed by Dr. Harding and Dr. Gesner on ripple-marked flags of the lower coal-measures in Nova Scotia (No. 2, [Fig. 447]), evidently made by quadrupeds walking on the ancient beach, or out of the water, just as the recent Menopoma is sometimes observed to do. The remains of a second and smaller species of Dendrerpeton, D. Oweni, were also found accompanying the larger one, and still retaining some of its dermal appendages; and in the same tree were the bones of a third small lizard-like reptile, Hylonomus Lyelli, seven inches long, with stout hind limbs, and fore limbs comparatively slender, supposed by Dr. Dawson to be capable of walking and running on land.[[7]]
![[Fig. 447]](#5745294564006620949_fig447.jpg.id-4453021147502183772.wrap-0.html.html){kind=link}
In a second specimen of an erect stump of a hollow tree 15 inches in diameter, the ribbed bark of which showed that it was a Sigillaria, and which belonged to the same forest as the specimen examined by us in 1852, Dr. Dawson obtained not only fifty specimens of Pupa vetusta (Fig. 442), and nine skeletons of reptiles belonging to four species, but also several examples of an articulated animal resembling the recent centipede or gally-worm, a creature which feeds on decayed vegetable matter (see Fig. 441). Under the microscope, the head, with the eyes, mandible, and labrum, are well seen. It is interesting, as being the earliest known representative of the myriapods, none of which had previously been met with in rocks older than the oolite or lithographic slate of Germany.
Some years after the discovery of the first Pupa, Dr. Dawson, carefully examining the same great section containing so many buried forests in the cliffs of Nova Scotia, discovered another bed, separated from the tree containing Dendrerpeton by a mass of strata more than 1200 feet thick. As there were 21 seams of coal in this intervening mass, the length of time comprised in the interval is not to be measured by the mere thickness of the sandstones and shales. This lower bed is an underclay seven feet thick, with stigmarian rootlets, and the small land-shells occurring in it are in all stages of growth. They are chiefly confined to a layer about two inches thick, and are unmixed with any aquatic shells. They were all originally entire when imbedded, but are most of them now crushed, flattened, and distorted by pressure; they must have been accumulated, says Dr. Dawson, in mud deposited in a pond or creek.
The surface striæ of Pupa vetusta, when magnified 50 diameters, present exactly the same appearance as a portion corresponding in size of the common English Pupa juniperi, and the internal hexagonal cells, magnified 500 diameters, show the internal structure of the fossil and recent Pupa to be identical. In 1866[[8]] Dr. Dawson discovered in this lower bed, so full of the Pupa, another land-shell of the genus Helix (sub-genus Zonites), see Fig. 443.
None of the reptiles obtained from the coal-measures of the South Joggins are of a higher grade than the Labyrinthodonts, but some of these were of very great size, two caudal vertebræ found by Mr. Marsh in 1862 measuring two and a half inches in diameter, and implying a gigantic aquatic reptile with a powerful swimming tail.
Except some obscure traces of an insect found by Dr. Dawson in a coprolite of a terrestrial reptile occurring in a fossil tree, no specimen of this class has been brought to light in the Joggins. But Mr. James Barnes found in a bed of shale at Little Grace Bay, Cape Breton, the wing of an Ephemera, which must have measured seven inches from tip to tip of the expanded wings—larger than any known living insect of the Neuropterous family.
That we should have made so little progress in obtaining a knowledge of the terrestrial fauna of the Coal is certainly a mystery, but we have no reason to wonder at the extreme rarity of insects, seeing how few are known in the carboniferous rocks of Europe, worked for centuries before America was discovered, and now quarried on so enormous a scale. These European rocks have not yet produced a single land-shell, in spite of the millions of tons of coal annually extracted, and the many hundreds of soils replete with the fossil roots of trees, and the erect trunks and stumps preserved in the position in which they grew. In many large coal-fields we continue as much in the dark respecting the invertebrate air-breathers then living, as if the coal had been thrown down in mid-ocean. The early date of the carboniferous strata cannot explain the enigma, because we know that while the land supported a luxuriant vegetation, the contemporaneous seas swarmed with life—with Articulata, Mollusca, Radiata, and Fishes. The perplexity in which we are involved when we attempt to solve this problem may be owing partly to our want of diligence as collectors, but still more perhaps to ignorance of the laws which govern the fossilisation of land-animals, whether of high or low degree.
Carboniferous Rain-prints.—At various levels in the coal measures of Nova Scotia, ripple-marked sandstones, and shales with rain-prints, were seen by Dr. Dawson and myself, but still more perfect impressions of rain were discovered by Mr. Brown, near Sydney, in the adjoining island of cape Breton. They consist of very delicate markings on greenish slates, accompanied by worm-tracks (a, b, Fig. 444), such as are often seen between high and low water mark on the recent mud of the Bay of Fundy.
The great humidity of the climate of the Coal period had been previously inferred from the number of its ferns and the continuity of its forests for hundreds of miles; but it is satisfactory to have at length obtained such positive proofs of showers of rain, the drops of which resembled in their average size those which now fall from the clouds. From such data we may presume that the atmosphere of the Carboniferous period corresponded in density with that now investing the globe, and that different currents of air varied then as now in temperature, so as to give rise, by their mixture, to the condensation of aqueous vapour.
Folding and Denudation of the Beds indicated by the Nova Scotia Coal-strata.—The series of events which are indicated by the great section of the coal-strata in Nova Scotia consist of a gradual and long-continued subsidence of a tract which throughout most of the period was in the state of a delta, though occasionally submerged beneath a sea of moderate depth. Deposits of mud and sand were first carried down into a shallow sea on the low shores of which the footprints of reptiles were sometimes impressed (see [p. 407]).
Though no regular seams of coal were formed, the characteristic imbedded coal-plants are of the genera Cyclopteris and Alethopteris, agreeing with species occurring at much higher levels, and distinct from those of the antecedent Devonian group. The Lepidodendron corrugatum (see Fig. 446), a plant predominating in the Lower Carboniferous group of Europe, is also conspicuous in these shallow-water beds, together with many fishes and entomostracans. A more rapid rate of subsidence sometimes converted part of the sea into deep clear water, in which there was a growth of coral which was afterwards turned into crystalline limestone, and parts of it, apparently by the action of sulphuric acid, into gypsum. In spite of continued sinking, amounting to several thousand feet, the sea might in time have been rendered shallow by the growth of coral, had not its conversion into land or swampy ground been accelerated by the pouring in of sand and the advance of the delta accompanied with such fluviatile and brackish-water formations as are common in lagoons.
The amount to which the bed of the sea sank down in order to allow of the formation of so vast a thickness of rock of sedimentary and organic origin is expressed by the total thickness of the Carboniferous strata, including the coal-measures, No. 1, and the rocks which underlie them, No. 2, Fig. 447.
After the strata No. 2 had been elaborated, the conditions proper to a great delta exclusively prevailed, the subsidence still continuing so that one forest after another grew and was submerged until their under-clays with roots, and usually seams of coal, were left at more than eighty distinct levels. Here and there, also, deposits bearing testimony to the existence of fresh or brackish-water lagoons, filled with calcareo-bituminous mud, were formed. In these beds (h and i, [Fig. 439]) are found fresh-water bivalves or mussels allied to Anodon, though not identical with that or any living genus, and called Naiadites carbonarius by Dawson. They are associated with small entomostracous crustaceans of the genus Cythere, and scales of small fishes. Occasionally some of the calamite brakes and forests of Sigillariæ and Coniferæ were exposed in the flood season, or sometimes, perhaps, by slight elevatory movements to the denuding action of the river or the sea.
![[Fig. 439]](#5745294564006620949_fig439.jpg.id-5453770771137636979.wrap-0.html.html){kind=link}
In order to interpret the great coast section exposed to view on the shores of the Bay of Fundy, the student must, in the first place, understand that the newest or last-mentioned coal formations would have been the only ones known to us (for they would have covered all the others), had there not been two great movements in opposite directions, the first consisting of a general sinking of three miles, which took place during the Carboniferous Period, and the second an upheaval of more limited horizontal extent, by which the anticlinal axis A was formed. That the first great change of level was one of subsidence is proved by the fact that there are shallow-water deposits at the base of the Carboniferous series, or in the lowest beds of No. 2.
Subsequent movements produced in the Nova Scotia and the adjoining New Brunswick coal-fields the usual anticlinal and synclinal flexures. In order to follow these, we must survey the country for about thirty miles round the South Joggins, or the region where the erect trees described in the foregoing pages are seen. As we pass along the cliffs for miles in a southerly direction, the beds containing these fossil trees, which were mentioned as dipping about 18° south, are less and less inclined, until they become nearly horizontal in the valley of a small river called the Shoulie, as ascertained by Dr. Dawson. After passing this synclinal line the beds begin to dip in an opposite or north-easterly direction, acquiring a steep dip where they rest unconformably on the edges of the Upper Silurian strata of the Cobequid Hills, as shown in Fig. 447. But if we travel northward towards Minudie from the region of the coal-seams and buried forests, we find the dip of the coal-strata increasing from an angle of 18° to one of more than 40°, lower beds being continually exposed to view until we reach the anticlinal axis A and see the lower Carboniferous formation, No. 2, at the surface. The missing rocks removed by denudation are expressed by the faint lines at A, and thus the student will see that, according to the principles laid down in the seventh chapter, we are enabled, by the joint operations of upheaval and denudation, to look, as it were, about three miles into the interior of the earth without passing beyond the limits of a single formation.
[1] Edward Hull, Quart. Geol. Journ., vol. xxiv, p. 327.
[2] Quart. Geol. Journ., vol. v, Mem., p. 17.
[3] Memoirs of the Geol. Survey, pp. 51, 255, etc.
[4] Dunker and V. Meyer, Palæont., vol. iv, p. 17.
[5] Acadian Geology, 2nd edit., 1868.
[6] Quart. Geol. Journ., vol. ix, p. 58.
[7] Dawson, Air-Breathers of the Coal in Nova Scotia, Montreal, 1863.
[8] Dawson, Acadian Geology, 1868, p. 385.
CHAPTER XXIV.
FLORA AND FAUNA OF THE CARBONIFEROUS PERIOD.
Vegetation of the Coal Period. — Ferns, Lycopodiaceæ, Equisetaceæ, Sigillariæ, Stigmariæ, Coniferæ. — Angiosperms. — Climate of the Coal Period. — Mountain Limestone. — Marine Fauna of the Carboniferous Period. — Corals. — Bryozoa, Crinoidea. — Mollusca. — Great Number of fossil Fish. — Foraminifera.
Vegetation of the Coal Period.—In the last chapter we have seen that the seams of coal, whether bituminous or anthracitic, are derived from the same species of plants, and Goppert has ascertained that the remains of every family of plants scattered through the shales and sandstones of the coal-measures are sometimes met with in the pure coal itself—a fact which adds greatly to the geological interest of this flora.
The coal-period was called by Adolphe Brongniart the age of Acrogens,[[1]] so great appears to have been the numerical preponderance of flowerless or cryptogamic plants of the families of ferns, club-mosses, and horse-tails. He reckoned the known species in 1849 at 500, and the number has been largely increased by recent research in spite of reductions owing to the discovery that different parts of even the same plants had been taken for distinct species. Notwithstanding these changes, Brongniart’s generalisation concerning this flora still holds true, namely, that the state of the vegetable world was then extremely different from that now prevailing, not only because the cryptogamous plants constituted nearly the whole flora, but also because they were, on the whole, more highly developed than any belonging to the same class now existing, and united some forms of structure now only found separately and in distinct orders. The only phænogamous plants were constitute any feature in the coal are the coniferæ; monocotyledonous angiosperms appear to have been very rare, and the dicotyledonous, with one or two doubtful exceptions, were wanting. For this we are in some measure prepared by what we have seen of the Secondary or Mesozoic floras if, consistently with the belief in the theory of evolution, we expect to find the prevalence of simpler and less specialised organisms in older rocks.
Ferns.—We are struck at the first glance with the similarity of the ferns to those now living. In the fossil genus Pecopteris, for example (Fig. 448), it is not easy to decide whether the fossils might not be referred to the same genera as those established for living ferns; whereas, in regard to some of the other contemporary families of plants, with the exception of the fir tribe, it is not easy to guess even the class to which they belong. The ferns of the Carboniferous period are generally without organs of fructification, but in the few instances in which these do occur in a fit state for microscopical investigations they agree with those of the living ferns.
Fig. 448: Pecopteris elliptica, Bunbury.[[2]] Frostburg.
Fig. 449: Caulopteris primæva, Lindley.
When collecting fossil specimens from the coal-measures of Frostburg, in Maryland, I found in the iron-shales several species with well-preserved rounded spots or marks of the sori (see Fig. 448). In the general absence of such characters they have been divided into genera distinguished chiefly by the branching of the fronds and the way in which the veins of the leaves are disposed. The larger portion are supposed to have been of the size of ordinary European ferns, but some were decidedly arborescent, especially the group called Caulopteris (see Fig. 449) by Lindley, and the Psaronius of the upper or newest coal-measures, before alluded to ([p. 393]). All the recent tree-ferns belong to one tribe (Polypodiaceæ), and to a small number only of genera in that tribe, in which the surface of the trunk is marked with scars, or cicatrices, left after the fall of the fronds. These scars resemble those of Caulopteris.
No less than 130 species of ferns are enumerated as having been obtained from the British coal-strata, and this number is more than doubled if we include the Continental and American species. Even if we make some reduction on the ground of varieties which have been mistaken, in the absence of their fructification, for species, still the result is singular, because the whole of Europe affords at present no more than sixty-seven indigenous species.
Lycopodiaceæ—Lepidodendron.—About forty species of fossil plants of the Coal have been referred to this genus, more than half of which are found in the British coal-measures. They consist of cylindrical stems or trunks, covered with leaf-scars. In their mode of branching, they are always dichotomous (see [Fig. 454]). They belong to the Lycopodiaceæ, bearing sporangia and spores similar to those of the living representatives of this family ([Fig. 457]); and although most of the Carboniferous species grew to the size of large trees, Mr. Carruthers has found by careful measurement that the volume of the fossil spores did not exceed that of the recent club-moss, a fact of some geological importance, as it may help to explain the facility with which these seeds may have been transported by the wind, causing the same wide distribution of the species of the fossil forests in Europe and America which we now observe in the geographical distribution of so many living families of cryptogamous plants.
![[Fig. 454]](#5745294564006620949_fig453.jpg.id-5893832561765750895.wrap-0.html.html){kind=link}
![[Fig. 457]](#5745294564006620949_fig457.jpg.id-3605171255723644103.wrap-0.html.html){kind=link}
The Figs. 453–455 represent a fossil Lepidodendron, 49 feet long, found in Jarrow Colliery, near Newcastle, lying in shale parallel to the planes of stratification. Fragments of others, found in the same shale, indicate, by the size of the rhomboidal scars which cover them, a still greater magnitude.
The living club-mosses, of which there are about 200 species, are most abundant in tropical climates. They usually creep on the ground, but some stand erect, as the Lycopodium densum from New Zealand (see Fig. 456), which attains a height of three feet.
In the Carboniferous strata of Coalbrook Dale, and in many other coal-fields, elongated cylindrical bodies, called fossil cones, named Lepidostrobus by M. Adolphe Brongniart, are met with. (See Fig. 457.) They often form the nucleus of concretionary balls of clay-ironstone, and are well preserved, exhibiting a conical axis, around which a great quantity of scales were compactly imbricated. The opinion of M. Brongniart that the Lepidostrobus is the fruit of Lepidodendron has been confirmed, for these strobili or fruits have been found terminating the tip of a branch of a well-characterised Lepidodendron in Coalbrook Dale and elsewhere.
Equisetaceæ.—To this family belong two fossil genera of the coal, Equisetites and Calamites. The Calamites were evidently closely related to the modern horse-tails (Equiseta) differing principally in their great size, the want of sheaths at the joints, and some details of fructification. They grew in dense brakes on sandy and muddy flats in the manner of modern Equisetaceæ, and their remains are frequent in the coal. Seven species of this plant occur in the great Nova Scotia section before described, where the stems of some of them five inches in diameter, and sometimes eight feet high, may be seen terminating downward in a tapering root (see Fig. 460).
Botanists are not yet agreed whether the Asterophyllites, a species of which is represented in Fig. 461, can form a separate genus from the Calamite, from which, however, according to Dr. Dawson, its foliage is distinguished by a true mid-rib, which is wanting in the leaves known to belong to some Calamites.
Figs. 462 and 463 represent leaves of Annularia and Sphenophyllum, common in the coal, and believed by Mr. Carruthers to be leaves of Calamites. Dr. Williamson, who has carefully studied the Calamites, thinks that they had a fistular pith, exogenous woody stem, and thick smooth bark, which last having always disappeared, leaves a fluted stem, as represented in Fig. 459.
Sigillaria.—A large portion of the trees of the Carboniferous period belonged to this genus, of which as many as 28 species are enumerated as British. The structure, both internal and external, was very peculiar, and, with reference to existing types, very anomalous. They were formerly referred, by M. Ad. Brongniart, to ferns, which they resemble in the scalariform texture of their vessels and, in some degree, in the form of the cicatrices left by the base of the leaf-stalks which have fallen off (see Fig. 464). But some of them are ascertained to have had long linear leaves, quite unlike those of ferns. They grew to a great height, from 30 to 60, or even 70 feet, with regular cylindrical stems, and without branches, although some species were dichotomous towards the top. Their fluted trunks, from one to five feet in diameter, appear to have decayed more rapidly in the interior than externally, so that they became hollow when standing; and when thrown prostrate, they were squeezed down and flattened. Hence, we find the bark of the two opposite sides (now converted into bright shining coal) constitute two horizontal layers, one upon the other, half an inch, or an inch, in their united thickness. These same trunks, when they are placed obliquely or vertically to the planes of stratification, retain their original rounded form, and are uncompressed, the cylinder of bark having been filled with sand, which now affords a cast of the interior.
Dr. Hooker inclined to the belief that the Sigillariæ may have been cryptogamous, though more highly developed than any flowerless plants now living. Dr. Dawson having found in some species what he regards as medullary rays, thinks with Brongniart that they have some relation to gymnogens, while Mr. Carruthers leans to the opinion that they belong to the Lycopodiaceæ.
Stigmaria.—This fossil, the importance of which has already been pointed out in [p. 398], was originally conjectured to be an aquatic plant. It is now ascertained to be the root of Sigillaria. The connection of the roots with the stem, previously suspected, on botanical grounds, by Brongniart, was first proved, by actual contact, in the Lancashire coal-field, by Mr. Binney. The fact has lately been shown, even more distinctly, by Mr. Richard Brown, in his description of the Stigmariæ occurring in the under-clays of the coal-seams of the Island of Cape Breton, in Nova Scotia. In a specimen of one of these, represented in Fig. 465, the spread of the roots was sixteen feet, and some of them sent out rootlets, in all directions, into the surrounding clay.
In the sea-cliffs of the South Joggins in Nova Scotia, I examined several erect Sigillariæ, in company with Dr. Dawson, and we found that from the lower extremities of the trunk they sent out Stigmariæ as roots. All the stools of the fossil trees dug out by us divided into four parts, and these again bifurcated, forming eight roots, which were also dichotomous when traceable far enough. The cylindrical rootlets formerly regarded as leaves are now shown by more perfect specimens to have been attached to the root by fitting into deep cylindrical pits. In the fossil there is rarely any trace of the form of these cavities, in consequence of the shrinkage of the surrounding tissues. Where the rootlets are removed, nothing remains on the surface of the Stigmaria but rows of mammillated tubercles (see Figs. 466, 467), which have formed the base of each rootlet.
These protuberances may possibly indicate the place of a joint at the lower extremity of the rootlet. Rows of these tubercles are arranged spirally round each root, which have always a medullary axis and woody system much resembling that of Sigillaria, the structure of the vessels being, like it, scalariform.
Coniferæ.—The coniferous trees of this period are referred to five genera; the woody structure of some of them showing that they were allied to the Araucarian division of pines, more than to any of our common European firs. Some of their trunks exceeded forty-four feet in height. Many, if not all of them, seem to have differed from living Coniferæ in having large piths; for Professor Williamson has demonstrated the fossil of the coal-measures called Sternbergia to be the pith of these trees, or rather the cast of cavities formed by the shrinking or partial absorption of the original medullary axis (see Figs. 468, 469). This peculiar type of pith is observed in living plants of very different families, such as the common Walnut and the White Jasmine, in which the pith becomes so reduced as simply to form a thin lining of the medullary cavity, across which transverse plates of pith extend horizontally, so as to divide the cylindrical hollow into discoid interspaces. When these interspaces have been filled up with inorganic matter, they constitute an axis to which, before their true nature was known, the provisional name of Sternbergia (d, d, Fig. 468) was given. In the above specimen the structure of the wood (b, Figs. 468 and 469) is coniferous, and the fossil is referable to Endlicher’s fossil genus Dadoxylon.
Fig. 468: Fragment of coniferous wood, Dadoxylon, of Endlicher, fractured longitudinally; from Coalbrook Dale.
W.C. Williamson[[3]]
The fossil named Trigonocarpon (Figs. 470 and 471), formerly supposed to be the fruit of a palm, may now, according to Dr. Hooker, be referred, like the Sternbergia, to the Coniferæ. Its geological importance is great, for so abundant is it in the coal-measures, that in certain localities the fruit of some species may be procured by the bushel; nor is there any part of the formation where they do not occur, except the under-clays and limestone. The sandstone, ironstone, shales, and coal itself, all contain them. Mr. Binney has at length found in the clay-ironstone of Lancashire several specimens displaying structure, and from these, says Dr. Hooker, we learn that the Trigonocarpon belonged to that large section of existing coniferous plants which bear fleshy solitary fruits, and not cones. It resembled very closely the fruit of the Chinese genus Salisburia, one of the Yew tribe, or Taxoid conifers.
Angiosperms.—The curious fossils called Antholithes by Lindley have usually been considered to be flower spikes, having what seems a calyx and linear petals (see Fig. 472). Dr. Hooker, after seeing very perfect specimens, also thought that they resembled the spike of a highly-organised plant in full flower, such as one of the Bromeliaceæ, to which Professor Lindley had at first compared them. Mr. Carruthers, who has lately examined a large series in different museums, considers it to be a dicotyledonous angiosperm allied to Orobanche (broom-rape), which grew, not on the soil, but parasitically on the trees of the coal forests.
In the coal-measures of Granton, near Edinburgh, a remarkable fossil (Fig. 473) was found and described in 1840,[[4]] by Dr. Robert Paterson. It was compressed between layers of bituminous shale, and consists of a stem bearing a cylindrical spike, a, which in the portion preserved in the slate exhibits two subdivisions and part of a third. The spike is covered on the exposed surface with the four-cleft calyces of the flowers arranged in parallel rows. The stem shows, at b, a little below the spike, remains of a lateral appendage, which is supposed to indicate the beginning of the spathe. The fossil has been referred to the Aroidiæ, and there is every probability that it is a true member of this order. There can at least be no doubt as to the high grade of its organisation, and that it belongs to the monocotyledonous angiosperms. Mr. Carruthers has carefully examined the original specimen in the Botanical Museum, Edinburgh, and thinks it may have been an epiphyte.
Climate of the Coal Period.—As to the climate of the Coal, the Ferns and the Coniferæ are perhaps the two classes of plants which may be most relied upon as leading us to safe conclusions, as the genera are nearly allied to living types. All botanists admit that the abundance of ferns implies a moist atmosphere. But the coniferæ, says Hooker, are of more doubtful import, as they are found in hot and dry, and in cold and dry climates; in hot and moist, and in cold and moist regions. In New Zealand the coniferæ attain their maximum in numbers, constituting 1/62 part of all the flowering plants; whereas in a wide district around the Cape of Good Hope they do not form 1/1600 of the phenogamic flora. Besides the conifers, many species of ferns flourish in New Zealand, some of them arborescent, together with many lycopodiums; so that a forest in that country may make a nearer approach to the carboniferous vegetation than any other now existing on the globe.
MARINE FAUNA OF THE CARBONIFEROUS PERIOD.
It has already been stated that the Carboniferous or Mountain Limestone underlies the coal-measures in the South of England and Wales, whereas in the North, and in Scotland, marine calcareous rocks partly of the age of the Mountain Limestone alternate with shales and sandstones, containing seams of coal. In its most calcareous form the Mountain Limestone is destitute of land-plants, and is loaded with marine remains—the greater part, indeed, of the rock being made up bodily of crinoids, corals, and bryozoa with interspersed mollusca.
Corals.—The corals deserve especial notice, as the cup-and-star corals, which have the most massive and stony skeletons, display peculiarities of structure by which they may be distinguished generally, as MM. Milne Edwards and Haime first pointed out, from all species found in strata newer than the Permian. There is, in short, an ancient or Palæozoic, and a modern or Neozoic type, if, by the latter term, we designate (as proposed by Professor E. Forbes) all strata from the triassic to the most modern, inclusive. The accompanying diagrams (Figs. 474, 475) may illustrate these types.
| Vertical section of Campophyllum flexuosum, (Cyathophyllum, Goldfuss); from the Devonian of the Eifel. The lamellæ are seen around the inside of the cup; the walls consist of cellular tissue; and large transverse plates, called tubulæ, divide the interior into chambers. Arrangement of the lamellæ in Polycoelia profunda, Germar, sp.; from the Magnesian Limestone, Durham. This diagram shows the quadripartite arrangement of the primary septa, characteristic of palæozoic corals, there being four principal and eight intermediate lamellæ, the whole number in this type being always a multiple of four. Stauria astræiformis, Milne Edwards. Young group, natural size. Upper Silurian, Gothland. The lamellæ or septal system in each cup are divided by four prominent ridges into four groups. | |
| Parasmilia centralis, Mantell, sp. Vertical section. Upper Chalk, Gravesend. In this type the lamellæ are massive, and extend to the axis or columella composed of loose cellular tissue, without any transverse plates like those in Fig. 474, a. Cyathina Bowerbankii, Ed. and H. Transverse section, enlarged. Gault, Folkestone. In this coral the primary septa are a multiple of six. The twelve principal plates reach the columella, and between each pair there are three secondaries, in all forty-eight. The short intermediate plates which proceed from the columella are not counted. They are called pali. Fungia patellaris, Lamarck. Recent; very young state. Diagram of its six primary and six secondary septa, magnified. The sextuple arrangement is always more manifest in the young than in the adult state. |
- Vertical section of Campophyllum flexuosum, (Cyathophyllum, Goldfuss); from the Devonian of the Eifel. The lamellæ are seen around the inside of the cup; the walls consist of cellular tissue; and large transverse plates, called tubulæ, divide the interior into chambers.
- Arrangement of the lamellæ in Polycoelia profunda, Germar, sp.; from the Magnesian Limestone, Durham. This diagram shows the quadripartite arrangement of the primary septa, characteristic of palæozoic corals, there being four principal and eight intermediate lamellæ, the whole number in this type being always a multiple of four.
- Stauria astræiformis, Milne Edwards. Young group, natural size. Upper Silurian, Gothland. The lamellæ or septal system in each cup are divided by four prominent ridges into four groups.
- Parasmilia centralis, Mantell, sp. Vertical section. Upper Chalk, Gravesend. In this type the lamellæ are massive, and extend to the axis or columella composed of loose cellular tissue, without any transverse plates like those in Fig. 474, a.
- Cyathina Bowerbankii, Ed. and H. Transverse section, enlarged. Gault, Folkestone. In this coral the primary septa are a multiple of six. The twelve principal plates reach the columella, and between each pair there are three secondaries, in all forty-eight. The short intermediate plates which proceed from the columella are not counted. They are called pali.
- Fungia patellaris, Lamarck. Recent; very young state. Diagram of its six primary and six secondary septa, magnified. The sextuple arrangement is always more manifest in the young than in the adult state.
It will be seen that the more ancient corals have what is called a quadripartite arrangement of the chief plates or lamellæ—parts of the skeleton which support the organs of reproduction. The number of these lamellæ in the Palæozoic type is 4, 8, 16, etc.; while in the Neozoic type the number is 6, 12, 24, or some other multiple of six; and this holds good, whether they be simple forms, as in Figs. 474, a, and 475, a, or aggregate clusters of corallites, as in 474, c. But further investigations have shown in this, as in all similar grand generalisations in natural history, that there are excepions to the rule. Thus in the Lower Greensand Holocystis elegans (Ed. and H.) and other forms have the Palæozoic type, and Dr. Duncan has shown to what extent the Neozoic forms penetrate downward into the Carboniferous and Devonian rocks.
From a great number of lamelliferous corals met with in the Mountain Limestone, two species (Figs. 476, 477) have been selected, as having a very wide range, extending from the eastern borders of Russia to the British Isles, and being found almost everywhere in each country. These fossils, together with numerous species of Zaphrentis, Amplexus, Cyathophyllum, Clisiophyllum, Syringopora, and Michelinia,[[5]] form a group of rugose corals widely different from any that followed them.
Bryozoa and Crinoidea.—Of the Bryozoa, the prevailing forms are Fenestella, Hemitrypa, and Polypora, and these often form considerable beds. Their net-like fronds are easily recognised. Crinoidea are also numerous in the Mountain Limestone (see Figs. 478, 479), two genera, Pentremites and Codonaster, being peculiar to this formation in Europe and North America.
In the greater part of them, the cup or pelvis, Figure 479, b, is greatly developed in size in proportion to the arms, although this is not the case in Fig. 478. The genera Poteriocrinus, Cyathocrinus, Pentremites, Actinocrinus, and Platycrinus, are all of them characteristic of this formation. Other Echinoderms are rare, a few Sea-Urchins only being known: these have a complex structure, with many more plates on their surface than are seen in the modern genera of the same group. One genus, the Palæchinus (Fig. 480), is the analogue of the modern Echinus, but has four, five, or six rows of plates in the interambulacral region or area, whereas the modern genera have only two. The other, Archæocidaris, represents, in like manner, the Cidaris of the present seas.
Mollusca.—The British Carboniferous mollusca enumerated by Mr. Etheridge[[6]] comprise 653 species referable to 86 genera, occurring chiefly in the Mountain Limestone. Of this large number only 40 species are common to the underlying Devonian rocks, 9 of them being Cephalopods, 7 Gasteropods, and the rest bivalves, chiefly Brachiopoda (or Palliobranchiates). This latter group constitutes the larger part of the Carboniferous Mollusca, 157 species being known in Great Britain alone, and it will be found to increase in importance in the fauna of the primary rocks the lower we descend in the series. Perhaps the most characteristic shells of the formation are large species of Productus, such as P. giganteus, p. hemisphericus, P. semireticulatus (Fig. 481), and P. scabriculus. Large plaited spirifers, as Spirifera striata, S. rotundata, and S. trigonalis (Fig. 482), also abound; and smooth species, such as Spirifera glabra (Fig. 483), with its numerous varieties.
Among the brachiopoda, Terebratula hastata (Fig. 484) deserves mention, not only for its wide range, but because it often retains the pattern of the original coloured stripes which ornamented the living shell. These coloured bands are also preserved in several lamellibranchiate bivalves, as in Aviculopecten (Fig. 485), in which dark stripes alternate with a light ground. In some also of the spiral univalves the pattern of the original painting is distinctly retained, as in Pleurotomaria (Fig. 486), which displays wavy blotches, resembling the colouring in many recent trochidæ.
Some few of the carboniferous mollusca, such as Avicula, Nucula (sub-genus Ctenodonta), Solemya, and Lithodomus, belong no doubt to existing genera; but the majority, though often referred to as living types, such as Isocardia, Turritella, and Buccinum, belong really to forms which appear to have become extinct at the close of the Palæozoic epoch. Euomphalus is a characteristic univalve shell of this period. In the interior it is divided into chambers (Fig. 487, d), the septa or partitions not being perforated as in foraminiferous shells, or in those having siphuncles, like the Nautilus. The animal appears to have retreated at different periods of its growth from the internal cavity previously formed, and to have closed all communication with it by a septum. The number of chambers is irregular, and they are generally wanting in the innermost whorl. The animal of the recent Turritella communis partitions off in like manner as it advances in age a part of its spire, forming a shelly septum.
More than twenty species of the genus Bellerophon (see Fig. 488), a shell like the living Argonaut without chambers, occur in the Mountain Limestone. The genus is not met with in strata of later date. It is most generally regarded as belonging to the pelagic Nucleobranchiata and the family Atlantidæ, partly allied to the Glass-Shell, Carinaria; but by some few it is thought to be a simple form of Cephalopod.
The carboniferous Cephalopoda do not depart so widely from the living type (the Nautilus) as do the more ancient Silurian representatives of the same order; yet they offer some remarkable forms. Among these is Orthoceras, a siphuncled and chambered shell, like a Nautilus uncoiled and straightened (Fig. 489). Some species of this genus are several feet long. The Goniatite is another genus, nearly allied to the Ammonite, from which it differs in having the lobes of the septa free from lateral denticulations, or crenatures; so that the outline of these is angular, continuous, and uninterrupted. The species represented in Fig. 490 is found in most localities, and presents the zigzag character of the septal lobes in perfection. The dorsal position of the siphuncle, however, clearly distinguishes the Goniatite from the Nautilus, and proves it to have belonged to the family of the Ammonites, from which, indeed, some authors do not believe it to be generically distinct.
Fossil Fish.—The distribution of these is singularly partial; so much so, that M. De Koninck of Liége, the eminent palæontologist, once stated to me that, in making his extensive collection of the fossils of the Mountain Limestone of Belgium, he had found no more than four or five examples of the bones or teeth of fishes. Judging from Belgian data, he might have concluded that this class of vertebrata was of extreme rarity in the Carboniferous seas; whereas the investigation of other countries has led to quite a different result. Thus, near Clifton, on the Avon, as well as at numerous places around the Bristol basin from the Mendip Hills to Tortworth, there is a celebrated “bone-bed,” almost entirely made up of ichthyolites. It occurs at the base of the Lower Limestone shales immediately resting upon the passage beds of the Old Red Sandstone. Similar bone-beds occur in the Carboniferous Limestone of Armagh, in Ireland, where they are made up chiefly of the teeth of fishes of the Placoid order, nearly all of them rolled as if drifted from a distance. Some teeth are sharp and pointed, as in ordinary sharks, of which the genus Cladodus afford an illustration; but the majority, as in Psammodus and Cochliodus, are, like the teeth of the Cestracion of Port Jackson (see [Fig. 261]), massive palatal teeth fitted for grinding. (See Figs. 491, 492.)
![[Fig. 261]](#5745294564006620949_fig261.jpg.id-5822349377446198851.wrap-0.html.html){kind=link}
There are upward of seventy other species of fossil fish known in the Mountain Limestone of the British Islands. The defensive fin-bones of these creatures are not infrequent at Armagh and Bristol; those known as Oracanthus, Ctenocanthus, and Onchus are often of a very large size. Ganoid fish, such as Holoptychius, also occur; but these are far less numerous. The great Megalichthys Hibberti appears to range from the Upper Coal-measures to the lowest Carboniferous strata.
Foraminifera.—In the upper part of the Mountain Limestone group in the S.W. of England, near Bristol, limestones having a distinct oolitic structure alternate with shales. In these rocks the nucleus of every minute spherule is seen, under the microscope, to consist of a small rhizopod or foraminifer. This division of the lower animals, which is represented so fully at later epochs by the Nummulites and their numerous minute allies, appears in the Mountain Limestone to be restricted to a very few species, among which Textularia, Nodosaria, Endothyra, and Fusulina (Fig. 493), have been recognised. The first two genera are common to this and all the after periods; the third has been found in the Upper Silurian, but is not known above the Carboniferous strata; the fourth (Fig. 493) is characteristic of the Mountain Limestone in the United States, Arctic America, Russia, and Asia Minor, but is also known in the Permian.
[1] For botanical nomenclature see [p. 304].
[2] Sir C. Bunbury, Quart. Geol. Journ., vol. ii, 1845.
[3] Manchester Phil. Mem., vol. ix, 1851.
[4] Trans. of Bot. Soc. of Edinburgh, vol. i, 1844.
[5] For figures of these corals, see Palæontographical Society’s Monographs, 1852.
[6] Quart. Geol. Journ., vol. xxiii, p. 674, 1867.
CHAPTER XXV.
DEVONIAN OR OLD RED SANDSTONE GROUP.
Classification of the Old Red Sandstone in Scotland and in Devonshire. — Upper Old Red Sandstone in Scotland, with Fish and Plants. — Middle Old Red Sandstone. — Classification of the Ichthyolites of the Old Red, and their Relation to Living Types. — Lower Old Red Sandstone, with Cephalaspis and Pterygotus. — Marine or Devonian Type of Old Red Sandstone. — Table of Devonian Series. — Upper Devonian Rocks and Fossils. — Middle. — Lower. — Eifel Limestone of Germany. — Devonian of Russia. — Devonian Strata of the United States and Canada. — Devonian Plants and Insects of Canada.
Classification of the two Types of Old Red Sandstone.—We have seen that the Carboniferous strata are surmounted by the Permian and Trias, both originally included in England under the name “New Red Sandstone,” from the prevailing red colour of the strata. Under the coal came other red sandstones and shales which were distinguished by the title of “Old Red Sandstone.” Afterwards the name of “Devonian” was given by Sir R. Murchison and Professor Sedgwick to marine fossiliferous strata which, in the south of England, occupy a similar position between the overlying coal and the underlying Silurian formations.
It may be truly said that in the British Isles the rocks of this age present themselves in their mineral aspect, and even to some extent in their fossil contents, under two very different forms; the one as distinct from the other as are often lacustrine or fluviatile from marine strata. It has indeed been suggested that by far the greater part of the deposits belonging to what may be termed the Old Red Sandstone type are of fresh-water origin. The number of land-plants, the character of the fishes, and the fact that the only shell yet discovered belongs to the genus Anodonta, must be allowed to lend no small countenance to this opinion. In this case the difficulty of classification when the strata of this type are compared in different regions, even where they are contiguous, may arise partly from their having been formed in distinct hydrographical basins, or in the neighbourhood of the land in shallow parts of the sea into which large bodies of fresh-water entered, and where no marine mollusca or corals could flourish. Under such geographical conditions the limited extent of some kinds of sediment, as well as the absence of those marine forms by which we are able to identify or contrast marine formations, may be explained, while the great thickness of the rocks, which might seem at first sight to require a corresponding depth of water, can often be shown to have been due to the gradual sinking down of the bottom of the estuary or sea where the sediment was accumulated.
Another active cause of local variation in Scotland was the frequency of contemporaneous volcanic eruptions; some of the rocks derived from this source, as between the Grampians and the Tay, having formed islands in the sea, and having been converted into shingle and conglomerate, before the upper portions of the red shales and sandstones were superimposed.
The dearth of calcareous matter over wide areas is characteristic of the Old Red Sandstone. This is, no doubt, in great part due to the absence of shells and corals; but why should these be so generally wanting in all sedimentary rocks the colour of which is determined by the red oxide of iron? Some geologists are of opinion that the waters impregnated with this oxide were prejudicial to living beings, others that strata permeated with this oxide would not preserve such fossil remains.
In regard to the two types, the Old Red Sandstone and the Devonian, I shall first treat of them separately, and then allude to the proofs of their having been to a great extent contemporaneous. That they constitute a series of rocks intermediate in date between the lowest Carboniferous and the uppermost Silurian is not disputed by the ablest geologists; and it can no longer be contended that the Upper, Middle, and Lower Old Red Sandstone preceded in date the three divisions to which, by aid of the marine shells, the Devonian rocks have been referred, while, on the other hand, we have not yet data for enabling us to affirm to what extent the subdivisions of the one series may be the equivalents in time of those of the other.
Upper Old Red Sandstone.—The highest beds of the series in Scotland, lying immediately below the coal in Fife, are composed of yellow sandstone well seen at Dura Den, near Coupar, in Fife, where, although the strata contain no mollusca, fish have been found abundantly, and have been referred to the genera Holoptychius, Pamphractus, Glyptopomus, and many others. In the county of Cork, in Ireland, a similar yellow sandstone occurs containing fish of genera characteristic of the Scotch Old Red Sandstone, as for example Coccosteus (a form represented by many species in the Old Red Sandstone and by one only in the Carboniferous group), and Glytolepis and Asterolepis, both exclusively confined to the “Old Red.” In the same Irish sandstone at Kiltorkan has been found an Anodonta or fresh-water mussel, the only shell hitherto discovered in the Old Red Sandstone of the British Isles (see Fig. 494).
In the same formation are found the fern (Fig. 496) and the Lepidodendron (Fig. 495), and other species of plants, some of which, Professor Heer remarks, agree specifically with species from the lower carboniferous beds. This induces him to lean to the opinion long ago advocated by Sir Richard Griffiths, that the yellow sandstone, in spite of its fish remains, should be classed as Lower Carboniferous, an opinion which I am not yet prepared to adopt. Between the Mountain Limestone and the yellow sandstone in the south-west of Ireland there intervenes a formation no less than 5000 feet thick, called the “Carboniferous slate,” and at the base of this, in some places, are local deposits, such as the Glengariff Grits, which appear to be beds of passage between the Carboniferous and Old Red Sandstone groups.
It is a remarkable result of the recent examination of the fossil flora of Bear Island, latitude 74° 30′ N., that Professor Heer has described as occurring in that part of the Arctic region (nearly twenty-six degrees to the north of the Irish locality) a flora agreeing in several of its species with that of the yellow sandstones of Ireland. This Bear Island flora is believed by Professor Heer to comprise species of plants some of which ascend even to the higher stages of the European Carboniferous formation, or as high as the Mountain Limestone and Millstone Grit. Palæontologists have long maintained that the same species which have a wide range in space are also the most persistent in time, which may prepare us to find that some plants having a vast geographical range may also have endured from the period of the Upper Devonian to that of the Millstone Grit.
Outliers of the Upper “Old Red” occur unconformably on older members of the group, and the formation represented at Whiteness, near Arbroath, a, [Fig. 55,] may probably be one of these outliers, though the want of organic remains renders this uncertain. It is not improbable that the beds given in this section as Nos. 1, 2, and 3, may all belong to the early part of the period of the Upper Old Red, as some scales of Holoptychius nobilissimus have been found scattered through these beds, No. 2, in Strathmore. Another nearly allied Holoptychius occurs in Dura Den, see Fig. 498 of this fish and also Fig. 497 of one of its scales, as these last are often the only parts met with; being scattered in Forfarshire through red-coloured shales and sandstones, as are scales of a large species of the same genus in a corresponding matrix in Herefordshire.[[1]] The number of fish obtained from the British Upper Old Red Sandstone amounts to fifteen species referred to eleven genera.
![[Fig. 55,]](#5745294564006620949_fig55.jpg.id-7176558459861297802.wrap-0.html.html){kind=link}
Sir R. Murchison groups with this upper division of the Old Red of Scotland certain light-red and yellow sandstones and grits which occur in the northernmost part of the mainland, and extend also into the Orkney and Shetland Islands. They contain Calamites and other plants which agree generically with Carboniferous forms.
Middle Old Red Sandstone.—In the northern part of Scotland there occur a great series of bituminous schists and flagstones, to the fossil fish of which attention was first called by the late Hugh Miller. They were afterwards described by Agassiz, and the rocks containing them were examined by Sir R. Murchison and Professor Sedgwick, in Caithness, Cromarty, Moray, Nairn, Gamrie in Banff, and the Orkneys and Shetlands, in which great numbers of fossil fish have been found. These were at first supposed to be the oldest known vertebrate animals, as in Cromarty the beds in which they occur seem to form the base of the Old Red system resting almost immediately on the crystalline or metamorphic rocks. But in fact these fish-bearing beds, when they are traced from north to south, or to the central parts of Scotland, thin out, so that their relative age to the Lower Old Red Sandstone, presently to be mentioned, was not at first detected, the two formations not appearing in superposition in the same district. In Caithness, however, many hundred feet below the fish-zone of the middle division, remains of Pteraspis were found by Mr. Peach in 1861. This genus has never yet been found in either of the two higher divisions of the Old Red Sandstone, and confirms Sir R. Murchison’s previous suspicion that the rocks in which it occurs belong to the Lower “Old Red,” or agree in age with the Arbroath paving-stone.[[2]]
Fossil Fish of the Middle Old Red Sandstone.—The Devonian fish were referred by Agassiz to two of his great orders, namely, the Placoids and Ganoids. Of the first of these, which in the Recent period comprise the shark, the dog-fish, and the ray, no entire skeletons are preserved, but fin-spines, called ichthyodorulites, and teeth occur. On such remains the genera Onchus, Odontacanthus, and Ctenodus, a supposed cestraciont, and some others, have been established.
By far the greater number of the Old Red Sandstone fishes belong to a sub-order of Ganoids instituted by Huxley in 1861, and for which he has proposed the name of Crossopterygidæ,[[3]] or the fringe-finned, in consideration of the peculiar manner in which the fin-rays of the paired fins are arranged so as to form a fringe round a central lobe, as in the Polypterus (see a, Fig. 499), a genus of which there are several species now inhabiting the Nile and other African rivers. The reader will at once recognise in Osteolepis (Fig. 500), one of the common fishes of the Old Red Sandstone, many points of analogy with Polypterus. They not only agree in the structure of the fin, at first pointed out by Huxley, but also in the position of the pectoral, ventral, and anal fins, and in having an elongated body and rhomboidal scales. On the other hand, the tail is more symmetrical in the recent fish, which has also an apparatus of dorsal finlets of a very abnormal character, both as to number and structure. As to the dorsals of Osteolepis, they are regular in structure and position, having nothing remarkable about them, except that there are two of them, which is comparatively unusual in living fish.
Among the “fringe-finned” Ganoids we find some with rhomboidal scales, such as Osteolepis, Fig. 500; others with cycloidal scales, as Holoptychius, before mentioned (see Fig. 498). In the genera Dipterus and Diplopterus, as Hugh Miller pointed out, and in several other of the fringe-finned genera, as in Gyroptychius and Glyptolepis, the two dorsals are placed far backward, or directly over the ventral and anal fins. The Asterolepis was a ganoid fish of gigantic dimensions. A. Asmusii, Eichwald, a species characteristic of the Old Red Sandstone of Russia, as well as that of Scotland, attained the length of between twenty and thirty feet. It was clothed with strong bony armour, embossed with star-like tubercles, but it had only a cartilaginous skeleton. The mouth was furnished with two rows of teeth, the outer ones small and fish-like, the inner larger and with a reptilian character. The Asterolepis occurs also in the Devonian rocks of North America.
If we except the Placoids already alluded to, and a few other families of doubtful affinities, all the Old Red Sandstone fishes are Ganoids, an order so named by Agassiz from the shining outer surface of their scales; but Professor Huxley has also called our attention to the fact that, while a few of the primary and the great majority of the secondary Ganoids resemble the living bony pike, Lepidosteus, or the Amia, genera now found in North American rivers, and one of them, Lepidosteus, extending as far south as Guatemala, the Crossopterygii, or fringe-finned Ichthyolites, of the Old Red are closely related to the African Polypterus, which is represented by five or six species now inhabiting the Nile and the rivers of Senegal. These North American and African Ganoids are quite exceptional in the living creation; they are entirely confined to the northern hemisphere, unless some species of Polypterus range to the south of the line in Africa; and, out of about 9000 living species of fish known to M. Günther, and of which more than 6000 are now preserved in the British Museum, they probably constitute no more than nine.
If many circumstances favour the theory of the fresh-water origin of the Old Red Sandstone, this view of its nature is not a little confirmed by our finding that it is in Llake Superior and the other inland Canadian seas of fresh water, and in the Mississippi and African rivers, that we at present find those fish which have the nearest affinity to the fossil forms of this ancient formation.
Among the anomalous forms of Old Red fishes not referable to Huxley’s Crossopterygii is the Pterichthys, of which five species have been found in the middle division of the Old Red of Scotland. Some writers have compared their shelly covering to that of Crustaceans, with which, however, they have no real affinity. The wing-like appendages, whence the genus is named, were first supposed by Hugh Miller to be paddles, like those of the turtle; and there can now be no doubt that they do really correspond with the pectoral fins.
The number of species of fish already obtained from the middle division of the Old Red Sandstone in Great Britain is about 70, and the principal genera, besides Osteolepis and Pterichthys, already mentioned, are Glyptolepis, Diplacanthus, Dendrodus, Coccosteus, Cheirancanthus, and Acanthoides.
Lower Old Red Sandstone.—The third or lowest division south of the Grampians consists of grey paving-stone and roofing-slate, with associated red and grey shales; these strata underlie a dense mass of conglomerate. In these grey beds several remarkable fish have been found of the genus named by Agassiz Cephalaspis, or “buckler-headed,” from the extraordinary shield which covers the head (see Fig. 502), and which has often been mistaken for that of a trilobite, such as Asaphus. A species of Pteraspis, of the same family, has also been found by the Reverend Hugh Mitchell in beds of corresponding age in Perthshire; and Mr. Powrie enumerates no less than five genera of the family Acanthodidæ, the spines, scales, and other remains of which have been detected in the grey flaggy sandstones.[[4]]
In the same formation at Carmylie, in Forfarshire, commonly known as the Arbroath paving-stone, fragments of a huge crustacean have been met with from time to time. They are called by the Scotch quarrymen the “Seraphim,” from the wing-like form and feather-like ornament of the thoracic appendage, the part most usually met with. Agassiz, having previously referred some of these fragments to the class of fishes, was the first to recognise their crustacean character, and, although at the time unable correctly to determine the true relation of the several parts, he figured the portions on which he founded his opinion, in the first plate of his “Poissons Fossiles du Vieux Grès Rouge.”
| Carapace, showing the large sessile eyes at the anterior angles. The metastoma or post-oral plate (serving the office of a lower lip). Chelate appendages (antennules). First pair of simple palpi (antennæ). Second pair of simple palpi (mandibles). Third pair of simple palpi (first maxillæ). Pair of swimming feet with their broad basal joints, whose serrated edges serve the office of maxillæ. Thoracic plate covering the first two thoracic segments, which are indicated by the figures 1, 2, and a dotted line. 1-6. Thoracic segments. 7-12. Abdominal segments. 13. Telson, or tail-plate.) |
- Carapace, showing the large sessile eyes at the anterior angles.
- The metastoma or post-oral plate (serving the office of a lower lip).
- Chelate appendages (antennules).
- First pair of simple palpi (antennæ).
- Second pair of simple palpi (mandibles).
- Third pair of simple palpi (first maxillæ).
- Pair of swimming feet with their broad basal joints, whose serrated edges serve the office of maxillæ.
- Thoracic plate covering the first two thoracic segments, which are indicated by the figures 1, 2, and a dotted line. 1-6. Thoracic segments. 7-12. Abdominal segments. 13. Telson, or tail-plate.)
A restoration in correct proportion to the size of the fragments of P. anglicus (Fig. 504), from the Lower Old Red Sandstone of Perthshire and Forfarshire, would give us a creature measuring from five to six feet in length, and more than one foot across.
The largest crustaceans living at the present day are the Inachus Kaempferi, of De Haan, from Japan (a brachyurous or short-tailed crab), chiefly remarkable for the extraordinary length of its limbs; the fore-arm measuring four feet in length, and the others in proportion, so that it covers about 25 square feet of ground; and the Limulus Moluccanus, the great King Crab of China and the Eastern seas, which, when adult, measures 1½ foot across its carapace, and is three feet in length.
Besides some species of Pterygotus, several of the allied genus Eurypterus occur in the Lower Old Red Sandstone, and with them the remains of grass-like plants so abundant in Forfarshire and Kincardineshire as to be useful to the geologist by enabling him to identify the inferior strata at distant points. Some botanists have suggested that these plants may be of the family Fluviales, and of fresh-water genera. They are accompanied by fossils, called “berries” by the quarrymen, which they compared to a compressed blackberry (see Figs. 505, 506), and which were called “Parka” by Dr. Fleming. They are now considered by Mr. Powrie to be the eggs of crustaceans, which is highly probable, for they have not only been found with Pterygotus anglicus in Forfarshire and Perthshire, but also in the Upper Silurian strata of England, in which species of the same genus, Pterygotus, occur.
The grandest exhibitions, says Sir R. Murchison, of the Old Red Sandstone in England and Wales appear in the escarpments of the Black Mountains and in the Fans of Brecon and Carmarthen, the one 2862, and the other 2590 feet above the sea. The mass of red and brown sandstone in these mountains is estimated at not less than 10,000 feet, clearly intercalated between the Carboniferous and Silurian strata. No shells or corals have ever been found in the whole series, not even where the beds are calcareous, forming irregular courses of concretionary lumps called “corn-stones,” which may be described as mottled red and green earthy limestones. The fishes of this lowest English Old Red are Cephalaspis and Pteraspis, specifically different from species of the same genera which occur in the uppermost Ludlow or Silurian tilestones. Crustaceans also of the genus Eurypterus are met with.
Marine or Devonian Type.—We may now speak of the marine type of the British strata intermediate between the Carboniferous and Silurian, in treating of which we shall find it much more easy to identify the Upper, Middle, and Lower divisions with strata of the same age in other countries. It was not until the year 1836 that Sir R. Murchison and Professor Sedgwick discovered that the culmiferous or anthracitic shales and sandstones of North Devon, several thousand feet thick, belonged to the coal, and that the beds below them, which are of still greater thickness, and which, like the carboniferous strata, had been confounded under the general name “graywacke,” occupied a geological position corresponding to that of the Old Red Sandstone already described. In this reform they were aided by a suggestion of Mr. Lonsdale, who, after studying the Devonshire fossils, perceived that they belonged to a peculiar palæontological type of intermediate character between the Carboniferous and Silurian.
It is in the north of Devon that these formations may best be studied, where they have been divided into an Upper, Middle, and Lower Group, and where, although much contorted and folded, they have for the most part escaped being altered by intrusive trap-rocks and by granite, which in Dartmoor and the more southern parts of the same county have often reduced them to a crystalline or metamorphic state.
DEVONIAN SERIES IN NORTH DEVON.
| UPPER DEVONIAN OR PILTON GROUP |
(a) Sandy slates and
schists with fossils, 36 species out of 110 common to the
Carboniferous group (Pilton, Barnstaple, etc.), resting on soft
schists in which fossils are very abundant (Croyde, etc.), and
which pass down into (b) Yellow, brown, and red sandstone, with land plants (Cyclopteris, etc.) and marine shells. One zone, characterised by the abundance of cucullæa (Baggy Point, Marwood, Sloly, etc.) resting on hard grey and reddish sandstone and micaceous flags, no fossils yet found (Dulverton, Pickwell, Down, etc.) |
| MIDDLE DEVONIAN OR ILFRACOMBE GROUP. | (a) Green glossy slates of
considerable thickness, no fossils yet recorded from these beds
(Mortenoe, Lee Bay, etc.). (b) Slates and schists, with several irregular courses of limestone containing shells and corals like those of the Plymouth Limestone (Combe Martin, Ilfracombe, etc.). |
| LOWER DEVONIAN OR LYNTON GROUP. | (a) Hard, greenish, red, and purple
sandstone—no fossils yet found (Hangman Hill, etc.). (b) Soft slates with subordinate sandstones—fossils numerous at various horizons—Orthis, Corals, Encrinites, etc. (Valley of Rocks, Lynmouth, etc.). |
The above table exhibits the sequence of the strata or subdivisions as seen both on the sea-coast of the British Channel and in the interior of Devon. It will be seen that in all main points it agrees with the table drawn up in 1864 for the sixth edition of my “Elements.” Mr. Etheridge[[5]] has since published an excellent account of the different subdivisions of the rocks and their fossils, and has also pointed out their relation to the corresponding marine strata of the Continent. The slight modifications introduced in my table since 1864 are the result of a tour made in 1870 in company with Mr. T. Mck. Hughes, when we had the advantage of Mr. Etheridge’s memoir as our guide.
The place of the sandstones of the Foreland is not yet clearly made out, as they are cut off by a great fault and disturbance.
Upper Devonian Rocks.—The slates and sandstones of Barnstaple (a and b of the preceding section) contain the shell Spirifera disjuncta, Sowerby (S. Verneuilii, Murch.), (see Fig. 508), which has a very wide range in Europe, Asia Minor, and even China; also Strophalosia caperata, together with the large trilobite Phacops latifrons, Bronn. (See Fig. 509), which is all but world-wide in its distribution. The fossils are numerous, and comprise about 150 species of mollusca, a fifth of which pass up into the overlying Carboniferous rocks. To this Upper Devonian belong a series of limestones and slates well developed at Petherwyn, in Cornwall, where they have yielded 75 species of fossils. The genus of Cephalopoda called Clymenia (Fig. 510) is represented by no less than eleven species, and strata occupying the same position in Germany are called Clymenien-Kalk, or sometimes Cypridinen-Schiefer, on account of the number of minute bivalve shells of the crustacean called Cypridina serrato-striata (Fig. 511), which is found in these beds, in the Rhenish provinces, the Harz, Saxony, and Silesia, as well as in Cornwall and Belgium.
Middle Devonian Rocks.—We come next to the most typical portion of the Devonian system, including the great limestones of Plymouth and Torbay, replete with shells, trilobites, and corals. Of the corals 51 species are enumerated by Mr. Etheridge, none of which pass into the Carboniferous formation. Among the genera we find Favosites, Heliolites, and Cyathophyllum. The two former genera are very frequent in Silurian rocks: some few even of the species are said to be common to the Devonian and Silurian groups, as, for example, Favosites cervicornis (Fig. 513), one of the commonest of all the Devonshire fossils. The Cyathophyllum cæspitosum (Fig. 514) and Heliolites pyriformis (Fig. 512) are species peculiar to this formation.
With the above are found no less than eleven genera of stone-lilies or crinoids, some of them, such as Cupressocrinites, distinct from any Carboniferous forms. The mollusks, also, are no less characteristic; of 68 species of Brachiopoda, ten only are common to the Carboniferous Limestone. The Stringocephalus Burtini (Fig. 515) and Uncites Gryphus (Fig. 516) may be mentioned as exclusively Middle Devonian genera, and extremely characteristic of the same division in Belgium. The Stringocephalus is also so abundant in the Middle Devonian of the banks of the Rhine as to have suggested the name of Stringocephalus Limestone.
The only two species of Brachiopoda common to the Silurian and Devonian formations are Atrypa reticularis (Fig. 532), which seems to have been a cosmopolite species, and Strophomena rhomboidalis.
Among the peculiar lamellibranchiate bivalves common to the Plymouth limestone of Devonshire and the Continent, we find the Megalodon (Fig. 517). There are also twelve genera of Gasteropods which have yielded 36 species, four of which pass to the Carboniferous group, namely Macrocheilus, Acroculia, Euomphalus, and Murchisonia. Pteropods occur, such as Conularia (Fig. 518), and Cephalopods, such as Cyrtoceras, Gyroceras, Orthoceras, and others, nearly all of genera distinct from those prevailing in the Upper Devonian Limestone, or Clymenien-kalk of the Germans already mentioned. Although but few species of Trilobites occur, the characteristic Bronteus flabellifer (Fig. 519) is far from rare, and all collectors are familiar with its fan-like tail. In this same group, called, as before stated, the Stringocephalus, or Eifel Limestone, in Germany, several fish remains have been detected, and among others the remarkable genus Coccosteus, covered with its tuberculated bony armour; and these ichthyolites serve, as Sir R. Murchison observes (Siluria, p. 362), to identify this middle marine Devonian with the Old Red Sandstone of Britain and Russia.
Beneath the Eifel Limestone (the great central and typical member of “the Devonian” on the Continent) lie certain schists called by German writers “Calceola-schiefer,” because they contain in abundance a fossil body of very curious structure, Calceola sandalina (Fig. 520), which has been usually considered a brachiopod, but which some naturalists have lately referred to a Goniophyllum, supposing it to be an abnormal form of the order Zoantharia rugosa (see [Fig. 474]), differing from all other corals in being furnished with a strong operculum. This is by no means a rare fossil in the slaty limestone of South Devon, and, like the Eifel form, is confined to the middle group of this country.
![[Fig. 474]](#5745294564006620949_fig474.jpg.id-7654506444138731805.wrap-0.html.html){kind=link}
Lower Devonian Rocks.—A great series of sandstones and glossy slates, with Crinoids, Brachiopods, and some corals, occurring on the coast at Lynmouth and the neighbourhood, and called the Lynton Group (see Table [p. 449], form the lowest member of the Devonian in North Devon. Among the 18 species of all classes enumerated by Mr. Etheridge, two-thirds are common to the Middle Devonian, but only one, the ubiquitous Atrypa reticularis, can with certainty be identified with Silurian species. Among the characteristic forms are Alveolites suborbicularis, also common to this formation in the Rhine, and Orthis arcuata, very widely spread in the North Devon localities. But we may expect a large addition to the number of fossils whenever these strata shall have been carefully searched. The Spirifer Sandstone of Sandberger, as exhibited in the rocks bordering the Rhine between Coblentz and Caub, belong to this Lower division, and the same broad-winged Spirifers distinguish the Devonian strata of North America.
Among the Trilobites of this era several large species of Homalonotus (Fig. 522) are conspicuous. The genus is still better known as a Silurian form, but the spinose species appear to belong exclusively to the “Lower Devonian,” and are found in Britain, Europe, and the Cape of Good Hope.
Devonian of Russia.—The Devonian strata of Russia extend, according to Sir R. Murchison, over a region more spacious than the British Isles; and it is remarkable that, where they consist of sandstone like the “Old Red” of Scotland and Central England, they are tenanted by fossil fishes often of the same species and still oftener of the same genera as the British, whereas when they consist of limestone they contain shells similar to those of Devonshire, thus confirming, as Sir Roderick has pointed out, the contemporaneous origin which had been previously assigned to formations exhibiting two very distinct mineral types in different parts of Britain.[[6]] The calcareous and the arenaceous rocks of Russia above alluded to alternate in such a manner as to leave no doubt of their having been deposited in different parts of the same great period.
Devonian Strata in the United States and Canada.—Between the Carboniferous and Silurian strata there intervenes, in the United States and Canada, a great series of formations referable to the Devonian group, comprising some strata of marine origin abounding in shells and corals, and others of shallow-water and littoral origin in which terrestrial plants abound. The fossils, both of the deep and shallow water strata, are very analogous to those of Europe, the species being in some cases the same. In Eastern Canada Sir W. Logan has pointed out that in the peninsula of Gaspe, south of the estuary of St. Lawrence, a mass of sandstone, conglomerate, and shale referable to this period occurs, rich in vegetable remains, together with some fish-spines. Far down in the sandstones of Gaspe, Dr. Dawson found, in 1869, an entire specimen of the genus Cephalaspis, a form so characteristic, as we have already seen, of the Scotch Lower Old Red Sandstone. Some of the sandstones are ripple-marked, and towards the upper part of the whole series a thin seam of coal has been observed, measuring, together with some associated carbonaceous shale, about three inches in thickness. It rests on an under-clay in which are the roots of Psilophyton (see Fig. 523). At many other levels rootlets of this same plant have been shown by Principal Dawson to penetrate the clays, and to play the same part as do the rootlets of Stigmaria in the coal formation.
We had already learnt from the works of Göppert, Unger, and Bronn that the European plants of the Devonian epoch resemble generically, with few exceptions, those already known as Carboniferous; and Dr. Dawson, in 1859, enumerated 32 genera and 69 species which he had then obtained from the State of New York and Canada. A perusal of his catalogue,[[7]] comprising Coniferæ, Sigillariæ, Calamites, Asterophyllites, Lepidodendra, and ferns of the genera Cyclopteris, Neuropteris, Sphenopteris, and others, together with fruits, such as Cardiocarpum and Trigonocarpum, might dispose geologists to believe that they were presented with a list of Carboniferous fossils, the difference of the species from those of the coal-measures, and even a slight admixture of genera unknown in Europe, being naturally ascribed to geographical distribution and the distance of the New from the Old World. But fortunately the coal formation is fully developed on the other side of the Atlantic, and is singularly like that of Europe, both lithologically and in the species of its fossil plants. There is also the most unequivocal evidence of relative age afforded by superposition, for the Devonian strata in the United States are seen to crop out from beneath the Carboniferous on the borders of Pennsylvania and New York, where both formations are of great thickness.
The number of American Devonian plants has now been raised by Dr. Dawson to 120, to which we may add about 80 from the European flora of the same age, so that already the vegetation of this period is beginning to be nearly half as rich as that of the coal-measures which have been studied for so much longer a time and over so much wider an area. The Psilophyton above alluded to is believed by Dr. Dawson to be a lycopodiaceous plant, branching dichotomously (see P. princeps, Fig. 523), with stems springing from a rhizome, which last has circular areoles, much resembling those of Stigmaria, and like it sending forth cylindrical rootlets. The extreme points of some of the branchlets are rolled up so as to resemble the croziers or circinate vernation of ferns; the leaves or bracts, a, supposed to belong to the same plant, are described by Dawson as having inclosed the fructification. The remains of Psilophyton princeps have been traced through all the members of the Devonian series in America, and Dr. Dawson has lately recognised it in specimens of Old Red Sandstone from the north of Scotland.
The monotonous character of the Carboniferous flora might be explained by imagining that we have only the vegetation handed down to us of one set of stations, consisting of wide swampy flats. But Dr. Dawson supposes that the geographical conditions under which the Devonian plants grew were more varied, and had more of an upland character. If so, the limitation of this more ancient flora, represented by so many genera and species, to the gymnospermous and cryptogamous orders, and the absence or extreme rarity of plants of higher grade, lead us naturally to speculate on the theory of progressive development, however difficult it may be to avail ourselves of this explanation, so long as we meet with even a few exceptional cases of what may seem to be monocotyledonous or dicotyledonous exogens.
Devonian Insects of Canada.—The earliest known insects were brought to light in 1865 in the Devonian strata of St. John’s, New Brunswick, and are referred by Mr. Scudder to four species of Neuroptera. One of them is a gigantic Ephemera, and measured five inches in expanse of wing.
Like many other ancient animals, says Dr. Dawson, they show a remarkable union of characters now found in distinct orders of insects, or constitute what have been named “synthetic types.” Of this kind is a stridulating or musical apparatus like that of the cricket in an insect otherwise allied to the Neuroptera. This structure, as Dr. Dawson observes, if rightly interpreted by Mr. Scudder, introduces us to the sounds of the Devonian woods, bringing before our imagination the trill and hum of insect life that enlivened the solitudes of these strange old forests.
[1] Siluria, 4th ed., p. 265.
[2] Siluria, 4th ed., p. 258.
[3] Abridged from crossotos, a fringe, and pteryx, a fin.
[4] Powrie, Geol. Quart. Journ., vol. xx, p. 417.
[5] Quart. Geol. Journ., vol. xxiii., 1867.
[6] Murchison’s Siluria, p. 329.
[7] Quart. Geol. Journ., vol. xv, p. 477, 1859; also vol. xviii, p. 296, 1862.
CHAPTER XXVI.
SILURIAN GROUP.
Classification of the Silurian Rocks. — Ludlow Formation and Fossils. — Bone-bed of the Upper Ludlow. — Lower Ludlow Shales with Pentamerus. — Oldest known Remains of fossil Fish. — Table of the progressive Discovery of Vertebrata in older Rocks. — Wenlock Formation, Corals, Cystideans and Trilobites. — Llandovery Group or Beds of Passage. — Lower Silurian Rocks. — Caradoc and Bala Beds. — Brachiopoda. — Trilobites. — Cystideæ. — Graptolites. — Llandeilo Flags. — Arenig or Stiper-stones Group. — Foreign Silurian Equivalents in Europe. — Silurian Strata of the United States. — Canadian Equivalents. — Amount of specific Agreement of Fossils with those of Europe.
Classification of the Silurian Rocks.—We come next in descending order to that division of Primary or Palæozoic rocks which immediately underlie the Devonian group or Old Red Sandstone. For these strata Sir Roderick Murchison first proposed the name of Silurian when he had studied and classified them in that part of Wales and some of the contiguous counties of England which once constituted the kingdom of the Silures, a tribe of ancient Britons. The following table will explain the two principal divisions, Upper and Lower, of the Silurian rocks, and the minor subdivisions usually adopted, comprehending all the strata originally embraced in the Silurian system by Sir Roderick Murchison. The formations below the Arenig or Stiper-stones group are treated of in the next chapter, when the “Primordial” or Cambrian group is described.
| UPPER SILURIAN ROCKS. | |
| Thickness in feet | |
| 1. LUDLOWFORMATION: a. Upper Ludlowbeds | 780 |
| b. Lower Ludlow beds: | 1,050 |
| 2. WENLOCKFORMATION: a. Wenlock limestone andshale | above 4,000 |
| b. Woolhope limestone and shale, and Denbighshire grits: | |
| 3. LLANDOVERYFORMATION (Beds of passage between Upper andLower Silurian): a. Upper Llandovery(May-Hill beds): | 800 |
| b. Lower Llandovery: | 600–1,000 |
| LOWER SILURIAN ROCKS. | |
| 1. BALA AND CARADOCBEDS, including volcanic rocks: | 12,000 |
| 2. LLANDEILO FLAGS,including volcanic rocks: | 4,500 |
| 3. ARENIG ORSTIPER-STONES GROUP, includingvolcanic rocks: | above 10,000 |
1. Ludlow Formation.—This member of the Upper Silurian group, as will be seen by above table, is of great thickness, and subdivided into two parts—the Upper Ludlow and the Lower Ludlow. Each of these may be distinguished near the town of Ludlow, and at other places in Shropshire and Herefordshire, by peculiar organic remains; but out of more than 500 species found in the Ludlow formation as a whole, not more than five species per hundred are common to the overlying Devonian. The student may refer to the excellent tables given in the last edition of Sir R. Murchison’s Siluria for a list of the organic remains of all classes distributed through the different subdivisions of the Upper and Lower Silurian.
a. Upper Ludlow: Downton Sandstone.—At the top of this subdivision there occur beds of fine-grained yellowish sandstone and hard reddish grits which were formerly referred by Sir R. Murchison to the Old Red Sandstone, under the name of “Tilestones.” In mineral character this group forms a transition from the Silurian to the Old Red Sandstone, the strata of both being conformable; but it is now ascertained that the fossils agree in great part specifically, and in general character entirely, with those of the underlying Upper Ludlow rocks. Among these are Orthoceras bullatum, Platyschisma helicites, Bellerophon trilobatus, Chonetes lata, etc., with numerous defenses of fishes.
These beds, therefore, now generally called the “Downton Sandstone,” are classed as the newest member of the Upper Silurian. They are well seen at Downton Castle, near Ludlow, where they are quarried for building, and at Kington, in Herefordshire. In the latter place, as well as at Ludlow, crustaceans of the genera Pterygotus (for genus see [Fig. 504]) and Eurypterus are met with.
![[Fig. 504]](#5745294564006620949_fig504.jpg.id-4764380856666244426.wrap-0.html.html){kind=link}
Bone-bed of the Upper Ludlow.—At the base of the Downton sandstones there occurs a bone-bed which deserves especial notice as affording the most ancient example of fossil fish occurring in any considerable quantity. It usually consists of one or two thin layers of brown bony fragments near the junction of the Old Red Sandstone and the Ludlow rocks, and was first observed by Sir R. Murchison near the town of Ludlow, where it is three or four inches thick. It has since been traced to a distance of 45 miles from that point into Gloucestershire and other counties, and is commonly not more than an inch thick, but varies to nearly a foot. Near Ludlow two bone-beds are observable, with 14 feet of intervening strata full of Upper Ludlow fossils.[[1]] At that point immediately above the upper fish-bed numerous small globular bodies have been found, which were determined by Dr. Hooker to be the sporangia of a cryptogamic land-plant, probably lycopodiaceous.
Most of the fish have been referred by Agassiz to his placoid order, some of them to the genus Onchus, to which the spine (Fig. 524) and the minute scales (Fig. 525) are supposed to belong. It has been suggested, however, that Onchus may be one of those Acanthodian fish referred by Agassiz to his Ganoid order, which are so characteristic of the base of the Old Red Sandstone in Forfarshire, although the species of the Old Red are all different from these of the Silurian beds now under consideration.
The jaw and teeth of another predaceous genus (Fig. 526) have also been detected, together with some specimens of Pteraspis Ludensis. As usual in bone-beds, the teeth and bones are, for the most part, fragmentary and rolled.
Grey Sandstone and Mudstone, etc.—The next subdivision of the Upper Ludlow consists of grey calcareous sandstone, or very commonly a micaceous stone, decomposing into soft mud, and contains, besides the shells mentioned aon page 459, Lingula cornea, Orthis orbicularis, a round variety of O. elegantula, Modiolopsis platyphylla, Grammysia cingulata, all characteristic of the Upper Ludlow. The lowest or mud-stone beds contain Rhynchonella navicula (Fig. 528), which is common to this bed and the Lower Ludlow. As usual in Palæozoic strata older than the coal, the brachiopodous mollusca greatly outnumber the lamellibranchiate (see [p. 470]); but the latter are by no means unrepresented. Among other genera, for example, we observe Avicula and Pterinea, Cardiola, Ctenodonta (sub-genus of Nucula), Orthonota, Modiolopsis, and Palæarca.
Some of the Upper Ludlow sandstones are ripple-marked, thus affording evidence of gradual deposition; and the same may be said of the accompanying fine argillaceous shales, which are of great thickness, and have been provincially named “mud-stones.” In some of these shales stems of crinoidea are found in an erect position, having evidently become fossil on the spots where they grew at the bottom of the sea. The facility with which these rocks, when exposed to the weather, are resolved into mud, proves that, notwithstanding their antiquity, they are nearly in the state in which they were first thrown down.
b. Lower Ludlow Beds.—The chief mass of this formation consists of a dark grey argillaceous shale with calcareous concretions, having a maximum thickness of 1000 feet. In some places, and especially at Aymestry, in Herefordshire, a subcrystalline and argillaceous limestone, sometimes 50 feet thick, overlies the shale. Sir R. Murchison therefore classes this Aymestry limestone as holding an intermediate position between the Upper and Lower Ludlow, but Mr. Lightbody remarks that at Mocktrie, near Leintwardine, the Lower Ludlow shales, with their characteristic fossils, occur both above and below a similar limestone. This limestone around Aymestry and Sedgeley is distinguished by the abundance of Pentamerus Knightii, Sowerby (Fig. 529), also found in the Lower Ludlow and Wenlock shale. This genus of brachiopoda was first found in Silurian strata, and is exclusively a palæozoic form. The name was derived from pente, five, and meros, a part, because both valves are divided by a central septum, making four chambers, and in one valve the septum itself contains a small chamber, making five. The size of these septa is enormous compared with those of any other brachiopod shell; and they must nearly have divided the animal into two equal halves; but they are, nevertheless, of the same nature as the septa or plates which are found in the interior of Spirifera, Terebratula, and many other shells of this order. Messrs. Murchison and De Verneuil discovered this species dispersed in myriads through a white limestone of Upper Silurian age, on the banks of the Is, on the eastern flank of the Urals in Russia, and a similar species is frequent in Sweden.
Three other abundant shells in the Aymestry limestone are, first, Lingula Lewisii (Fig. 530); second, Rhynchonella Wilsoni, Sowerby (Fig. 531), which is also common to the Lower Ludlow and Wenlock limestone; third, Atrypa reticularis, Linn. (Fig. 532), which has a very wide range, being found in every part of the Upper Silurian system, and even ranging up into the Middle Devonian series.
The Aymestry Limestone contains many shells, especially brachiopoda, corals, trilobites, and other fossils, amounting on the whole to 74 species, all except three or four being common to the beds either above or below.
The Lower Ludlow Shale contains, among other fossils, many large cephalopoda not known in newer rocks, as the Phragmoceras of Broderip, and the Lituites of Breynius (see Figs. 533, 534). The latter is partly straight and partly convoluted in a very flat spire. The Orthoceras Ludense (Fig. 535), as well as the cephalopod last mentioned, occurs in this member of the species.
A species of Graptolite, G. priodon, Bronn ([Fig. 545]), occurs plentifully in the Lower Ludlow. This fossil, referred, though somewhat doubtfully, to a form of hydrozoid or sertularian polyp, has not yet been met with in strata above the Silurian.
![[Fig. 545]](#5745294564006620949_fig545.jpg.id-8514546600579344270.wrap-0.html.html){kind=link}
Star-fish, as Sir R. Murchison points out, are by no means rare in the Lower Ludlow rock. These fossils, of which six extinct genera are now known in the Ludlow series, represented by 18 species, remind us of various living forms now found in our British seas, both of the families Asteriadæ and Ophiuridæ.
Oldest known Fossil Fish.—Until 1859 there was no example of a fossil fish older than the bone-bed of the Upper Ludlow, but in that year a specimen of Pteraspis was found at Church Hill, near Leintwardine, in Shropshire, by Mr. J. E. Lee of Caerleon, F.G.S., in shale below the Aymestry limestone, associated with fossil shells of the Lower Ludlow formation—shells which differ considerably from those characterising the Upper Ludlow already described. This discovery is of no small interest as bearing on the theory of progressive development, because, according to Professor Huxley, the genus Pteraspis is allied to the sturgeon, and therefore by no means of low grade in the piscine class.
It is a fact well worthy of notice that no remains of vertebrata have yet been met with in any strata older than the Lower Ludlow.
When we reflect on the hundreds of Mollusks, Echinoderms, Trilobites, Corals, and other fossils already obtained from more ancient Silurian formations, Upper, Middle, and Lower, we may well ask whether any set of fossiliferous rocks newer in the series were ever studied with equal diligence, and over so vast an area, without yielding a single ichthyolite. Yet we must hesitate before we accept, even on such evidence, so sweeping a conclusion, as that the globe, for ages after it was inhabited by all the great classes of invertebrata, remained wholly untenanted by vertebrate animals.
Dates of the Discovery of different Classes of Fossil Vertebrata; showing the gradual progress made in tracing them to rocks of higher antiquity.
| Year | Formations | Geographical localities | |
| Mammalia | 1798 | Upper Eocene | Paris (Gypsum of Montmartre).1 |
| 1818 | Lower Oolite | Stonesfield.2 | |
| 1847 | Upper Trias | Stuttgart.3 | |
| Aves | 1782 | Upper Eocene | Paris (Gypsum of Montmartre).4 |
| 1839 | Lower Eocene | Isle of Sheppey (London Clay).5 | |
| 1854 | Lower Eocene | Woolwich Beds.6 | |
| 1855 | Lower Eocene | Mendon (Plastic Clay).7 | |
| 1858 | Chloritic Series, or Upper Greensand | Cambridge.8 | |
| 1863 | Upper Oolite | Solenhofen.9 | |
| Reptilia | 1710 | Permian (or Zechstein) | Thuringia.10 |
| 1844 | Carboniferous | Saarbrück, nearTrèves.11 | |
| Pisces | 1709 | Permian (or Kupferschiefer) | Thuringia.12 |
| 1793 | Carboniferous (Mountain Limestone) | Glasgow.13 | |
| 1828 | Devonian | Caithness.14 | |
| 1840 | Upper Ludlow | Ludlow.15 | |
| 1859 | Lower Ludlow | Leintwardine.16 |
1. George Cuvier, Bulletin Soc. Philom. xx.
2. In 1818, Cuvier, visiting the Museum of Oxford, decided on the mammalian character of a jaw from Stonesfield. See also [p. 347.]
3. Prof. Plieninger. See [p. 368.]
4. Cuvier, Ossemens Foss. Art. “Oiseaux.”
5. Prof. Owen, Geol. Trans., 2nd series, vol. vi, p. 203, 1839.
6. Upper part of the Woolwich beds. Prestwich, Quart. Geol. Journ., vol. x, p. 157.
7. Gastornis Parisiensis. Owen, Quart. Geol. Journ., vol. xii, p. 204, 1856.
8. Coprolitic bed, in the Upper Greensand. See [p. 299.]
9. The Archæopteryx macrura, Owen. See [p. 338.]
10. The fossil monitor of Thuringia (Protosaurus Speneri, V. Meyer) was figured by Spener of Berlin in 1810. (Miscel. Berlin.)
11. See [p. 406.]
12. Memorabilia Saxoniæ Subterr., Leipsic, 1709.
13. History of Rutherglen by Rev. David Ure, 1793.
14. Sedgwick and Murchison, Geol. Trans., 2nd series, vol. ii, p. 141, 1828.
15. Sir R. Murchison. See [p. 459.]
16. See [p. 461.]
Obs.—The evidence derived from foot-prints, though often to be relied on, is omitted in the above table, as being less exact than that founded on bones and teeth.
In the preceding Table a few dates are set before the reader of the discovery of different classes of animals in ancient rocks, to enable him to perceive at a glance how gradual has been our progress in tracing back the signs of vertebrata to formations of high antiquity. Such facts may be useful in warning us not to assume too hastily that the point which our retrospect may have reached at the present moment can be regarded as fixing the date of the first introduction of any one class of beings upon the earth.
2. Wenlock Formation.—We next come to the Wenlock formation, which has been divided (see Table, [ p. 458]) into Wenlock limestone, Wenlock shale, and Woolhope limestone and Denbighshire grits.
a. Wenlock Limestone.—This limestone, otherwise well known to collectors by the name of the Dudley Limestone, forms a continuous ridge in Shropshire, ranging for about 20 miles from S.W. to N.E., about a mile distant from the nearly parallel escarpment of the Aymestry limestone. This ridgy prominence is due to the solidity of the rock, and to the softness of the shales above and below it. Near Wenlock it consists of thick masses of grey subcrystalline limestone, replete with corals, encrinites, and trilobites. It is essentially of a concretionary nature; and the concretions, termed “ball-stones” in Shropshire, are often enormous, even 80 feet in diameter. They are of pure carbonate of lime, the surrounding rock being more or less argillaceous[[2]] Sometimes in the Malvern Hills this limestone, according to Professor Phillips, is oolitic.
Among the corals, in which this formation is so rich, 53 species being known, the “chain-coral,” Halysites catenularius (Fig. 536), may be pointed out as one very easily recognised, and widely spread in Europe, ranging through all parts of the Silurian group, from the Aymestry limestone to near the bottom of the Llandeilo rocks. Another coral, the Favosites Gothlandica (Fig. 537), is also met with in profusion in large hemispherical masses, which break up into columnar and prismatic fragments, like that here figured (Fig. 537, b). Another common form in the Wenlock limestone is the Omphyma turbinatum (Fig. 538), which, like many of its modern companions, reminds us of some cup-corals; but all the Silurian genera belong to the palæozoic type before mentioned ([p. 432]), exhibiting the quadripartite arrangement of the septalamellæ within the cup.
Among the numerous Crinoids, several peculiar species of Cyathocrinus (for genus see [Figs. 478], 479) contribute their calcareous stems, arms, and cups towards the composition of the Wenlock limestone. Of Cystideans there are a few very remarkable forms, most of them peculiar to the Upper Silurian formation, as, for example, the Pseudocrinites, which was furnished with pinnated fixed arms,[[3]] as represented in Fig. 539.
![[Figs. 478]](#5745294564006620949_fig478.jpg.id-1459245806508636084.wrap-0.html.html){kind=link}
The Brachiopoda are, many of them, of the same species as those of the Aymestry limestone; as, for example, Atrypa reticularis (Fig. [532]), and Strophomena depressa (Fig. 540); but the latter species ranges also from the Ludlow rocks, through the Wenlock shale, to the Caradoc Sandstone.
![[532]](#5745294564006620949_fig532.jpg.id-6781383957360580150.wrap-0.html.html){kind=link}
The crustaceans are represented almost exclusively by Trilobites, which are very conspicuous, 22 being peculiar. The Calymene Blumenbachii (Fig. 541), called the ”Dudley Trilobite,” was known to collectors long before its true place in the animal kingdom was ascertained. It is often found coiled up like the common Oniscus or wood-louse, and this is so usual a circumstance among certain genera of trilobites as to lead us to conclude that they must have habitually resorted to this mode of protecting themselves when alarmed. The other common species is the Phacops caudatus (Asaphus caudatus), Brong. (see Fig. 542), and this is conspicuous for its large size and flattened form. Sphærexochus mirus (Fig. 543) is almost a globe when rolled up, the forehead or glabellum of this species being extremely inflated. The Homalonotus, a form of Trilobite in which the tripartite division of the dorsal crust is almost lost (see Fig. 544), is very characteristic of this division of the Silurian series.
Wenlock Shale.—This, observes Sir R. Murchison, is infinitely the largest and most persistent member of the Wenlock formation, for the limestone often thins out and disappears. The shale, like the Lower Ludlow, often contains elliptical concretions of impure earthy limestone.
In the Malvern district it is a mass of finely levigated argillaceous matter, attaining, according to Professor Phillips, a thickness of 640 feet, but it is sometimes more than 1000 feet thick in Wales, and is worked for flag-stones and slates. The prevailing fossils, besides corals and trilobites, and some crinoids, are several small species of Orthis, Cardiola, and numerous thin-shelled species of Orthoceratites.
About six species of Graptolite, a peculiar group of sertularian fossils before alluded to (p. [ 463]) as being confined to Silurian rocks, occur in this shale. Of fossils of this genus, which is very characteristic of the Lower Silurian, I shall again speak in the sequel (p. [474]).
b. Woolhope Beds.—Though not always recognised as a separate subdivision of the Wenlock, the Woolhope beds, which underlie the Wenlock shale, are of great importance. Usually they occur as massive or nodular limestones, underlaid by a fine shale or flag-stone; and in other cases, as in the noted Denbighshire sandstones, as a coarse grit of very great thickness. This grit forms mountain ranges through North and South Wales, and is generally marked by the great sterility of the soil where it occurs. It contains the usual Wenlock fossils, but with the addition of some common in the uppermost Ludlow rock, such as Chonetes lata and Bellerophon trilobatus. The chief fossils of the Woolhope limestone are Illænus Barriensis, Homalonotus delphinocephalus (Fig. 544), Strophomena imbrex, and Rhynchonella Wilsoni ([Fig. 531]). The latter attains in the Woolhope beds an unusual size for the species, the specimens being sometimes twice as large as those found in the Wenlock limestone.
![[Fig. 531]](#5745294564006620949_fig531.jpg.id-6059506990440818405.wrap-0.html.html){kind=link}
In some places below the Wenlock formation there are shales of a pale or purple colour, which near Tarannon attain a thickness of about 1000 feet; they can be traced through Radnor and Montgomery to North Wales, according to Messrs. Jukes and Aveline. By the latter geologist they have been identified with certain shales above the May-Hill Sandstone, near Llandovery, but, owing to the extreme scarcity of fossils, their exact position remains doubtful.
3. Llandovery Group—Beds of Passage.—We now come to beds respecting the classification of which there has been much difference of opinion, and which in fact must be considered as beds of passage between Upper and Lower Silurian. I formerly adopted the plan of those who class them as Middle Silurian, but they are scarcely entitled to this distinction, since after about 1400 Silurian species have been compared the number peculiar to the group in question only gives them an importance equal to such minor subdivisions as the Ludlow or Bala groups. I therefore prefer to regard them as the base of the Upper Silurian, to which group they are linked by more than twice as many species as to the Lower Silurian. By this arrangement the line of demarkation between the two great divisions, though confessedly arbitrary, is less so than by any other. They are called Llandovery Rocks, from a town in South Wales, in the neighbourhood of which they are well developed, and where, especially at a hill called Noeth Grug, in spite of several faults, their relations to one another can be clearly seen.
a. Upper Llandovery or May-Hill Sandstone.—The May-Hill group, which has also been named ”Upper Llandovery,” by Sir R. Murchison, ranges from the west of the Longmynd to Builth, Llandovery, and Llandeilo, and to the sea in Marlow’s Bay, where it is seen in the cliffs. It consists of brownish and yellow sandstones with calcareous nodules, having sometimes a conglomerate at the base derived from the waste of the Lower Silurian rocks. These May-Hill beds were formerly supposed to be part of the Caradoc formation, but their true position was determined by Professor Sedgwick[[4]] to be at the base of the Upper Silurian proper. The more calcareous portions of the rock have been called the Pentamerus limestone, because Pentamerus oblongus (Fig. 546) is very abundant in them. It is usually accompanied by P. (Stricklandinia) lirata (Fig. 547); both forms have a wide geographical range, being also met with in the same part of the Silurian series in Russia and the United States.
About 228 species of fossils are known in the May-Hill division, more than half of which are Wenlock species. They consist of trilobites of the genera Illænus and Calymene; Brachiopods of the genera Orthis, Atrypa, Leptæna, Pentamerus, Strophomena, and others; Gasteropods of the genera Turbo, Murchisonia (for genus, see [Fig. 567]), and Bellerophon; and Pteropods of the genus Conularia. The Brachiopods, of which there are 66 species, are almost all Upper Silurian.
![[Fig. 567]](#5745294564006620949_fig567.jpg.id-7405143371278444683.wrap-0.html.html){kind=link}
Among the fossils of the May-Hill shelly sandstone at Malvern, Tentaculites annulatus (Fig. 548), an annelid, probably allied to Serpula, is found.
Lower Llandovery Rocks.—Below the May-Hill Group are the Lower Llandovery Rocks, which consist chiefly of hard slaty rocks, and beds of conglomerate from 600 to 1000 feet in thickness. The fossils, which are somewhat rare in the lower beds, consist of 128 known species, only eleven of which are peculiar, 83 being common to the May-Hill group above, and 93 common to the rocks below. Stricklandinia (Pentamerus) levis, which is common in the Lower Llandovery, becomes rare in the Upper, while Pentamerus oblongus (Fig. 546), which is the characteristic shell of the Upper Llandovery, occurs but seldom in the Lower.
LOWER SILURIAN ROCKS.
The Lower Silurian has been divided into, first, the Bala Group; second, the Llandeilo flags; and, third, the Arenig or Lower Llandeilo formation.
Bala and Caradoc Beds.—The Caradoc sandstone was originally so named by Sir R. I. Murchison from the mountain called Caer Caradoc, in Shropshire; it consists of shelly sandstones of great thickness, and sometimes containing much calcareous matter. The rock is frequently laden with the beautiful trilobite called by Murchison Trinucleus Caractaci (see Fig. 553), which ranges from the base to the summit of the formation, usually accompanied by Strophomena grandis (see Fig. 551), and Orthis vespertilio (Fig. 550), with many other fossils.
Brachiopoda.—Nothing is more remarkable in these beds, and in the Silurian strata generally of all countries, than the preponderance of brachiopoda over other forms of mollusca. Their proportional numbers can by no means be explained by supposing them to have inhabited seas of great depth, for the contrast between the palæozoic and the present state of things has not been essentially altered by the late discoveries made in our deep-sea dredgings. We find the living brachiopoda so rare as to form about one forty-fourth of the whole bivalve fauna, whereas in the Lower Silurian rocks of which we are now about to treat, and where the brachiopoda reach their maximum, they are represented by more than twice as many species as the Lamellibranchiate bivalves.
There may, indeed, be said to be a continued decrease of the proportional number of this lower tribe of mollusca as we proceed from older to newer rocks. In the British Devonian, for example, the Brachiopoda number 99, the Lamellibranchiata 58; while in the Carboniferous their proportions are more than reversed, the Lamellibranchiata numbering 334 species, and the Brachiopoda only 157. In the Secondary or Cainozoic formations the preponderance of the higher grade of bivalves becomes more and more marked, till in the tertiary strata it approaches that observed in the living creation.
While on this subject, it may be useful to the student to know that a Brachiopod differs from ordinary bivalves, mussels, cockles, etc., in being always equal-sided and never quite equi-valved; the form of each valve being symmetrical, it may be divided into two equal parts by a line drawn from the apex to the centre of the margin.
Trilobites.—In the Bala and Caradoc beds the trilobites reach their maximum, being represented by 111 species referred to 23 genera.
Burmeister, in his work on the organisation of trilobites, supposes that they swam at the surface of the water in the open sea and near coasts, feeding on smaller marine animals, and to have had the power of rolling themselves into a ball as a defence against injury. He was also of opinion that they underwent various transformations analogous to those of living crustaceans. M. Barrande, author of an admirable work on the Silurian rocks of Bohemia, confirms the doctrine of their metamorphosis, having traced more than twenty species through different stages of growth from the young state just after its escape from the egg to the adult form. He has followed some of them from a point in which they show no eyes, no joints, or body rings, and no distinct tail, up to the complete form with the full number of segments. This change is brought about before the animal has attained a tenth part of its full dimensions, and hence such minute and delicate specimens are rarely met with. Some of his figures of the metamorphoses of the common Trinucleus are copied in Figs. 552 and 553. It was not till 1870 that Mr. Billings was enabled, by means of a specimen found in Canada, to prove that the trilobite was provided with eight legs.
It has been ascertained that a great thickness of slaty and crystalline rocks of South Wales, as well as those of Snowdon and Bala, in North Wales, which were first supposed to be of older date than the Silurian sandstones and mudstones of Shropshire, are in fact identical in age, and contain the same organic remains. At Bala, in Merionethshire, a limestone rich in fossils occurs, in which two genera of star-fish, Protaster and Palæaster, are found; the fossil specimen of the latter (Fig. 554) being almost as uncompressed as if found just washed up on the sea-beach. Besides the star-fish there occur abundance of those peculiar bodies called Cystideæ. They are the Sphæronites of old authors, and were considered by Professor E. Forbes as intermediate between the crinoids and echinoderms. The Echinosphæronite here represented (Fig. 555) is characteristic of the Caradoc beds in Wales, and of their equivalents in Sweden and Russia.
With it have been found several other genera of the same family, such as Sphæronites, Hemicosmites, etc. Among the mollusca are Pteropods of the genus Conularia of large size (for genus, see [Fig. 518]). About eleven species of Graptolite are reckoned as belonging to this formation; they are chiefly found in peculiar localities where black mud abounded. The formation, when traced into South Wales and Ireland, assumes a greatly altered mineral aspect, but still retains its characteristic fossils. The known fauna of the Bala group comprises 565 species, 352 of which are peculiar, and 93, as before stated, are common to the overlying Llandovery rocks. It is worthy of remark that, when it occurs under the form of trappean tuff (volcanic ashes of De la Beche), as in the crest of Snowdon, the peculiar species which distinguish it from the Llandeilo beds are still observable. The formation generally appears to be of shallow-water origin, and in that respect is contrasted with the group next to be described. Professor Ramsay estimates the thickness of the Bala Beds, including the contemporaneous volcanic rocks, stratified and unstratified, as being from 10,000 to 12,000 feet.
![[Fig. 518]](#5745294564006620949_fig518.jpg.id-5447728915902423040.wrap-0.html.html){kind=link}
Llandeilo Flags.—The Lower Silurian strata were originally divided by Sir R. Murchison into the upper group already described, under the name of Caradoc Sandstone, and a lower one, called, from a town in Carmarthenshire, the Llandeilo flags. The last mentioned strata consist of dark-coloured micaceous flags, frequently calcareous, with a great thickness of shales, generally black, below them. The same beds are also seen at Builth, in Radnorshire, where they are interstratified with volcanic matter.
A still lower part of the Llandeilo rocks consists of a black carbonaceous slate of great thickness, frequently containing sulphate of alumina, and sometimes, as in Dumfriesshire, beds of anthracite. It has been conjectured that this carbonaceous matter may be due in great measure to large quantities of imbedded animal remains, for the number of Graptolites included in these slates was certainly very great. In Great Britain eleven genera and about 40 species of Graptolites occur in the Llandeilo flags and underlying Arenig beds. The double Graptolites, or those with two rows of cells, such as Diplograpsus (Fig. 557), are conspicuous.
The brachiopoda of the Llandeilo flags, which number 47 species, are in the main the same as those of the Caradoc Sandstone, but the other mollusca are in great part of different species.
In Europe generally, as, for example, in Sweden and Russia, no shells are so characteristic of this formation as Orthoceratites, usually of great size, and with a wide siphuncle placed on one side instead of being central (see Fig. 560).
Among other Cephalopods in the Llandeilo flags is Cyrtoceras; in the same beds also are found Bellerophon (see [Fig. 488]) and some Pteropod shells (Conularia, Theca, etc.), also in spots where sand abounded, lamellibranchiate bivalves of large size. The Crustaceans were plentifully represented by the Trilobites, which appear to have swarmed in the Silurian seas just as crabs and shrimps do in our own; no less than 263 species have been found in the British Silurian fauna. The genera Asaphus (Fig. 561), Ogygia (Fig. 562), and Trinucleus ([Figs. 552] and 553) form a marked feature of the rich and varied Trilobitic fauna of this age.
![[Fig. 488]](#5745294564006620949_fig488.jpg.id-3635873337779625570.wrap-0.html.html){kind=link}
![[Figs. 552]](#5745294564006620949_fig552.jpg.id-6763195411028365390.wrap-0.html.html){kind=link}
Beneath the black slates above described of the Llandeilo formation, Graptolites are still found in great variety and abundance, and the characteristic genera of shells and trilobites of the Lower Silurian rocks are still traceable downward, in Shropshire, Cumberland, and North and South Wales, through a vast depth of shaly beds, in some districts interstratified with trappean formations of contemporaneous origin; these consist of tuffs and lavas, the tuffs being formed of such materials as are ejected from craters and deposited immediately on the bed of the ocean, or washed into it from the land. According to Professor Ramsay, their thickness is about 3300 feet in North Wales, including those of the Lower Llandeilo. The lavas are feldspathic, and of porphyritic structure, and, according to the same authority, of an aggregate thickness of 2500 feet.
Arenig or Stiper-Stones Group (Lower Llandeilo of Murchison).—Next in the descending order are the shales and sandstones in which the quartzose rocks called Stiper-Stones in Shropshire occur. Originally these Stiper-Stones were only known as arenaceous quartzose strata in which no organic remains were conspicuous, except the tubular burrows of annelids (see Fig. 563, Arenicolites linearis), which are remarkably common in the Lowest Silurian in Shropshire, and in the State of New York, in America. They have already been alluded to as occurring by thousands in the Silurian strata unconformably overlying the Cambrian, in the mountain of Queenaig, in Sutherlandshire ([Fig. 82]). I have seen similar burrows now made on the retiring of the tides in the sands of the Bristol Channel, near Minehead, by lob-worms which are dug out by fishermen and used as bait. When the term Silurian was given by Sir R. Murchison, in 1835, to the whole series, he considered the Stiper-Stones as the base of the Silurian system, but no fossil fauna had then been obtained, such as could alone enable the geologist to draw a line between this member of the series and the Llandeilo flags above, or a vast thickness of rock below, which was seen to form the Longmynd hills, and was called ”unfossiliferous graywacke.” Professor Sedgwick had described, in 1843, strata now ascertained to be of the same age as largely developed in the Arenig mountain, in Merionethshire; and the Skiddaw slates in the Lake-District of Cumberland, studied by the same author, were of corresponding date, though the number of fossils was, in both cases, too few for the determination of their true chronological relations. The subsequent researches of Messrs. Sedgwick and Harkness, in Cumberland, and of Sir R. I. Murchison and the Government surveyors in Shropshire, have increased the species to more than sixty. These were examined by Mr. Salter, and shown in the third edition of ”Siluria” (p. 52, 1859) to be quite distinct from the fossils of the overlying Llandeilo flags. Among these the Obolella plumbea, Æglina binodosa, Ogygia Selwynii, and Didymograpsus geminus (Fig. 564), and D. Hirundo, are characteristic.
![[Fig. 82]](#5745294564006620949_fig82.jpg.id-8745414716520178724.wrap-0.html.html){kind=link}
But, although the species are distinct, the genera are the same as those which characterise the Silurian rocks above, and none of the characteristic primordial or Cambrian forms, presently to be mentioned, are intermixed. The same may be said of a set of beds underlying the Arenig rocks at Ramsay Island and other places in the neighbourhood of St. David’s. These beds, which have only lately become known to us through the labours of Dr. Hicks,[[5]] present already twenty new species, the greater part of them allied generically to the Arenig rocks. This Arenig group may therefore be conveniently regarded as the base of the great Silurian system, a system which, by the thickness of its strata and the changes in animal life of which it contains the record, is more than equal in value to the Devonian, or Carboniferous, or other principal divisions, whether of primary or secondary date.
It would be unsafe to rely on the mere thickness of the strata, considered apart from the great fluctuations in organic life which took place between the era of the Llandeilo and that of the Ludlow formation, especially as the enormous pile of Silurian rocks observed in Great Britain (in Wales more particularly) is derived in great part from igneous action, and is not confined to the ordinary deposition of sediment from rivers or the waste of cliffs.
In volcanic archipelagoes, such as the Canaries, we see the most active of all known causes, aqueous and igneous, simultaneously at work to produce great results in a comparatively moderate lapse of time. The outpouring of repeated streams of lava—the showering down upon land and sea of volcanic ashes—the sweeping seaward of loose sand and cinders, or of rocks ground down to pebbles and sand, by rivers and torrents descending steeply inclined channels—the undermining and eating away of long lines of sea-cliff exposed to the swell of a deep and open ocean—these operations combine to produce a considerable volume of superimposed matter, without there being time for any extensive change of species. Nevertheless, there would seem to be a limit to the thickness of stony masses formed even under such favourable circumstances, for the analogy of tertiary volcanic regions lends no countenance to the notion that sedimentary and igneous rocks 25,000, much less 45,000 feet thick, like those of Wales, could originate while one and the same fauna should continue to people the earth. If, then, we allow that about 25,000 feet of matter may be ascribed to one system, such as the Silurian, as above described, we may be prepared to discover in the next series of subjacent rocks a distinct assemblage of species, or even in great part of genera, of organic remains. Such appears to be the fact, and I shall therefore conclude with the Arenig beds my enumeration of the Silurian formations in Great Britain, and proceed to say something of their foreign equivalents, before treating of rocks older than the Silurian.
Silurian Strata of the Continent of Europe.—When we turn to the continent of Europe, we discover the same ancient series occupying a wide area, but in no region as yet has it been observed to attain great thickness. Thus, in Norway and Sweden, the total thickness of strata of Silurian age is considerably less than 1000 feet, although the representatives both of the Upper and Lower Silurian of England are not wanting there. In Russia the Silurian strata, so far as they are yet known, seem to be even of smaller vertical dimensions than in Scandinavia, and they appear to consist chiefly of the Llandovery group, or of a limestone containing Pentamerus oblongus, below which are strata with fossils corresponding to those of the Llandeilo beds of England. The lowest rock with organic remains yet discovered is ”the Ungulite or Obolus grit” of St. Petersburg, probably coeval with the Llandeilo flags of Wales.
The shales and grits near St. Petersburg, above alluded to, contain green grains in their sandy layers, and are in a singularly unaltered state, taking into account their high antiquity. The prevailing Brachiopods consist of the Obolus Shells of the lowest known Fossiliferous Beds in Russia.
or Ungulite of Pander, and a Siphonotreta (Figs. 565, 566). Notwithstanding the antiquity of this Russian formation, it should be stated that both of these genera of brachiopods have been also found in the Upper Silurian of England, i.e., in the Wenlock limestone.
Among the green grains of the sandy strata above-mentioned, Professor Ehrenberg announced in 1854 his discovery of remains of foraminifera. These are casts of the cells; and among five or six forms three are considered by him as referable to existing genera (e.g., Textularia, Rotalia, and Guttulina).
Silurian Strata of the United States.—The Silurian formations can be advantageously studied in the States of New York, Ohio, and other regions north and south of the great Canadian lakes. Here they are often found, as in Russia, nearly in horizontal position, and are more rich in well-preserved fossils than in almost any spot in Europe. In the State of New York, where the succession of the beds and their fossils have been most carefully worked out by the Government surveyors, the subdivisions given in the first column of the table below have been adopted.
Subdivisions of the Silurian Strata of New York.
(Strata below the Oriskany sandstone or base of the Devonian.)
| New York Names | British equivalents |
| 1. Upper Pentamerus
Limestone 2. Encrinal Limestone 3. Delthyris Shaly Limestone 4. Pentamerus and Tentaculite Limestones 5. Water Lime Group 6. Onondaga Salt Group 7. Niagara Group | Upper Silurian (or Ludlow and Wenlock formations |
| 8. Clinton Group 9. Medina Sandstone 10. Oneida Conglomerate 11. Gray Sandstone | Beds of Passage, Llandovery Group. |
| 12. Hudson River Group 13. Trenton Limestone 14. Black-River Limestone 15. Bird’s-eye Limestone 16. Chazy Limestone 17. Calciferous Sandstone | Lower Silurian (or Caradoc and
Bala, Llandeilo and Arenig Formations). |
In the second column of the same table I have added the supposed British equivalents. All Palæontologists, European and American, such as MM. De Verneuil, D. Sharpe, Professor Hall, E. Billings, and others, who have entered upon this comparison, admit that there is a marked general correspondence in the succession of fossil forms, and even species, as we trace the organic remains downward from the highest to the lowest beds; but it is impossible to parallel each minor subdivision.
That the Niagara Limestone, over which the river of that name is precipitated at the great cataract, together with its underlying shales, corresponds to the Wenlock limestone and shale of England there can be no doubt. Among the species common to this formation in America and Europe are Calymene Blumenbachii, Homalonotus delphinocephalus ([Fig. 544]), with several other trilobites; Rhynchonella Wilsoni, [Fig. 531], and Retzia cuneata; Orthis elegantula, Pentamerus galeatus, with many more brachiopods; Orthoceras annulatum, among the cephalopodous shells; and Favosites gothlandica, with other large corals.
![[Fig. 544]](#5745294564006620949_fig544.jpg.id-2238044425444715705.wrap-0.html.html){kind=link}
![[Fig. 531]](#5745294564006620949_fig531.jpg.id-2590802927678770343.wrap-0.html.html){kind=link}
The Clinton Group, containing Pentamerus oblongus and Stricklandinia, and related more nearly by its fossil species with the beds above than with those below, is the equivalent of the Llandovery Group or beds of passage.
The Hudson River Group, and the Trenton Limestone, agree palæontologically with the Caradoc or Bala group, containing in common with them several species of trilobites, such as Asaphus (Isotelus) gigas, Trinucleus concentricus ([Fig. 553]); and various shells, such as Orthis striatula, Orthis biforata (or O. lynx), O. porcata (O. occidentalis of Hall), and Bellerophon bilobatus. In the Trenton limestone occurs Murchisonia gracilis, Fig. 567, a fossil also common to the Llandeilo beds in England.
![[Fig. 553]](#5745294564006620949_fig552.jpg.id-8596041145674966108.wrap-0.html.html){kind=link}
Mr. D. Sharpe, in his report on the mollusca collected by me from these strata in North America,[[6]] has concluded that the number of species common to the Silurian rocks on both sides of the Atlantic is between 30 and 40 per cent; a result which, although no doubt liable to future modification, when a larger comparison shall have been made, proves, nevertheless, that many of the species had a wide geographical range. It seems that comparatively few of the gasteropods and lamellibranchiate bivalves of North America can be identified specifically with European fossils, while no less than two-fifths of the brachiopoda, of which my collection chiefly consisted, are the same. In explanation of these facts, it is suggested that most of the recent brachiopoda (especially the orthidiform ones) are inhabitants of deep water, and that they may have had a wider geographical range than shells living near shore. The predominance of bivalve mollusca of this peculiar class has caused the Silurian period to be sometimes styled ”the age of brachiopods.”
In Canada, as in the State of New York, the Potsdam Sandstone underlies the above-mentioned calcareous rocks, but contains a different suite of fossils, as will be hereafter explained. In parts of the globe still more remote from Europe the Silurian strata have also been recognised, as in South America, Australia, and India. In all these regions the facies of the fauna, or the types of organic life, enable us to recognise the contemporaneous origin of the rocks; but the fossil species are distinct, showing that the old notion of a universal diffusion throughout the ”primæval seas” of one uniform specific fauna was quite unfounded, geographical provinces having evidently existed in the oldest as in the most modern times.
[1] Murchison’s Siluria, p. 140.
[2] Murchison’s Siluria, chap. vi.
[3] E. Forbes, Mem. Geol. Surv., vol. ii, p. 496.
[4] Quart. Geol. Journ., vol. iv, p. 215, 1853.
[5] Trans. Brit. Assoc., 1866. Proc. Liverpool Geol. Soc., 1869.
[6] Quart. Geol. Journ., vol. iv.
CHAPTER XXVII.
CAMBRIAN AND LAURENTIAN GROUPS.
Classification of the Cambrian Group, and its Equivalent in Bohemia. — Upper Cambrian Rocks. — Tremadoc Slates and their Fossils. — Lingula Flags. — Lower Cambrian Rocks. — Menevian Beds. — Longmynd Group. — Harlech Grits with large Trilobites. — Llanberis Slates. — Cambrian Rocks of Bohemia. — Primordial Zone of Barrande. — Metamorphosis of Trilobites. — Cambrian Rocks of Sweden and Norway. — Cambrian Rocks of the United States and Canada. — Potsdam Sandstone. — Huronian Series. — Laurentian Group, upper and lower. — Eozoon Canadense, oldest known Fossil. — Fundamental Gneiss of Scotland.
CAMBRIAN GROUP.
The characters of the Upper and Lower Silurian rocks were established so fully, both on stratigraphical and palæontological data, by Sir Roderick Murchison after five years’ labour, in 1839, when his “Silurian System” was published, that these formations could from that period be recognised and identified in all other parts of Europe and in North America, even in countries where most of the fossils differed specifically from those of the classical region in Britain, where they were first studied.
While Sir R. I. Murchison was exploring in 1833, in Shropshire and the borders of Wales, the strata which in 1835 he first called Silurian, Professor Sedgwick was surveying the rocks of North Wales, which both these geologists considered at that period as of older date, and for which in 1836 Sedgwick proposed the name of Cambrian. It was afterwards found that a large portion of the slaty rocks of North Wales, which had been considered as more ancient than the Llandeilo beds and Stiper-Stones before alluded to, were, in reality, not inferior in position to those Lower Silurian beds of Murchison, but merely extensive undulations of the same, bearing fossils identical in species, though these were generally rarer and less perfectly preserved, owing to the changes which the rocks had undergone from metamorphic action. To such rocks the term “Cambrian” was no longer applicable, although it continued to be appropriate to strata inferior to the Stiper-Stones, and which were older than those of the Lower Silurian group as originally defined. It was not till 1846 that fossils were found in Wales in the Lingula flags, the place of which will be seen in the table below. By this time Barrande had already published an account of a rich collection of fossils which he had discovered in Bohemia, portions of which he recognised as of corresponding age with Murchison’s Upper and Lower Silurian, while others were more ancient, to which he gave the name of “Primordial,” for the fossils were sufficiently distinct to entitle the rocks to be referred to a new period. They consisted chiefly of trilobites of genera distinct from those occurring in the overlying Silurian formations. These peculiar genera were afterwards found in rocks holding a corresponding position in Wales, and I shall retain for them the term Cambrian, as recent discoveries in our own country seem to carry the first fauna of Barrande, or his primordial type, even into older strata than any which he found to be fossiliferous in Bohemia.
The term primordial was intended to express M. Barrande’s own belief that the fossils of the rocks so-called afforded evidence of the first appearance of vital phenomena on this planet, and that consequently no fossiliferous strata of older date would or could ever be discovered. The acceptance of such a nomenclature would seem to imply that we despaired of extending our discoveries of new and more ancient fossil groups at some future day when vast portions of the globe, hitherto unexplored, should have been thoroughly surveyed. Already the discovery of the Laurentian Eozoon in Canada, presently to be mentioned, discountenances such views.
The following table will show the succession of the strata in England and Wales which belong to the Cambrian group or the fossiliferous rocks older than the Arenig or Lower Llandeilo rocks:
| UPPER CAMBRIAN | |
| TREMADOC SLATES | (Primordial of Barrande in part) |
| LINGULA FLAGS | (Primordial of Barrande) |
| LOWER CAMBRIAN | |
| MENEVIAN BEDS | (Primordial of Barrande) |
| LONGMYNDGROUP | a. Harlech Grits b. Llanberis Slates |
Tremadoc Slates.—The Tremadoc slates of Sedgwick are more than 1000 feet in thickness, and consist of dark earthy slates occurring near the little town of Tremadoc, situated on the north side of Cardigan Bay, in Carnarvonshire. These slates were first examined by Sedgwick in 1831, and were re-examined by him and described in 1846,[[1]] after some fossils had been found in the underlying Lingula flags by Mr. Davis. The inferiority in position of these Lingula flags to the Tremadoc beds was at the same time established. The overlying Tremadoc beds were traced by their pisolitic ore from Tremadoc to Dolgelly. No fossils proper to the Tremadoc slates were then observed, but subsequently, thirty-six species of all classes have been found in them, thanks to the researches of Messrs. Salter, Homfray, and Ash. We have already seen that in the Arenig or Stiper-Stones group, where the species are distinct, the genera agree with Silurian types; but in these Tremadoc slates, where the species are also peculiar, there is about an equal admixture of Silurian types with those which Barrande has termed “primordial.” Here, therefore, it may truly be said that we are entering upon a new domain of life in our retrospective survey of the past. The trilobites of new species, but of Lower Silurian genera, belong to Ogygia, Asaphus, and Cheirurus; whereas those belonging to primordial types, or Barrande’s first fauna as well as to the Lingula flags of Wales, comprise Dikelocephalus, Conocoryphe (for genera see [Fig. 577] and 581),[[2]] Olenus, and Angelina.
![[Fig. 577]](#5745294564006620949_fig576.jpg.id-1327423574967720545.wrap-0.html.html){kind=link}
In the Tremadoc slates are found Bellerophon, Orthoceras, and Cyrtoceras, all specifically distinct from Lower Silurian fossils of the same genera: the Pteropods Theca (Fig. 568) and Conularia range throughout these slates; there are no Graptolites. The Lingula (Lingulella) Davisii ranges from the top to the bottom of the formation, and links it with the zone next to be described. The Tremadoc slates are very local, and seem to be confined to a small part of North Wales; and Professor Ramsay supposes them to lie unconformably on the Lingula flags, and that a long interval of time elapsed between these formations. Cephalopoda have not yet been found lower than this group, but it will be observed that they occur here associated with genera of Trilobites considered by Barrande as characteristically Primordial, some of which belong to all the divisions of the British Cambrian about to be mentioned. This renders the absence of cephalopoda of less importance as bearing on the theory of development.
Lingula Flags.—Next below the Tremadoc slates in North Wales lie micaceous flagstones and slates, in which, in 1846, Mr. E. Davis discovered the Lingula (Lingulella), Fig. 570, named after him, and from which was derived the name of Lingula flags. These beds, which are palæontologically the equivalents of Barrande’s primordial zone, are represented by more than 5000 feet of strata, and have been studied chiefly in the neighbourhood of Dolgelly, Ffestiniog, and Portmadoc in North Wales, and at St. David’s in South Wales. They have yielded about forty species of fossils, of which six only are common to the overlying Tremadoc rocks, but the two formations are closely allied by having several characteristic “primordial” genera in common. Dikelocephalus, Olenus (Fig. 571), and Conocoryphe are prominent forms, as is also Hymenocaris (Fig. 569), a genus of phyllopod crustacean entirely confined to the Lingula Flags. According to Mr. Belt, who has devoted much attention to these beds, there are already palæontological data for subdividing the Lingula Flags into three sections.[[3]]
In Merionethshire, according to Professor Ramsay, the Lingula Flags attain their greatest development; in Carnarvonshire they thin out so as to have lost two-thirds of their thickness in eleven miles, while in Anglesea and on the Menai Straits both they and the Tremadoc beds are entirely absent, and the Lower Silurian rests directly on Lower Cambrian strata.
LOWER CAMBRIAN.
Menevian Beds.—Immediately beneath the Lingula Flags there occurs a series of dark grey and black flags and slates alternating at the upper part with some beds of sandstone, the whole reaching a thickness of from 500 to 600 feet. These beds were formerly classed, on purely lithological grounds, as the base of the Lingula Flags, but Messrs. Hicks and Salter, to whose exertions we owe almost all our knowledge of the fossils, have pointed out[[4]] that the most characteristic genera found in them are quite unknown in the Lingula Flags, while they possess many of the strictly Lower Cambrian genera, such as Microdiscus and Paradoxides. They therefore proposed to place them, and it seems to me with good reason, at the top of the Lower Cambrian under the term “Menevian,” Menevia being the classical name of St. David’s. The beds are well exhibited in the neighbourhood of St. David’s in South Wales, and near Dolgelly and Maentwrog in North Wales. They are the equivalents of the lowest part of Barrande’s Primordial Zone (Étage C). More than forty species have been found in them, and the group is altogether very rich in fossils for so early a period.
The trilobites are of large size; Paradoxides Davidis (see Fig. 572), the largest trilobite known in England, 22 inches or nearly two feet long, is peculiar to the Menevian Beds. By referring to the Bohemian trilobite of the same genus ([Fig. 576]), the reader will at once see how these fossils (though of such different dimensions) resemble each other in Bohemia and Wales, and other closely allied species from the two regions might be added, besides some which are common to both countries. The Swedish fauna, presently to be mentioned, will be found to be still more nearly connected with the Welsh Menevian. In all these countries there is an equally marked difference between the Cambrian fossils and those of the Upper and Lower Silurian rocks. The trilobite with the largest number of rings, Erinnys venulosa, occurs here in conjunction with Agnostus and Microdiscus, the genera with the smallest number. Blind trilobites are also found as well as those which have the largest eyes, such as Microdiscus on the one hand, and Anoplenus on the other.
![[Fig. 576]](#5745294564006620949_fig576.jpg.id-5151932741773332694.wrap-0.html.html){kind=link}
LONGMYND GROUP.
Older than the Menevian Beds are a thick series of olive green, purple, red and grey grits and conglomerates found in North and South Wales, Shropshire, and parts of Ireland and Scotland. They have been called by Professor Sedgwick the Longmynd or Bangor Group, comprising, first, the Harlech and Barmouth sandstones; and secondly, the Llanberis slates.
Harlech Grits.—The sandstones of this period attain in the Longmynd hills a thickness of no less than 6000 feet without any interposition of volcanic matter; in some places in Merionethshire they are still thicker. Until recently these rocks possessed but a very scanty fauna.
With the exception of five species of annelids (see [Fig. 460]) brought to light by Mr. Salter in Shropshire, and Dr. Kinahan in Wicklow, and an obscure crustacean form, Palæopyge Ramsayi, they were supposed to be barren of organic remains. Now, however, through the labours of Mr. Hicks,[[5]] they have yielded at St. David’s a rich fauna of trilobites, brachiopods, phyllopods, and pteropods, showing, together with other fossils, a by no means low state of organisation at this early period. Already the fauna amounts to 20 species referred to 17 genera.
![[Fig. 460]](#5745294564006620949_fig460.jpg.id-807595128452038795.wrap-0.html.html){kind=link}
A new genus of trilobite called Plutonia Sedgwickii, not yet figured and described, has been met with in the Harlech grits. It is comparable in size to the large Paradoxides Davidis before mentioned, has well-developed eyes, and is covered all over with tubercles. In the same strata occur other genera of trilobites, namely, Conocoryphe, Paradoxides, Microdiscus, and the Pteropod Theca ([Fig. 568]), all represented by species peculiar to the Harlech grits. The sands of this formation are often rippled, and were evidently left dry at low tides, so that the surface was dried by the sun and made to shrink and present sun-cracks. There are also distinct impressions of rain-drops on many surfaces, like those in [Fig. 444] and 445.
![[Fig. 568]](#5745294564006620949_fig568.jpg.id-5588239332358236557.wrap-0.html.html){kind=link}
![[Fig. 444]](#5745294564006620949_fig444.jpg.id-3475905229632828247.wrap-0.html.html){kind=link}
Lanberis Slates.—The slates of Llanberis and Penrhyn in Carnarvonshire, with their associated sandy strata, attain a great thickness, sometimes about 3000 feet. They are perhaps not more ancient than the Harlech and Barmouth beds last mentioned, for they may represent the deposits of fine mud thrown down in the same sea, on the borders of which the sands above-mentioned were accumulating. In some of these slaty rocks in Ireland, immediately opposite Anglesea and Carnarvon, two species of fossils have been found, to which the late Professor E. Forbes gave the name of Oldhamia. The nature of these organisms is still a matter of discussion among naturalists.
Cambrian Rocks of Bohemia (Primordial zone of Barrande).—In the year 1846, as before stated, M. Joachim Barrande, after ten years’ exploration of Bohemia, and after collecting more than a thousand species of fossils, had ascertained the existence in that country of three distinct faunas below the Devonian. To his first fauna, which was older than any then known in this country, he gave the name of Étage C; his two first stages A and B consisting of crystalline and metamorphic rocks and unfossiliferous schists. This Étage C or primordial zone proved afterwards to be the equivalent of those subdivisions of the Cambrian groups which have been above described under the names of Menevian and Lingula Flags. The second fauna tallies with Murchison’s Lower Silurian, as originally defined by him when no fossils had been discovered below the Stiper-Stones. The third fauna agrees with the Upper Silurian of the same author. Barrande, without government assistance, had undertaken single-handed the geological survey of Bohemia, the fossils previously obtained from that country having scarcely exceeded 20 in number, whereas he had already acquired, in 1850, no less than 1100 species, namely, 250 crustaceans (chiefly Trilobites), 250 Cephalopods, 160 gasteropods and pteropods, 130 acephalous mollusks, 210 brachiopods, and 110 corals and other fossils. These numbers have since been almost doubled by subsequent investigations in the same country.
In the primordial zone C, he discovered trilobites of the genera Paradoxides, Conocoryphe, Ellipsocephalus, Sao, Arionellus, Hydrocephalus, and Agnostus. M. Barrande pointed out that these primordial trilobites have a peculiar facies of their own dependent on the multiplication of their thoracic segments and the diminution of their caudal shield or pygidium.
Fossils of the lowest Fossiliferous Beds in Bohemia, or
“Primordial Zone” of Barrande.
One of the “primordial” or Upper Cambrian Trilobites of the genus Sao, a form not found as yet elsewhere in the world, afforded M. Barrande a fine illustration of the metamorphosis of these creatures, for he traced them through no less than twenty stages of their development. A few of these changes have been selected for representation in Figure 580, that the reader may learn the gradual manner in which different segments of the body and the eyes make their appearance.
In Bohemia the primordial fauna of Barrande derived its importance exclusively from its numerous and peculiar trilobites. Besides these, however, the same ancient schists have yielded two genera of brachiopods, Orthis and Orbicula, a Pteropod of the genus Theca, and four echinoderms of the cystidean family.
Cambrian of Sweden and Norway.—The Cambrian beds of Wales are represented in Sweden by strata the fossils of which have been described by a most able naturalist, M. Angelin, in his “Palæontologica Suecica” (1852-4). The “alum-schists,” as they are called in Sweden, are horizontal argillaceous rocks which underlie conformably certain Lower Silurian strata in the mountain called Kinnekulle, south of the great Wener Lake in Sweden. These schists contain trilobites belonging to the genera Paradoxides, Olenus, Agnostus, and others, some of which present rudimentary forms, like the genus last mentioned, without eyes, and with the body segments scarcely developed, and others, again, have the number of segments excessively multiplied, as in Paradoxides. Such peculiarities agree with the characters of the crustaceans met with in the Cambrian strata of Wales; and Dr. Torell has recently found in Sweden the Paradoxides Hicksii, a well-known Lower Cambrian fossil.
At the base of the Cambrian strata in Sweden, which in the neighbourhood of Lake Wener are perfectly horizontal, lie ripple-marked quartzose sandstones with worm-tracks and annelid borings, like some of those found in the Harlech grits of the Longmynd. Among these are some which have been referred doubtfully to plants. These sandstones have been called in Sweden “fucoid sandstones.” The whole thickness of the Cambrian rocks of Sweden does not exceed 300 feet from the equivalents of the Tremadoc beds to these sandstones, which last seem to correspond with the Longmynd, and are regarded by Torell as older than any fossiliferous primordial rocks in Bohemia.
Cambrian of the United States and Canada (Potsdam Sandstone).—This formation, as we learn from Sir W. Logan, is 700 feet thick in Canada; the upper part consists of sandstone containing fucoids, and perforated by small vertical holes, which are very characteristic of the rock, and appear to have been made by annelids (Scolithus linearis). The lower portion is a conglomerate with quartz pebbles. I have seen the Potsdam sandstone on the banks of the St. Lawrence, and on the borders of Lake Champlain, where, as at Keesville, it is a white quartzose fine-grained grit, almost passing into quartzite. It is divided into horizontal ripple-marked beds, very like those of the Lingula Flags of Britain, and replete with a small round-shaped Obolella, in such numbers as to divide the rock into parallel planes, in the same manner as do the scales of mica in some micaceous sandstones. Among the shells of this formation in Wisconsin are species of Lingula and Orthis, and several trilobites of the primordial genus Dikelocephalus (Fig. 581). On the banks of the St. Lawrence, near Beauharnois and elsewhere, many fossil footprints have been observed on the surface of the rippled layers. They are supposed by Professor Owen to be the trails of more than one species of articulate animal, probably allied to the King Crab, or Limulus.
Recent investigations by the naturalists of the Canadian survey have rendered it certain that below the level of the Potsdam Sandstone there are slates and schists extending from New York to Newfoundland, occupied by a series of trilobitic forms similar in genera, though not in species, to those found in the European Upper Cambrian strata.
Huronian Series.—Next below the Upper Cambrian occur strata called the Huronian by Sir W. Logan, which are of vast thickness, consisting chiefly of quartzite, with great masses of greenish chloritic slate, which sometimes include pebbles of crystalline rocks derived from the Laurentian formation, next to be described. Limestones are rare in this series, but one band of 300 feet in thickness has been traced for considerable distances to the north of Lake Huron. Beds of greenstone are intercalated conformably with the quartzose and argillaceous members of this series. No organic remains have yet been found in any of the beds, which are about 18,000 feet thick, and rest unconformably on the Laurentian rocks.
LAURENTIAN GROUP.
In the course of the geological survey carried on under the direction of Sir W.E. Logan, it has been shown that, northward of the river St. Lawrence, there is a vast series of crystalline rocks of gneiss, mica-schist, quartzite, and limestone, more than 30,000 feet in thickness, which have been called Laurentian, and which are already known to occupy an area of about 200,000 square miles. They are not only more ancient than the fossiliferous Cambrian formations above described, but are older than the Huronian last mentioned, and had undergone great disturbing movements before the Potsdam sandstone and the other “primordial” or Cambrian rocks were formed. The older half of this Laurentian series is unconformable to the newer portion of the same.
Upper Laurentian or Labrador Series.—The Upper Group, more than 10,000 feet thick, consists of stratified crystalline rocks in which no organic remains have yet been found. They consist in great part of feldspars, which vary in composition from anorthite to andesine, or from those kinds in which there is less than one per cent of potash and soda to those in which there is more than seven per cent of these alkalies, the soda preponderating greatly. These feldsparites sometimes form mountain masses almost without any admixture of other minerals; but at other times they include augite, which passes into hypersthene. They are often granitoid in structure. One of the varieties is the same as the apolescent labradorite rock of Labrador. The Adirondack Mountains in the State of New York are referred to the same series, and it is conjectured that the hypersthene rocks of Skye, which resemble this formation in mineral character, may be of the same geological age.
Lower Laurentian.—This series, about 20,000 feet in thickness, is, as before stated, unconformable to that last mentioned; it consists in great part of gneiss of a reddish tint with orthoclase feldspar. Beds of nearly pure quartz, from 400 to 600 feet thick, occur in some places. Hornblendic and micaceous schists are often interstratified, and beds of limestone, usually crystalline. Beds of plumbago also occur. That this pure carbon may have been of organic origin before metamorphism has naturally been conjectured.
There are several of these limestones which have been traced to great distances, and one of them is from 700 to 1500 feet thick. In the most massive of them Sir W. Logan observed, in 1859, what he considered to be an organic body much resembling the Silurian fossil called Stromatopora rugosa. It had been obtained the year before by Mr. J. MacMullen at the Grand Calumet, on the river Ottawa. This fossil was examined in 1864 by Dr. Dawson of Montreal, who detected in it, by aid of the microscope, the distinct structure of a Rhizopod or Foraminifer. Dr. Carpenter and Professor T. Rupert Jones have since confirmed this opinion, comparing the structure to that of the well-known nummulite. It appears to have grown one layer over another, and to have formed reefs of limestone as do the living coral-building polyp animals. Parts of the original skeleton, consisting of carbonate of lime, are still preserved; while certain inter-spaces in the calcareous fossil have been filled up with serpentine and white augite. On this oldest of known organic remains Dr. Dawson has conferred the name of Eozoon Canadense (see Figs. 582, 583); its antiquity is such that the distance of time which separated it from the Upper Cambrian period, or that of the Potsdam sandstone, may, says Sir W. Logan, be equal to the time which elapsed between the Potsdam sandstone and the nummulitic limestones of the Tertiary period. The Laurentian and Huronian rocks united are about 50,000 feet in thickness, and the Lower Laurentian was disturbed before the newer series was deposited. We may naturally expect the other proofs of unconformability will hereafter be detected at more than one point in so vast a succession of strata.
Fig. 582. a. Chambers of lower tier communicating at +, and separated from adjoining chambers at O by an intervening septum, traversed by passages. b. Chambers of an upper tier. c. Walls of the chambers traversed by fine tubules. (These tubules pass with uniform parallelism from the inner to the outer surface, opening at regular distances from each other.) d. Intermediate skeleton, composed of homogeneous shell substance, traversed by f. Stoloniferous passages connecting the chambers of the two tiers. e. Canal system in intermediate skeleton, showing the arborescent saceodic prolongations. (Fig. 583 shows these bodies in a decalcified state.) f. Stoloniferous passages.
Fig. 583. Decalcified portion of natural rock, showing canal system and the several layers; the acuteness of the planes prevents more than one or two parallel tiers being observed.
The mineral character of the Upper Laurentian differs, as we have seen, from that of the Lower, and the pebbles of gneiss in the Huronian conglomerates are thought to prove that the Laurentian strata were already in a metamorphic state before they were broken up to supply materials for the Huronian. Even if we had not discovered the Eozoon, we might fairly have inferred from analogy that as the quartzites were once beds of sand, and the gneiss and mica-schist derived from shales and argillaceous sandstones, so the calcareous masses, from 400 to 1000 feet and more in thickness, were originally of organic origin. This is now generally believed to have been the case with the Silurian, Devonian, Carboniferous, Oolitic, and Cretaceous limestones and those nummulitic rocks of tertiary date which bear the closest affinity to the Eozoon reefs of the Lower Laurentian. The oldest stratified rock in Scotland is that called by Sir R. Murchison “the fundamental gneiss,” which is found in the north-west of Ross-shire, and in Sutherlandshire (see [Fig. 82]), and forms the whole of the adjoining island of Lewis, in the Hebrides. It has a strike from north-west to south-east, nearly at right angles to the metamorphic strata of the Grampians. On this Laurentian gneiss, in parts of the western Highlands, the Lower Cambrian and various metamorphic rocks rest unconformably. It seems highly probable that this ancient gneiss of Scotland may correspond in date with part of the great Laurentian group of North America.
![[Fig. 82]](#5745294564006620949_fig82.jpg.id-597649194980080654.wrap-0.html.html){kind=link}
[1] Quart. Geol. Journ., vol. iii, p. 156.
[2] This genus has been substituted for Barrande’s Conocephalus, as the latter term had been preoccupied by the entomologists.
[3] Geol. Mag., vol iv.
[4] British Association Report 1865, 1866, 1868 and Quart. Geol. Journ., vols. xxi, xxv.
[5] Brit. Assoc. Report, 1868.
CHAPTER XXVIII.
VOLCANIC ROCKS.
External Form, Structure, and Origin of Volcanic Mountains. — Cones and Craters. — Hypothesis of “Elevation Craters” considered. — Trap Rocks. — Name whence derived. — Minerals most abundant in Volcanic Rocks. — Table of the Analysis of Minerals in the Volcanic and Hypogene Rocks. — Similar Minerals in Meteorites. — Theory of Isomorphism. — Basaltic Rocks. — Trachytic Rocks. — Special Forms of Structure. — The columnar and globular Forms. — Trap Dikes and Veins. — Alteration of Rocks by volcanic Dikes. — Conversion of Chalk into Marble. — Intrusion of Trap between Strata. — Relation of trappean Rocks to the Products of active Volcanoes.
The aqueous or fossiliferous rocks having now been described, we have next to examine those which may be called volcanic, in the most extended sense of that term. In the diagram (Fig. 584) suppose a, a to represent the crystalline formations, such as the granitic and metamorphic; b, b the fossiliferous strata; and c, c the volcanic rocks. These last are sometimes found, as was explained in the first chapter, breaking through a and b, sometimes overlying both, and occasionally alternating with the strata b, b.
External Form, Structure, and Origin of Volcanic Mountains.—The origin of volcanic cones with crater-shaped summits has been explained in the “Principles of Geology” (Chapters 23 to 27), where Vesuvius, Etna, Santorin, and Barren Island are described. The more ancient portions of those mountains or islands, formed long before the times of history, exhibit the same external features and internal structure which belong to most of the extinct volcanoes of still higher antiquity; and these last have evidently been due to a complicated series of operations, varied in kind according to circumstances; as, for example, whether the accumulation took place above or below the level of the sea, whether the lava issued from one or several contiguous vents, and, lastly, whether the rocks reduced to fusion in the subterranean regions happened to have contained more or less silica, potash, soda, lime, iron, and other ingredients. We are best acquainted with the effects of eruptions above water, or those called subÆrial or supramarine; yet the products even of these are arranged in so many ways that their interpretation has given rise to a variety of contradictory opinions, some of which will have to be considered in this chapter.
Cones and Craters.—In regions where the eruption of volcanic matter has taken place in the open air, and where the surface has never since been subjected to great aqueous denudation, cones and craters constitute the most striking peculiarity of this class of formations. Many hundreds of these cones are seen in central France, in the ancient provinces of Auvergne, Velay, and Vivarais, where they observe, for the most part, a linear arrangement, and form chains of hills. Although none of the eruptions have happened within the historical era, the streams of lava may still be traced distinctly descending from many of the craters, and following the lowest levels of the existing valleys. The origin of the cone and crater-shaped hill is well understood, the growth of many having been watched during volcanic eruptions. A chasm or fissure first opens in the earth, from which great volumes of steam are evolved. The explosions are so violent as to hurl up into the air fragments of broken stone, parts of which are shivered into minute atoms. At the same time melted stone or lava usually ascends through the chimney or vent by which the gases make their escape. Although extremely heavy, this lava is forced up by the expansive power of entangled gaseous fluids, chiefly steam or aqueous vapour, exactly in the same manner as water is made to boil over the edge of a vessel when steam has been generated at the bottom by heat. Large quantities of the lava are also shot up into the air, where it separates into fragments, and acquires a spongy texture by the sudden enlargement of the included gases, and thus forms scoriæ, other portions being reduced to an impalpable powder or dust. The showering down of the various ejected materials round the orifice of eruption gives rise to a conical mound, in which the successive envelopes of sand and scoriæ form layers, dipping on all sides from a central axis. In the mean time a hollow, called a crater, has been kept open in the middle of the mound by the continued passage upward of steam and other gaseous fluids. The lava sometimes flows over the edge of the crater, and thus thickens and strengthens the sides of the cone; but sometimes it breaks down the cone on one side (see Fig. 585), and often it flows out from a fissure at the base of the hill, or at some distance from its base.
Some geologists had erroneously supposed, from observations made on recent cones of eruption, that lava which consolidates on steep slopes is always of a scoriaceous or vesicular structure, and never of that compact texture which we find in those rocks which are usually termed “trappean.” Misled by this theory, they have gone so far as to believe that if melted matter has originally descended a slope at an angle exceeding four or five degrees, it never, on cooling, acquires a stony compact texture. Consequently, whenever they found in a volcanic mountain sheets of stony materials inclined at angles of from 5° to 20° or even more than 30°, they thought themselves warranted in assuming that such rocks had been originally horizontal, or very slightly inclined, and had acquired their high inclination by subsequent upheaval. To such dome-shaped mountains with a cavity in the middle, and with the inclined beds having what was called a quâquâversal dip or a slope outward on all sides, they gave the name of “Elevation craters.”
As the late Leopold Von Buch, the author of this theory, had selected the Isle of Palma, one of the Canaries, as a typical illustration of this form of volcanic mountain, I visited that island in 1854, in company with my friend Mr. Hartung, and I satisfied myself that it owes its origin to a series of eruptions of the same nature as those which formed the minor cones, already alluded to. In some of the more ancient or Miocene volcanic mountains, such as Mont Dor and Cantal in central France, the mode of origin by upheaval as above described is attributed to those dome-shaped masses, whether they possess or not a great central cavity, as in Palma. Where this cavity is present, it has probably been due to one or more great explosions similar to that which destroyed a great part of ancient Vesuvius in the time of Pliny. Similar paroxysmal catastrophes have caused in historical times the truncation on a grand scale of some large cones in Java and elsewhere.[[1]]
Among the objections which may be considered as fatal to Von Buch’s doctrine of upheaval in these cases, I may state that a series of volcanic formations extending over an area six or seven miles in its shortest diameter, as in Palma, could not be accumulated in the form of lavas, tuffs, and volcanic breccias or agglomerates without producing a mountain as lofty as that which they now constitute. But assuming that they were first horizontal, and then lifted up by a force acting most powerfully in the centre and tilting the beds on all sides, a central crater having been formed by explosion or by a chasm opening in the middle, where the continuity of the rocks was interrupted, we should have a right to expect that the chief ravines or valleys would open towards the central cavity, instead of which the rim of the great crater in Palma and other similar ancient volcanoes is entire for more than three parts of the whole circumference.
If dikes are seen in the precipices surrounding such craters or central cavities, they certainly imply rents which were filled up with liquid matter. But none of the dislocations producing such rents can have belonged to the supposed period of terminal and paroxysmal upheaval, for had a great central crater been already formed before they originated, or at the time when they took place, the melted matter, instead of filling the narrow vents, would have flowed down into the bottom of the cavity, and would have obliterated it to a certain extent. Making due allowance for the quantity of matter removed by subaërial denudation in volcanic mountains of high antiquity, and for the grand explosions which are known to have caused truncation in active volcanoes, there is no reason for calling in the violent hypothesis of elevation craters to explain the structure of such mountains as Teneriffe, the Grand Canary, Palma, or those of central France, Etna, or Vesuvius, all of which I have examined. With regard to Etna, I have shown, from observations made by me in 1857, that modern lavas, several of them of known date, have formed continuous beds of compact stone even on slopes of 15, 36, and 38 degrees, and, in the case of the lava of 1852, more than 40 degrees. The thickness of these tabular layers varies from 1½ foot to 26 feet. And their planes of stratification are parallel to those of the overlying and underlying scoriæ which form part of the same currents.[[2]]
Nomenclature of Trappean Rocks.—When geologists first began to examine attentively the structure of the northern and western parts of Europe, they were almost entirely ignorant of the phenomena of existing volcanoes. They found certain rocks, for the most part without stratification, and of a peculiar mineral composition, to which they gave different names, such as basalt, greenstone, porphyry, trap tuff, and amygdaloid. All these, which were recognised as belonging to one family, were called “trap” by Bergmann, from trappa, Swedish for a flight of steps—a name since adopted very generally into the nomenclature of the science; for it was observed that many rocks of this class occurred in great tabular masses of unequal extent, so as to form a succession of terraces or steps. It was also felt that some general term was indispensable, because these rocks, although very diversified in form and composition, evidently belonged to one group, distinguishable from the Plutonic as well as from the non-volcanic fossiliferous rocks.
By degrees familiarity with the products of active volcanoes convinced geologists more and more that they were identical with the trappean rocks. In every stream of modern lava there is some variation in character and composition, and even where no important difference can be recognised in the proportions of silica, alumina, lime, potash, iron, and other elementary materials, the resulting materials are often not the same, for reasons which we are as yet unable to explain. The difference also of the lavas poured out from the same mountain at two distinct periods, especially in the quantity of silica which they contain, is often so great as to give rise to rocks which are regarded as forming distinct families, although there may be every intermediate gradation between the two extremes, and although some rocks, forming a transition from the one class to the other, may often be so abundant as to demand special names. These species might be multiplied indefinitely, and I can only afford space to name a few of the principal ones, about the composition and aspect of which there is the least discordance of opinion.
Minerals most abundant in Volcanic Rocks.—The minerals which form the chief constituents of these igneous rocks are few in number. Next to quartz, which is nearly pure silica or silicic acid, the most important are those silicates commonly classed under the several heads of feldspar, mica, hornblende or augite, and olivine. In Table 28.1, in drawing up which I have received the able assistance of Mr. David Forbes, the chemical analysis of these minerals and their varieties is shown, and he has added the specific gravity of the different mineral species, the geological application of which in determining the rocks formed by these minerals will be explained in the sequel (p.504).
Analysis of Minerals most abundant in the Volcanic and Hypogene Rocks.
| THE QUARTZ GROUP | ||||
| QUARTZ | 100·0 2·6 | Silica Specific gravity | ||
| TRIDYMITE | 100·0 2·3 | Silica Specific gravity | ||
| THE FELDSPAR GROUP | ||||
| ORTHOCLASE. —— Carisbad, in granite (bulk) | 65·23 16·26 0·27 nil trace nil 14·66 1·45 nil 2·55 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Sanadine,Drachenfels in trachyte (Rammelsberg) | 65·87 18·53 nil nil 0·95 0·30 10·32 3·49 W. 0·44 2·55 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| ALBITE. —— Arendal, in granite (G. Rose) | 68·46 19·30 nil 0·28 0·68 nil nil 11·27 nil 2·61 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| OLIGOCLASE. —— Ytterby, in granite (Berzelius) | 61·55 23·80 nil nil 3·18 0·80 0·38 9·67 nil 2·65 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Teneriffe, intrachyte (Deville) | 61·55 22·03 nil nil 2·81 0·47 3·44 7·74 nil 2·59 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| LABRADORITE. —— Hitteroe, in Labrador-rock (Waage) | 51·39 29·42 2·90 nil 9·44 0·37 1·10 5·03 W. 0·71 2·72 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Iceland, involcanic (Damour) | 52·17 29·22 1·90 nil 13·11 nil nil 3·40 nil 2·71 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| ANORTHITE. —— Harzburg, in diorite (Streng) | 45·37 34·81 0·59 nil 16·52 0·83 0·40 1·45 W. 0·87 2·74 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Hecla, in volcanic(Waltershausen) | 45·14 32·10 2·03 0·78 18·32 nil 0·22 1·06 nil 2·74 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| LEUCITE. —— Vesuvius, 1811, in lava (Rammelsberg) | 56·10 23·22 nil nil nil nil 20·59 0·57 nil 2·48 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| NEPHELINE. —— Miask, in Miascite (Scheerer) | 44·30 33·25 0·82 nil 0·32 0·07 5·82 16·02 nil 2·59 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Vesuvius, involcanic (Arfvedson) | 44·11 33·73 nil nil nil nil nil 20·46 W. 0·62 2·60 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| THE MICA GROUP | ||||
| MUSCOVITE. —— Finland, in grante (Rose) | 46·36 36·80 4·53 nil nil nil 9·22 nil F. 0·67 W. 1·84 2·90 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| LEPIDOLITE. —— Cornwall, in granite (Regnault) | 52·40 26·80 nil 1·50 nil nil 9·14 nil F. 4·18 Li. 4·85 2·90 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| BIOTITE. —— Bodennais (V. Kobel> | 40·86 15·13 13·00 nil nil 22·00 8·83 nil W. 0·44 2·70 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Vesuvius, involcanic (Chodnef) | 40·91 17·71 11·02 nil 0·30 19·04 9·96 nil nil 2·75 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| PHLOGOPITE. —— New York, in metamorphic limestone(Rammelsberg) | 41·96 13·47 nil 2·67 0·34 27·12 9·37 nil F. 2·93 W. 0·60 2·81 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| MARGARITE. —— Nexos (Smith) | 30·02 49·52 1·65 nil 10·82 0·48 1·25 W. 5·55 2·99 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia =Potash =Soda Other constituents Specific gravity | ||
| RAPIDOLITE. —— Pyrenees (Delesse) | 32·10 18·50 nil 0·06 nil 36·70 nil nil W. 12·10 2·61 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| TALC. —— Zillerthal (Delesse) | 63·00 nil nil trace nil 33·60 nil nil W. 3·10 2·78 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| THE AMPHIBOLE AND PYROXENEGROUP | ||||
| TREMOLITE. —— St. Gothard (Rammelsbeg) | 58·55 nil nil nil 13·90 26·63 nil nil F.W. 0·34 2·93 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| ACTINOLITE. —— Arendal, in granite (Rammelsberg) | 56·77 0·97 nil 5·88 13·56 21·48 nil nil W. 2·20 3·02 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| HORNBLENDE. —— Faymont, in diorite (Deville) | 41·99 11·66 nil 22·22 9·55 12·59 nil 1·02 W. 1·47 3·20 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Etna, in volcanic(Waltershausen) | 40·91 13·68 nil 17·49 13·44 13·19 nil nil W. 0·85 3·01 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| URALITE. —— Ural, (Rammelsberg) | 50·75 5·65 nil 17·27 11·59 12·28 nil nil W. 1·80 3·14 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| AUGITE. —— Bohemia, in dolerite (Rammelsberg) | 51·12 3·38 0·95 8·08 23·54 12·82 nil nil nil 3·35 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Vesuvius, in lavaof 1858 (Rammelsberg) | 49·61 4·42 nil 9·08 22·83 14·22 nil nil nil 3·25 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| DIALLAGE. —— Harz, in Gabbro (Rammelsberg) | 52·00 3·10 nil 9·36 16·29 18·51 nil nil W. 1·10 3·23 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| HYPERSTHENE. —— Labrador, in Labrador-Rock (Damour) | 51·36 0·37 nil 22·59 3·09 21·31 nil nil nil 3·39 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| THE OLIVINE GROUP | ||||
| BRONZITE. —— Greenland (V. Kobell) | 58·00 1·33 11·14 nil nil 29·66 nil nil nil 3·20 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| OLIVINE. —— Carlsbad, in basalt (Rammelsberg) | 39·34 nil nil 14·85 nil 45·81 nil nil nil 3·40 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Mount Somma, involcanic (Walmstedt) | 10·08 0·18 nil 15·74 nil 44·22 nil nil nil 3·33 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
In the “Other constituents” the following signs are used: F=Fluorine, Li=Lithia, W=Loss on igniting the mineral, in most instances only Water.
From the table above it will be observed that many minerals are omitted which, even if they are of common occurrence, are more to be regarded as accessory than as essential components of the rocks in which they are found.[[3]] Such are, for example, Garnet, Epidote, Tourmaline, Idocrase, Andalusite, Scapolite, the various Zeolites, and several other silicates of somewhat rarer occurrence. Magnetite, Titanoferrite, and Iron-pyrites also occur as normal constituents of various igneous rocks, although in very small amount, as also Apatite, or phosphate of lime. The other salts of lime, including its carbonate or calcite, although often met with, are invariably products of secondary chemical action.
The Zeolites, above mentioned, so named from the manner in which they froth up under the blow-pipe and melt into a glass, differ in their chemical composition from all the other mineral constituents of volcanic rocks, since they are hydrated silicates containing from 10 to 25 per cent of water. They abound in some trappean rocks and ancient lavas, where they fill up vesicular cavities and interstices in the substance of the rocks, but are rarely found in any quantity in recent lavas; in most cases they are to be regarded as secondary products formed by the action of water on the other constituents of the rocks. Among them the species Analcime, Stilbite, Natrolite, and Chabazite may be mentioned as of most common occurrence.
Quartz Group.—The microscope has shown that pure quartz is oftener present in lavas than was formerly supposed. It had been argued that the quartz in granite having a specific gravity of 2·6, was not of purely igneous origin, because the silica resulting from fusion in the laboratory has only a specific gravity of 2·3. But Mr. David Forbes has ascertained that the free quartz in trachytes, which are known to have flowed as lava, has the same specific gravity as the ordinary quartz of granite; and the recent researches of Von Rath and others prove that the mineral Tridymite, which is crystallised silica of specific gravity 2·3 (see Table, p. 499), is of common occurrence in the volcanic rocks of Mexico, Auvergne, the Rhine, and elsewhere, although hitherto entirely overlooked.
Feldspar Group.—In the Feldspar group (Table, p. 499) the five mineral species most commonly met with as rock constituents are: 1. Orthoclase, often called common or potash-feldspar. 2. Albite, or soda-feldspar, a mineral which plays a more subordinate part than was formerly supposed, this name having been given to much which has since been proved to be Oligoclase. 3. Oligoclase, or soda-lime feldspar, in which soda is present in much larger proportion than lime, and of which mineral andesite are andesine, is considered to be a variety. 4. Labradorite, or lime-soda-feldspar, in which the proportions of lime and soda are the reverse to what they are in Oligoclase. 5. Anorthite or lime-feldspar. The two latter feldspars are rarely if ever found to enter into the composition of rocks containing quartz.
In employing such terms as potash-feldspar, etc., it must, however, always be borne in mind that it is only intended to direct attention to the predominant alkali or alkaline earth in the mineral, not to assert the absence of the others, which in most cases will be found to be present in minor quantity. Thus potash-feldspar (orthoclase) almost always contains a little soda, and often traces of lime or magnesia; and in like manner with the others. The terms “glassy” and “compact” feldspars only refer to structure, and not to species or composition; the student should be prepared to meet with any of the above feldspars in either of these conditions: the glassy state being apparently due to quick cooling, and the compact to conditions unfavourable to crystallisation; the so-called “compact feldspar” is also very commonly found to be an admixture of more than one feldspar species, and frequently also contains quartz and other extraneous mineral matter only to be detected by the microscope.
Feldspars when arranged according to their system of crystallisation are monoclinic, having one axis obliquely inclined; or triclinic, having the three axes all obliquely inclined to each other. If arranged with reference to their cleavage they are orthoclastic, the fracture taking place always at a right angle; or plagioclastic, in which the cleavages are oblique to one another. Orthoclase is orthoclastic and monoclinic; all the other feldspars are plagioclastic and triclinic.
Minerals in Meteorites.—That variety of the Feldspar Group which is called Anorthite has been shown by Rammelsberg to occur in a meteoric stone, and his analysis proves it to be almost identical in its chemical proportions to the same mineral in the lavas of modern volcanoes. So also Bronzite (Enstatite) and Olivine have been met with in meteorites shown by analysis to come remarkably near to these minerals in ordinary rocks.
Mica Group.—With regard to the micas, the four principal species (Table, p. 499) all contain potash in nearly the same proportion, but differ greatly in the proportion and nature of their other ingredients. Muscovite is often called common or potash mica; Lepidolite is characterised by containing lithia in addition; Biotite contains a large amount of magnesia and oxide of iron; whilst Phlogopite contains still more of the former substance. In rocks containing quartz, muscovite or lepidolite are most common. The mica in recent volcanic rocks, gabbros, and diorites is usually Biotite, while that so common in metamorphic limestones is usually, if not always, Phlogopite.
Amphibole and Pyroxene Group.—The minerals included in the table under the Amphibole and Pyroxene Group differ somewhat in their crystallisation form, though they all belong to the monoclinic system. Amphibole is a general name for all the different varieties of Hornblende, Actinolite, Tremolite, etc., while Pyroxene includes Augite, Diallage, Malacolite, Sahlite, etc. The two divisions are so much allied in chemical composition and crystallographic characters, and blend so completely one into the other in Uralite (see [page 499]), that it is perhaps best to unite them in one group.
Theory of Isomorphism.—The history of the changes of opinion on this point is curious and instructive. Werner first distinguished augite from hornblende; and his proposal to separate them obtained afterwards the sanction of Haüy, Mohs, and other celebrated mineralogists. It was agreed that the form of the crystals of the two species was different, and also their structure, as shown by cleavage—that is to say, by breaking or cleaving the mineral with a chisel, or a blow of the hammer, in the direction in which it yields most readily. It was also found by analysis that augite usually contained more lime, less alumina, and no fluoric acid; which last, though not always found in hornblende, often enters into its composition in minute quantity. In addition to these characters, it was remarked as a geological fact, that augite and hornblende are very rarely associated together in the same rock. It was also remarked that in the crystalline slags of furnaces augitic forms were frequent, the hornblendic entirely absent; hence it was conjectured that hornblende might be the result of slow, and augite of rapid cooling. This view was confirmed by the fact that Mitscherlich and Berthier were able to make augite artificially, but could never succeed in forming hornblende. Lastly, Gustavus Rose fused a mass of hornblende in a porcelain furnace, and found that it did not, on cooling, assume its previous shape, but invariably took that of augite. The same mineralogist observed certain crystals called Uralite (see Table, [p. 499]) in rocks from Siberia, which possessed the cleavage and chemical composition of hornblende, while they had the external form of augite.
If, from these data, it is inferred that the same substance may assume the crystalline forms of hornblende or augite indifferently, according to the more or less rapid cooling of the melted mass, it is nevertheless certain that the variety commonly called augite, and recognised by a peculiar crystalline form, has usually more lime in it, and less alumina, than that called hornblende, although the quantities of these elements do not seem to be always the same. Unquestionably the facts and experiments above mentioned show the very near affinity of hornblende and augite; but even the convertibility of one into the other, by melting and recrystallising, does not perhaps demonstrate their absolute identity. For there is often some portion of the materials in a crystal which are not in perfect chemical combination with the rest. Carbonate of lime, for example, sometimes carries with it a considerable quantity of silex into its own form of crystal, the silex being mechanically mixed as sand, and yet not preventing the carbonate of lime from assuming the form proper to it. This is an extreme case, but in many others some one or more of the ingredients in a crystal may be excluded from perfect chemical union; and after fusion, when the mass recrystallises, the same elements may combine perfectly or in new proportions, and thus a new mineral may be produced. Or some one of the gaseous elements of the atmosphere, the oxygen for example, may, when the melted matter reconsolidates, combine with some one of the component elements.
The different quantity of the impurities or the refuse above alluded to, which may occur in all but the most transparent and perfect crystals, may partly explain the discordant results at which experienced chemists have arrived in their analysis of the same mineral. For the reader will often find that crystals of a mineral determined to be the same by physical characters, crystalline form, and optical properties, have been declared by skilful analysers to be composed of distinct elements. This disagreement seemed at first subversive of the atomic theory, or the doctrine that there is a fixed and constant relation between the crystalline form and structure of a mineral and its chemical composition. The apparent anomaly, however, which threatened to throw the whole science of mineralogy into confusion, was reconciled to fixed principles by the discoveries of Professor Mitscherlich at Berlin, who ascertained that the composition of the minerals which had appeared so variable was governed by a general law, to which he gave the name of isomorphism (from isos, equal, and morphe, form). According to this law, the ingredients of a given species of mineral are not absolutely fixed as to their kind and quality; but one ingredient may be replaced by an equivalent portion of some analogous ingredient. Thus, in augite, the lime may be in part replaced by portions of protoxide of iron, or of manganese, while the form of the crystal, and the angle of its cleavage planes, remain the same. These vicarious substitutions, however, of particular elements cannot exceed certain defined limits.
Basaltic Rocks.—The two principal families of trappean or volcanic rocks are the basalts and the trachytes, which differ chiefly from each other in the quantity of silica which they contain. The basaltic rocks are comparatively poor in silica, containing less than 50 per cent of that mineral, and none in a pure state or as free quartz, apart from the rest of the matrix. They contain a larger proportion of lime and magnesia than the trachytes, so that they are heavier, independently of the frequent presence of the oxides of iron which in some cases forms more than a fourth part of the whole mass. Abich has, therefore, proposed that we should weigh these rocks, in order to appreciate their composition in cases where it is impossible to separate their component minerals. Thus, basalt from Staffa, containing 47·80 per cent of silica, has a specific gravity of 2·95; whereas trachyte, which has 66 per cent of silica, has a specific gravity of only 2·68; trachytic porphyry, containing 69 per cent of silica, a specific gravity of only 2·58. If we then take a rock of intermediate composition, such as that prevailing in the Peak of Teneriffe, which Abich calls Trachyte-dolerite, its proportion of silica being intermediate, or 58 per cent, it weighs 2·78, or more than trachyte, and less than basalt.[[4]]
Basalt.—The different varieties of this rock are distinguished by the names of basalts, anamezites, and dolerites, names which, however, only denote differences in texture without implying any difference in mineral or chemical composition: the term Basalt being used only when the rock is compact, amorphous, and often semi-vitreous in texture, and when it breaks with a perfect conchoidal fracture; when, however, it is uniformly crystalline in appearance, yet very close-grained, the name Anamesite (from anamesos, intermediate) is employed, but if the rock be so coarsely crystallised that its different mineral constituents can be easily recognised by the eye, it is called Dolerite (from doleros, deceitful), in allusion to the difficulty of distinguishing it from some of the rocks known as Plutonic.
Melaphyre is often quite undistinguishable in external appearance from basalt, for although rarely so heavy, dark-coloured, or compact, it may present at times all these varieties of texture. Both these rocks are composed of triclinic feldspar and augite with more or less olivine, magnetic or titaniferous oxide of iron, and usually a little nepheline, leucite, and apatite; basalt usually contains considerably more olivine than melaphyre, but chemically they are closely allied, although the melaphyres usually contain more silica and alumina, with less oxides of iron, lime, and magnesia, than the basalts. The Rowley Hills in Staffordshire, commonly known as Rowley Ragstone, are melaphyre.
Greenstone.—This name has usually been extended to all granular mixtures, whether of hornblende and feldspar, or of augite and feldspar. The term diorite has been applied exclusively to compounds of hornblende and triclinic feldspar. Labrador-rock is a term used for a compound of labradorite or labrador-feldspar and hypersthene; when the hypersthene predominates it is sometimes known under the name of Hypersthene-rock. Gabbro and Diabase are rocks mainly composed of triclinic feldspars and diallage. All these rocks become sometimes very crystalline, and help to connect the volcanic with the Plutonic formations, which will be treated of in Chapter XXXI.
Trachytic Rocks.—The name trachyte (from [**Greek] trachus, rough) was originally given to a coarse granular feldspathic rock which was rough and gritty to the touch. The term was subsequently made to include other rocks, such as clinkstone and obsidian, which have the same mineral composition, but to which, owing to their different texture, the word in its original meaning would not apply. The feldspars which occur in Trachytic rocks are invariably those which contain the largest proportion of silica, or from 60 to 70 per cent of that mineral. Through the base are usually disseminated crystals of glassy feldspar, mica, and sometimes hornblende. Although quartz is not a necessary ingredient in the composition of this rock, it is very frequently present, and the quartz trachytes are very largely developed in many volcanic districts. In this respect the trachytes differ entirely from the members of the Basaltic family, and are more nearly allied to the granites.
Obsidian.—Obsidian, Pitchstone, and Pearlstone are only different forms of a volcanic glass produced by the fusion of trachytic rocks. The distinction between them is caused by different rates of cooling from the melted state, as has been proved by experiment. Obsidian is of a black or ash-grey colour, and though opaque in mass is transparent in thin edges.
Clinkstone or Phonolite.—Among the rocks of the trachytic family, or those in which the feldspars are rich in silica, that termed Clinkstone or Phonolite is conspicuous by its fissile structure, and its tendency to lamination, which is such as sometimes to render it useful as roofing-slate. It rings when struck with the hammer, whence its name; is compact, and usually of a greyish blue or brownish colour; is variable in composition, but almost entirely composed of feldspar. When it contains disseminated crystals of feldspar, it is called Clinkstone porphyry.
Volcanic Rocks distinguished by special Forms of Structure.—Many volcanic rocks are commonly spoken of under names denoting structure alone, which must not be taken to imply that they are distinct rocks, i.e., that they differ from one another either in mineral or chemical composition. Thus the terms Trachytic porphyry, Trachytic tuff, etc., merely refer to the same rock under different conditions of mechanical aggregation or crystalline development which would be more correctly expressed by the use of the adjective, as porphyritic trachyte, etc., but as these terms are so commonly employed it is considered advisable to direct the student’s attention to them.
Porphyry is one of this class, and very characteristic of the volcanic formations. When distinct crystals of one or more minerals are scattered through an earthy or compact base, the rock is termed a porphyry (see Fig. 586). Thus trachyte is usually porphyritic; for in it, as in many modern lavas, there are crystals of feldspar; but in some porphyries the crystals are of augite, olivine, or other minerals. If the base be greenstone, basalt, or pitchstone, the rock may be denominated greenstone-porphyry, pitchstone-porphyry, and so forth. The old classical type of this form of rock is the red porphyry of Egypt, or the well-known “Rosso antico.” It consists, according to Delesse, of a red feldspathic base in which are disseminated rose-coloured crystals of the feldspar called oligoclase, with some plates of blackish hornblende and grains of oxide of iron (iron-glance). Red quartziferous porphyry is a much more siliceous rock, containing about 70 or 80 per cent of silex, while that of Egypt has only 62 per cent.
Amygdaloid.—This is also another form of igneous rock, admitting of every variety of composition. It comprehends any rock in which round or almond-shaped nodules of some mineral, such as agate, chalcedony, calcareous spar, or zeolite, are scattered through a base of wacke, basalt, greenstone, or other kind of trap. It derives its name from the Greek word amygdalon, an almond. The origin of this structure cannot be doubted, for we may trace the process of its formation in modern lavas. Small pores or cells are caused by bubbles of steam and gas confined in the melted matter. After or during consolidation, these empty spaces are gradually filled up by matter separating from the mass, or infiltered by water permeating the rock. As these bubbles have been sometimes lengthened by the flow of the lava before it finally cooled, the contents of such cavities have the form of almonds. In some of the amygdaloidal traps of Scotland, where the nodules have decomposed, the empty cells are seen to have a glazed or vitreous coating, and in this respect exactly resemble scoriaceous lavas, or the slags of furnaces.
Fig. 587 represents a fragment of stone taken from the upper part of a sheet of basaltic lava in Auvergne. One-half is scoriaceous, the pores being perfectly empty; the other part is amygdaloidal, the pores or cells being mostly filled up with carbonate of lime, forming white kernels.
Lava.—This term has a somewhat vague signification, having been applied to all melted matter observed to flow in streams from volcanic vents. When this matter consolidates in the open air, the upper part is usually scoriaceous, and the mass becomes more and more stony as we descend, or in proportion as it has consolidated more slowly and under greater pressure. At the bottom, however, of a stream of lava, a small portion of scoriaceous rock very frequently occurs, formed by the first thin sheet of liquid matter, which often precedes the main current, and solidifies under slight pressure.
The more compact lavas are often porphyritic, but even the scoriaceous part sometimes contains imperfect crystals, which have been derived from some older rocks, in which the crystals pre-existed, but were not melted, as being more infusible in their nature. Although melted matter rising in a crater, and even that which enters a rent on the side of a crater, is called lava, yet this term belongs more properly to that which has flowed either in the open air or on the bed of a lake or sea. If the same fluid has not reached the surface, but has been merely injected into fissures below ground, it is called trap. There is every variety of composition in lavas; some are trachytic, as in the Peak of Teneriffe; a great number are basaltic, as in Vesuvius and Auvergne; others are andesitic, as those of Chili; some of the most modern in Vesuvius consist of green augite, and many of those of Etna of augite and labrador-feldspar.[[5]]
Scoriæ and Pumice may next be mentioned, as porous rocks produced by the action of gases on materials melted by volcanic heat. Scoriæ are usually of a reddish-brown and black colour, and are the cinders and slags of basaltic or augitic lavas. Pumice is a light, spongy, fibrous substance, produced by the action of gases on trachytic and other lavas; the relation, however, of its origin to the composition of lava is not yet well understood. Von Buch says that it never occurs where only labrador-feldspar is present.
Volcanic Ash or Tuff, Trap Tuff.—Small angular fragments of the scoriæ and pumice, above-mentioned, and the dust of the same, produced by volcanic explosions, form the tuffs which abound in all regions of active volcanoes, where showers of these materials, together with small pieces of other rocks ejected from the crater, and more or less burnt, fall down upon the land or into the sea. Here they often become mingled with shells, and are stratified. Such tuffs are sometimes bound together by a calcareous cement, and form a stone susceptible of a beautiful polish. But even when little or no lime is present, there is a great tendency in the materials of ordinary tuffs to cohere together. The term volcanic ash has been much used for rocks of all ages supposed to have been derived from matter ejected in a melted state from volcanic orifices. We meet occasionally with extremely compact beds of volcanic materials, interstratified with fossiliferous rocks. These may sometimes be tuffs, although their density or compactness is such as the cause them to resemble many of those kinds of trap which are found in ordinary dikes.
Wacke is a name given to a decomposed state of various trap rocks of the basaltic family, or those which are poor in silica. It resembles clay of a yellowish or brown colour, and passes gradually from the soft state to the hard dolerite, greenstone, or other trap rock from which it has been derived.
Agglomerate.—In the neighbourhood of volcanic vents, we frequently observe accumulations of angular fragments of rocks formed during eruptions by the explosive action of steam, which shatters the subjacent stony formations, and hurls them up into the air. They then fall in showers around the cone or crater, or may be spread for some distance over the surrounding country. The fragments consist usually of different varieties of scoriaceous and compact lavas; but other kinds of rock, such as granite or even fossiliferous limestones, may be intermixed; in short, any substance through which the expansive gases have forced their way. The dispersion of such materials may be aided by the wind, as it varies in direction or intensity, and by the slope of the cone down which they roll, or by floods of rain, which often accompany eruptions. But if the power of running water, or of the waves and currents of the sea, be sufficient to carry the fragments to a distance, it can scarcely fail to wear off their angles, and the formation then becomes a conglomerate. If occasionally globular pieces of scoriæ abound in an agglomerate, they may not owe their round form to attrition. When all the angular fragments are of volcanic rocks the mass is usually termed a volcanic breccia.
Laterite is a red or brick-like rock composed of silicate of alumina and oxide of iron. The red layers called “ochre beds,” dividing the lavas of the Giant’s Causeway, are laterites. These were found by Delesse to be trap impregnated with the red oxide of iron, and in part reduced to kaolin. When still more decomposed, they were found to be clay coloured by red ochre. As two of the lavas of the Giant’s Causeway are parted by a bed of lignite, it is not improbable that the layers of laterite seen in the Antrim cliffs resulted from atmospheric decomposition. In Madeira and the Canary Islands streams of lava of subaërial origin are often divided by red bands of laterite, probably ancient soils formed by the decomposition of the surfaces of lava-currents, many of these soils having been coloured red in the atmosphere by oxide of iron, others burnt into a red brick by the overflowing of heated lavas. These red bands are sometimes prismatic, the small prisms being at right angles to the sheets of lava. Red clay or red marl, formed as above stated by the disintegration of lava, scoriæ, or tuff, has often accumulated to a great thickness in the valleys of Madeira, being washed into them by alluvial action; and some of the thick beds of laterite in India may have had a similar origin. In India, however, especially in the Deccan, the term “laterite” seems to have been used too vaguely to answer the above definition. The vegetable soil in the gardens of the suburbs of Catania which was overflowed by the lava of 1669 was turned or burnt into a layer of red brick-coloured stone, or in other words, into laterite, which may now be seen supporting the old lava-current.
Columnar and Globular Structure.—One of the characteristic forms of volcanic rocks, especially of basalt, is the columnar, where large masses are divided into regular prisms, sometimes easily separable, but in other cases adhering firmly together. The columns vary, in the number of angles, from three to twelve; but they have most commonly from five to seven sides. They are often divided transversely, at nearly equal distances, like the joints in a vertebral column, as in the Giant’s Causeway, in Ireland. They vary exceedingly in respect to length and diameter. Dr. MacCulloch mentions some in Skye which are about 400 feet long; others, in Morven, not exceeding an inch. In regard to diameter, those of Ailsa measure nine feet, and those of Morven an inch or less.[[6]] They are usually straight, but sometimes curved; and examples of both these occur in the island of Staffa. In a horizontal bed or sheet of trap the columns are vertical; in a vertical dike they are horizontal.
It being assumed that columnar trap has consolidated from a fluid state, the prisms are said to be always at right angles to the cooling surfaces. If these surfaces, therefore, instead of being either perpendicular or horizontal, are curved, the columns ought to be inclined at every angle to the horizon; and there is a beautiful exemplification of this phenomenon in one of the valleys of the Vivarais, a mountainous district in the South of France, where, in the midst of a region of gneiss, a geologist encounters unexpectedly several volcanic cones of loose sand and scoriæ. From the crater of one of these cones, called La Coupe d’Ayzac, a stream of lava has descended and occupied the bottom of a narrow valley, except at those points where the river Volant, or the torrents which join it, have cut away portions of the solid lava. Fig. 588 represents the remnant of the lava at one of these points. It is clear that the lava once filled the whole valley up to the dotted line d a; but the river has gradually swept away all below that line, while the tributary torrent has laid open a transverse section; by which we perceive, in the first place, that the lava is composed, as usual in this country, of three parts: the uppermost, at a, being scoriaceous, the second b, presenting irregular prisms; and the third, c, with regular columns, which are vertical on the banks of the Volant, where they rest on a horizontal base of gneiss, but which are inclined at an angle of 45°, at g, and are nearly horizontal at f, their position having been everywhere determined, according to the law before mentioned, by the form of the original valley.
In Fig. 589, a view is given of some of the inclined and curved columns which present themselves on the sides of the valleys in the hilly region north of Vicenza, in Italy, and at the foot of the higher Alps.[[7]] Unlike those of the Vivarais, last mentioned, the basalt of this country was evidently submarine, and the present valleys have since been hollowed out by denudation.
The columnar structure is by no means peculiar to the trap rocks in which augite abounds; it is also observed in trachyte, and other feldspathic rocks of the igneous class, although in these it is rarely exhibited in such regular polygonal forms. It has been already stated that basaltic columns are often divided by cross-joints. Sometimes each segment, instead of an angular, assumes a spheroidal form, so that a pillar is made up of a pile of balls, usually flattened, as in the Cheese-grotto at Bertrich-Baden, in the Eifel, near the Moselle (Fig. 590). The basalt there is part of a small stream of lava, from 30 to 40 feet thick, which has proceeded from one of several volcanic craters, still extant, on the neighbouring heights.
In some masses of decomposing greenstone, basalt, and other trap rocks, the globular structure is so conspicuous that the rock has the appearance of a heap of large cannon balls. According to M. Delesse, the centre of each spheroid has been a centre of crystallisation, around which the different minerals of the rock arranged themselves symmetrically during the process of cooling. But it was also, he says, a centre of contraction, produced by the same cooling, the globular form, therefore, of such spheroids being the combined result of crystallisation and contraction.[[8]]
Mr. Scrope gives as an illustration of this structure a resinous trachyte or pitchstone-porphyry in one of the Ponza islands, which rise from the Mediterranean, off the coast of Terracina and Gaeta. The globes vary from a few inches to three feet in diameter, and are of an ellipsoidal form (see Fig. 591). The whole rock is in a state of decomposition, “and when the balls,” says Mr. Scrope, “have been exposed a short time to the weather, they scale off at a touch into numerous concentric coats, like those of a bulbous root, inclosing a compact nucleus. The laminæ of this nucleus have not been so much loosened by decomposition; but the application of a ruder blow will produce a still further exfoliation.”[[9]]
Volcanic or Trap Dikes.—The leading varieties of the trappean rocks—basalt, greenstone, trachyte, and the rest—are found sometimes in dikes penetrating stratified and unstratified formations, sometimes in shapeless masses protruding through or overlying them, or in horizontal sheets intercalated between strata. Fissures have already been spoken of as occurring in all kinds of rocks, some a few feet, others many yards in width, and often filled up with earth or angular pieces of stone, or with sand and pebbles. Instead of such materials, suppose a quantity of melted stone to be driven or injected into an open rent, and there consolidated, we have then a tabular mass resembling a wall, and called a trap dike. It is not uncommon to find such dikes passing through strata of soft materials, such as tuff, scoriæ, or shale, which, being more perishable than the trap, are often washed away by the sea, rivers, or rain, in which case the dike stands prominently out in the face of precipices, or on the level surface of a country (see Fig. 592).
In the islands of Arran and Skye, and in other parts of Scotland, where sandstone, conglomerate, and other hard rocks are traversed by dikes of trap, the converse of the above phenomenon is seen. The dike, having decomposed more rapidly than the containing rock, has once more left open the original fissure, often for a distance of many yards inland from the sea-coast. There is yet another case, by no means uncommon in Arran and other parts of Scotland, where the strata in contact with the dike, and for a certain distance from it, have been hardened, so as to resist the action of the weather more than the dike itself, or the surrounding rocks. When this happens, two parallel walls of indurated strata are seen protruding above the general level of the country and following the course of the dike. In Fig. 593, a ground plan is given of a ramifying dike of greenstone, which I observed cutting through sandstone on the beach near Kildonan Castle, in Arran. The larger branch varies from five to seven feet in width, which will afford a scale of measurement for the whole.
In the Hebrides and other countries, the same masses of trap which occupy the surface of the country far and wide, concealing the subjacent stratified rocks, are seen also in the sea-cliffs, prolonged downward in veins or dikes, which probably unite with other masses of igneous rock at a greater depth. The largest of the dikes represented in Fig. 594, and which are seen in part of the coast of Skye, is no less than 100 feet in width.
Every variety of trap-rock is sometimes found in dikes, as basalt, greenstone, feldspar-porphyry, and trachyte. The amygdaloidal traps also occur, though more rarely, and even tuff and breccia, for the materials of these last may be washed down into open fissures at the bottom of the sea, or during eruption on the land may be showered into them from the air. Some dikes of trap may be followed for leagues uninterruptedly in nearly a straight direction, as in the north of England, showing that the fissures which they fill must have been of extraordinary length.
Rocks altered by Volcanic Dikes.—After these remarks on the form and composition of dikes themselves, I shall describe the alterations which they sometimes produce in the rocks in contact with them. The changes are usually such as the heat of melted matter and of the entangled steam and gases might be expected to cause.
Plas-Newydd: Dike cutting through Shale.—A striking example, near Plas-Newydd, in Anglesea, has been described by Professor Henslow.[[10]] The dike is 134 feet wide, and consists of a rock which is a compound of feldspar and augite (dolerite of some authors). Strata of shale and argillaceous limestone, through which it cuts perpendicularly, are altered to a distance of 30, or even, in some places, of 35 feet from the edge of the dike. The shale, as it approaches the trap, becomes gradually more compact, and is most indurated where nearest the junction. Here it loses part of its schistose structure, but the separation into parallel layers is still discernible. In several places the shale is converted into hard porcelanous jasper. In the most hardened part of the mass the fossil shells, principally Producti, are nearly obliterated; yet even here their impressions may frequently be traced. The argillaceous limestone undergoes analogous mutations, losing its earthy texture as it approaches the dike, and becoming granular and crystalline. But the most extraordinary phenomenon is the appearance in the shale of numerous crystals of analcime and garnet, which are distinctly confined to those portions of the rock affected by the dike.[[11]] Some garnets contain as much as 20 per cent of lime, which they may have derived from the decomposition of the fossil shells or Producti. The same mineral has been observed, under very analogous circumstances, in High Teesdale, by Professor Sedgwick, where it also occurs in shale and limestone, altered by basalt.[[12]]
Antrim: Dike cutting through Chalk.—In several parts of the county of Antrim, in the north of Ireland, chalk with flints is traversed by basaltic dikes. The chalk is there converted into granular marble near the basalt, the change sometimes extending eight or ten feet from the wall of the dike, being greatest near the point of contact, and thence gradually decreasing till it becomes evanescent. “The extreme effect,” says Dr. Berger, “presents a dark brown crystalline limestone, the crystals running in flakes as large as those of coarse primitive (metamorphic) limestone; the next state is saccharine, then fine grained and arenaceous; a compact variety, having a porcelanous aspect and a bluish-grey colour, succeeds: this, towards the outer edge, becomes yellowish-white, and insensibly graduates into the unaltered chalk. The flints in the altered chalk usually assume a grey yellowish colour.”[[13]] All traces of organic remains are effaced in that part of the limestone which is most crystalline.
Fig. 595: Basaltic dikes in chalk in Island of Rathlin, Antrim. Ground-plan as seen on the beach. (Conybeare and Buckland[[14]])
Fig. 595 represents three basaltic dikes traversing the chalk, all within the distance of 90 feet. The chalk contiguous to the two outer dikes is converted into a finely granular marble, m, m, as are the whole of the masses between the outer dikes and the central one. The entire contrast in the composition and colour of the intrusive and invaded rocks, in these cases, renders the phenomena peculiarly clear and interesting. Another of the dikes of the north-east of Ireland has converted a mass of red sandstone into hornstone. By another, the shale of the coal-measures has been indurated, assuming the character of flinty slate; and in another place the slate-clay of the lias has been changed into flinty slate, which still retains numerous impressions of ammonites.[[15]]
It might have been anticipated that beds of coal would, from their combustible nature, be affected in an extraordinary degree by the contact of melted rock. Accordingly, one of the greenstone dikes of Antrim, on passing through a bed of coal, reduces it to a cinder for the space of nine feet on each side. At Cockfield Fell, in the north of England, a similar change is observed. Specimens taken at the distance of about thirty yards from the trap are not distinguishable from ordinary pit-coal; those nearer the dike are like cinders, and have all the character of coke; while those close to it are converted into a substance resembling soot.[[16]]
It is by no means uncommon to meet with the same rocks, even in the same districts, absolutely unchanged in the proximity of volcanic dikes. This great inequality in the effects of the igneous rocks may often arise from an original difference in their temperature, and in that of the entangled gases, such as is ascertained to prevail in different lavas, or in the same lava near its source and at a distance from it. The power also of the invaded rocks to conduct heat may vary, according to their composition, structure, and the fractures which they may have experienced, and perhaps, also, according to the quantity of water (so capable of being heated) which they contain. It must happen in some cases that the component materials are mixed in such proportions as to prepare them readily to enter into chemical union, and form new minerals; while in other cases the mass may be more homogeneous, or the proportions less adapted for such union.
We must also take into consideration, that one fissure may be simply filled with lava, which may begin to cool from the first; whereas in other cases the fissure may give passage to a current of melted matter, which may ascend for days or months, feeding streams which are overflowing the country above, or being ejected in the shape of scoriæ from some crater. If the walls of a rent, moreover, are heated by hot vapour before the lava rises, as we know may happen on the flanks of a volcano, the additional heat supplied by the dike and its gases will act more powerfully.
Intrusion of Trap between Strata.—Masses of trap are not unfrequently met with intercalated between strata, and maintaining their parallelism to the planes of stratification throughout large areas. They must in some places have forced their way laterally between the divisions of the strata, a direction in which there would be the least resistance to an advancing fluid, if no vertical rents communicated with the surface, and a powerful hydrostatic pressure were caused by gases propelling the lava upward.
Relation of Trappean Rocks to the Products of active Volcanoes.—When we reflect on the changes above described in the strata near their contact with trap dikes, and consider how complete is the analogy or often identity in composition and structure of the rocks called trappean and the lavas of active volcanoes, it seems difficult at first to understand how so much doubt could have prevailed for half a century as to whether trap was of igneous or aqueous origin. To a certain extent, however, there was a real distinction between the trappean formations and those to which the term volcanic was almost exclusively confined. A large portion of the trappean rocks first studied in the north of Germany, and in Norway, France, Scotland, and other countries, were such as had been formed entirely under water, or had been injected into fissures and intruded between strata, and which had never flowed out in the air, or over the bottom of a shallow sea. When these products, therefore, of submarine or subterranean igneous action were contrasted with loose cones of scoriæ, tuff, and lava, or with narrow streams of lava in great part scoriaceous and porous, such as were observed to have proceeded from Vesuvius and Etna, the resemblance seemed remote and equivocal. It was, in truth, like comparing the roots of a tree with its leaves and branches, which, although the belong to the same plant, differ in form, texture, colour, mode of growth, and position. The external cone, with its loose ashes and porous lava, may be likened to the light foliage and branches, and the rocks concealed far below, to the roots. But it is not enough to say of the volcano,
“Quantum vertice in auras
Ætherias, tantum radice in Tartara tendit,”
for its roots do literally reach downward to Tartarus, or to the regions of subterranean fire; and what is concealed far below is probably always more important in volume and extent than what is visible above ground.
We have already stated how frequently dense masses of strata have been removed by denudation from wide areas (see Chapter VI); and this fact prepares us to expect a similar destruction of whatever may once have formed the uppermost part of ancient submarine or subaërial volcanoes, more especially as those superficial parts are always of the lightest and most perishable materials. The abrupt manner in which dikes of trap usually terminate at the surface (see Fig. 596), and the water-worn pebbles of trap in the alluvium which covers the dike, prove incontestably that whatever was uppermost in these formations has been swept away. It is easy, therefore, to conceive that what is gone in regions of trap may have corresponded to what is now visible in active volcanoes.
As to the absence of porosity in the trappean formations, the appearances are in a great degree deceptive, for all amygdaloids are, as already explained, porous rocks, into the cells of which mineral matter such as silex, carbonate of lime, and other ingredients, have been subsequently introduced (see [p. 507]); sometimes, perhaps, by secretion during the cooling and consolidation of lavas. In the Little Cumbray, one of the Western Islands, near Arran, the amygdaloid sometimes contains elongated cavities filled with brown spar; and when the nodules have been washed out, the interior of the cavities is glazed with the vitreous varnish so characteristic of the pores of slaggy lavas. Even in some parts of this rock which are excluded from air and water, the cells are empty, and seem to have always remained in this state, and are therefore undistinguishable from some modern lavas.[[17]]
Dr. MacCulloch, after examining with great attention these and the other igneous rocks of Scotland, observes, “that it is a mere dispute about terms, to refuse to the ancient eruptions of trap the name of submarine volcanoes; for they are such in every essential point, although they no longer eject fire and smoke.” The same author also considers it not improbable that some of the volcanic rocks of the same country may have been poured out in the open air.[[18]]
It will be seen in the following chapters that in the earth’s crust there are volcanic tuffs of all ages, containing marine shells, which bear witness to eruptions at many successive geological periods. These tuffs, and the associated trappean rocks, must not be compared to lava and scoriæ which had cooled in the open air. Their counterparts must be sought in the products of modern submarine volcanic eruptions. If it be objected that we have no opportunity of studying these last, it may be answered, that subterranean movements have caused, almost everywhere in regions of active volcanoes, great changes in the relative level of land and sea, in times comparatively modern, so as to expose to view the effects of volcanic operations at the bottom of the sea.
[1] Principles, vol. ii, pp. 56 and 145.
[2] Memoir on Mount Etna, Phil. Trans., 1858.
[3] For analyses of these minerals see the Mineralogies of Dana and Bristow.
[4] Dr. Daubeny on Volcanoes, 2nd ed., pp. 14, 15.
[5] G. Hose, Ann. des Mines, tome viii, p. 32.
[6] MacCulloch Sys. of Geol., vol. ii, p. 137.
[7] Fortis, Mém. sur l’Hist. Nat. de l’Italie, tome 1., p. 233, plate 7.
[8] Delesse, sur les Roches Globuleuses, Mém. de la Soc. Géol. de France, 2 sér., tome iv.
[9] Scrope, Geol. Trans., 2nd series, vol. ii, p. 205.
[10] Cambridge Transactions, vol. i, p. 402.
[11] Ibid., vol. i, p. 410.
[12] Ibid., vol. ii, p. 175.
[13] Dr. Berger, Geol. Trans., 1st series, vol. iii, p. 172.
[14] Geol. Trans., 1st series, vol. iii, p. 210 and plate 10.
[15] Ibid., vol. iii, p. 213; and Playfair, Illus. of Hutt. Theory, s. 253.
[16] Sedgwick, Camb. Trans., vol. ii, p. 37.)
[17] MacCulloch, West. Islands, vol. ii, p. 487.
[18] Syst. of Geol., vol. ii, p. 114.
CHAPTER XXIX.
ON THE AGES OF VOLCANIC ROCKS.
Tests of relative Age of Volcanic Rocks. — Why ancient and modern Rocks cannot be identical. — Tests by Superposition and intrusion. — Test by Alteration of Rocks in Contact. — Test by Organic Remains. — Test of Age by Mineral Character. — Test by Included Fragments. — Recent and Post-pliocene volcanic Rocks. — Vesuvius, Auvergne, Puy de Côme, and Puy de Pariou. — Newer Pliocene volcanic Rocks. — Cyclopean Isles, Etna, Dikes of Palagonia, Madeira. — Older Pliocene volcanic Rocks. — Italy. — Pliocene Volcanoes of the Eifel. — Trass.
Having in the former part of this work referred the sedimentary strata to a long succession of geological periods, we have now to consider how far the volcanic formations can be classed in a similar chronological order. The tests of relative age in this class of rocks are four: first, superposition and intrusion, with or without alteration of the rocks in contact; second, organic remains; third, mineral characters; fourth, included fragments of older rocks.
Besides these four tests it may be said, in a general way, that volcanic rocks of Primary or Palæozoic antiquity differ from those of the Secondary or Mesozoic age, and these again from the Tertiary and Recent. Not, perhaps, that they differed originally in a greater degree than the modern volcanic rocks of one region, such as that of the Andes, differ from those of another, such as Iceland, but because all rocks permeated by water, especially if its temperature be high, are liable to undergo a slow transmutation, even when they do not assume a new crystalline form like that of the hypogene rocks.
Although subaërial and submarine denudation, as before stated, remove, in the course of ages, large portions of the upper or more superficial products of volcanoes, yet these are sometimes preserved by subsidence, becoming covered by the sea or by superimposed marine deposits. In this way they may be protected for ages from the waves of the sea, or the destroying action of rivers, while, at the same time, they may not sink so deep as to be exposed to that Plutonic action (to be spoken of in Chapter XXXI) which would convert them into crystalline rocks. But even in this case they will not remain unaltered, because they will be percolated by water often of high temperature, and charged with carbonate of lime, silex, iron, and other mineral ingredients, whereby gradual changes in the constitution of the rocks may be superinduced. Every geologist is aware how often silicified trees occur in volcanic tuffs, the perfect preservation of their internal structure showing that they have not decayed before the petrifying material was supplied.
The porous and vesicular nature of a large part, both of the basaltic and trachytic lavas, affords cavities in which silex and carbonate of lime are readily deposited. Minerals of the zeolite family, the composition of which has already been alluded to, [p. 500], occur in amygdaloids and other trap-rocks in great abundance, and Daubrée’s observations have proved that they are not always simple deposits of substances held in solution by the percolating waters, being occasionally products of the chemical action of that water on the rock through which they are filtered, and portions of which are decomposed. From these considerations it follows that the perfect identity of very ancient and very modern volcanic formations is scarcely possible.
Tests by Superposition.—If a volcanic rock rest upon an aqueous deposit, the volcanic must be the newest of the two; but the like rule does not hold good where the aqueous formation rests upon the volcanic, for melted matter, rising from below, may penetrate a sedimentary mass without reaching the surface, or may be forced in conformably between two strata, as b below D in Fig. 597, after which it may cool down and consolidate. Superposition, therefore, is not of the same value as a test of age in the unstratified volcanic rocks as in fossiliferous formations. We can only rely implicitly on this test where the volcanic rocks are contemporaneous, not where they are intrusive. Now, they are said to be contemporaneous if produced by volcanic action which was going on simultaneously with the deposition of the strata with which they are associated. Thus in the section at D (Fig. 597), we may perhaps ascertain that the trap b flowed over the fossiliferous bed c, and that, after its consolidation, a was deposited upon it, a and c both belonging to the same geological period. But, on the other hand, we must conclude the trap to be intrusive, if the stratum a be altered by b at the point of contact, or if, in pursuing b for some distance, we find at length that it cuts through the stratum a, and then overlies it as at E.
We may, however, be easily deceived in supposing the volcanic rock to be intrusive, when in reality it is contemporaneous; for a sheet of lava, as it spreads over the bottom of the sea, cannot rest everywhere upon the same stratum, either because these have been denuded, or because, if newly thrown down, they thin out in certain places, thus allowing the lava to cross their edges. Besides, the heavy igneous fluid will often, as it moves along, cut a channel into beds of soft mud and sand. Suppose the submarine lava F (Fig. 598) to have come in contact in this manner with the strata a, b, c, and that after its consolidation the strata d, e are thrown down in a nearly horizontal position, yet so as to lie unconformably to F, the appearance of subsequent intrusion will here be complete, although the trap is in fact contemporaneous. We must not, therefore, hastily infer that the rock F is intrusive, unless we find the overlying strata, d, e, to have been altered at their junction, as if by heat.
The test of age by superposition is strictly applicable to all stratified volcanic tuffs, according to the rules already explained in the case of sedimentary deposits (see [p. 124]).
Test of Age by Organic Remains.—We have seen how, in the vicinity of active volcanoes, scoriæ, pumice, fine sand, and fragments of rock are thrown up into the air, and then showered down upon the land, or into neighbouring lakes or seas. In the tuffs so formed shells, corals, or any other durable organic bodies which may happen to be strewed over the bottom of a lake or sea will be imbedded, and thus continue as permanent memorials of the geological period when the volcanic eruption occurred. Tufaceous strata thus formed in the neighbourhood of Vesuvius, Etna, Stromboli, and other volcanoes now in islands or near the sea, may give information of the relative age of these tuffs at some remote future period when the fires of these mountains are extinguished. By evidence of this kind we can establish a coincidence in age between volcanic rocks and the different primary, secondary, and tertiary fossiliferous strata.
The tuffs alluded to may not always be marine, but may include, in some places, fresh-water shells; in others, the bones of terrestrial quadrupeds. The diversity of organic remains in formations of this nature is perfectly intelligible, if we reflect on the wide dispersion of ejected matter during late eruptions, such as that of the volcano of Coseguina, in the province of Nicaragua, January 19, 1835. Hot cinders and fine scoriæ were then cast up to a vast height, and covered the ground as they fell to the depth of more than ten feet, for a distance of eight leagues from the crater, in a southerly direction. Birds, cattle, and wild animals were scorched to death in great numbers, and buried in ashes. Some volcanic dust fell at Chiapa, upward of 1200 miles, not to leeward of the volcano, as might have been anticipated, but to windward, a striking proof of a counter-current in the upper region of the atmosphere; and some on Jamaica, about 700 miles distant to the north-east. In the sea, also, at the distance of 1100 miles from the point of eruption, Captain Eden of the “Conway” sailed 40 miles through floating pumice, among which were some pieces of considerable size.[[1]]
Test of Age by Mineral Composition.—As sediment of homogeneous composition, when discharged from the mouth of a large river, is often deposited simultaneously over a wide space, so a particular kind of lava flowing from a crater during one eruption may spread over an extensive area; thus in Iceland, in 1783, the melted matter, pouring from Skaptar Jokul, flowed in streams in opposite directions, and caused a continuous mass the extreme points of which were 90 miles distant from each other. This enormous current of lava varied in thickness from 100 feet to 600 feet, and in breadth from that of a narrow river gorge to 15 miles.[[2]] Now, if such a mass should afterwards be divided into separate fragments by denudation, we might still, perhaps, identify the detached portions by their similarity in mineral composition. Nevertheless, this test will not always avail the geologist; for, although there is usually a prevailing character in lava emitted during the same eruption, and even in the successive currents flowing from the same volcano, still, in many cases, the different parts even of one lava-stream, or, as before stated, of one continuous mass of trap, vary much in mineral composition and texture.
In Auvergne, the Eifel, and other countries where trachyte and basalt are both present, the trachytic rocks are for the most part older than the basaltic. These rocks do, indeed, sometimes alternate partially, as in the volcano of Mont Dor, in Auvergne; and in Madeira trachytic rocks overlie an older basaltic series; but the trachyte occupies more generally an inferior position, and is cut through and overflowed by basalt. It can by no means be inferred that trachyte predominated at one period of the earth’s history and basalt at another, for we know that trachytic lavas have been formed at many successive periods, and are still emitted from many active craters; but it seems that in each region, where a long series of eruptions have occurred, the lavas containing feldspar more rich in silica have been first emitted, and the escape of the more augitic kinds has followed. The hypothesis suggested by Mr. Scrope may, perhaps, afford a solution of this problem. The minerals, he observes, which abound in basalt are of greater specific gravity than those composing the feldspathic lavas; thus, for example, hornblende, augite, and olivine are each more than three times the weight of water; whereas common feldspar and albite have each scarcely more than 2½ times the specific gravity of water; and the difference is increased in consequence of there being much more iron in a metallic state in basalt and greenstone than in trachyte and other allied feldspathic lavas. If, therefore, a large quantity of rock be melted up in the bowels of the earth by volcanic heat, the denser ingredients of the boiling fluid may sink to the bottom, and the lighter remaining above would in that case be first propelled upward to the surface by the expansive power of gases. Those materials, therefore, which occupy the lowest place in the subterranean reservoir will always be emitted last, and take the uppermost place on the exterior of the earth’s crust.
Test by Included Fragments.—We may sometimes discover the relative age of two trap-rocks, or of an aqueous deposit and the trap on which it rests, by finding fragments of one included in the other in cases such as those before alluded to, where the evidence of superposition alone would be insufficient. It is also not uncommon to find a conglomerate almost exclusively composed of rolled pebbles of trap, associated with some fossiliferous stratified formation in the neighbourhood of massive trap. If the pebbles agree generally in mineral character with the latter, we are then enabled to determine its relative age by knowing that of the fossiliferous strata associated with the conglomerate. The origin of such conglomerates is explained by observing the shingle beaches composed of trap-pebbles in modern volcanoes, as at the base of Etna.
Recent and Post-pliocene Volcanic Rocks.—I shall now select examples of contemporaneous volcanic rocks of successive geological periods, to show that igneous causes have been in activity in all past ages of the world. They have been perpetually shifting the places where they have broken out at the earth’s surface, and we can sometimes prove that those areas which are now the great theatres of volcanic action were in a state of perfect tranquillity at remote geological epochs, and that, on the other hand, in places where at former periods the most violent eruptions took place at the surface and continued for a great length of time, there has been an entire suspension of igneous action in historical times, and even, as in the British Isles, throughout a large part of the antecedent Tertiary Period.
In the absence of British examples of volcanic rocks newer than the Upper Miocene, I may state that in other parts of the world, especially in those where volcanic eruptions are now taking place from time to time, there are tuffs and lavas belonging to that part of the Tertiary era the antiquity of which is proved by the presence of the bones of extinct quadrupeds which co-existed with terrestrial, fresh-water, and marine mollusca of species still living. One portion of the lavas, tuffs, and trap-dikes of Etna, Vesuvius, and the island of Ischia has been produced within the historical era; another and a far more considerable part originated at times immediately antecedent, when the waters of the Mediterranean were already inhabited by the existing testacea, but when certain species of elephant, rhinoceros, and other quadrupeds now extinct, inhabited Europe.
Vesuvius.—I have traced in the “Principles of Geology” the history of the changes which the volcanic region of Campania is known to have undergone during the last 2000 years. The aggregate effect of igneous operations during that period is far from insignificant, comprising as it does the formation of the modern cone of Vesuvius since the year 79, and the production of several minor cones in Ischia, together with that of Monte Nuovo in the year 1538. Lava-currents have also flowed upon the land and along the bottom of the sea—volcanic sand, pumice, and scoriæ have been showered down so abundantly that whole cities were buried—tracts of the sea have been filled up or converted into shoals—and tufaceous sediment has been transported by rivers and land-floods to the sea. There are also proofs, during the same recent period, of a permanent alteration of the relative levels of the land and sea in several places, and of the same tract having, near Puzzuoli, been alternately upheaved and depressed to the amount of more than twenty feet. In connection with these convulsions, there are found, on the shores of the Bay of Baiæ, recent tufaceous strata, filled with articles fabricated by the hands of man, and mingled with marine shells.
It has also been stated ([p. 206]), that when we examine this same region, it is found to consist largely of tufaceous strata, of a date anterior to human history or tradition, which are of such thickness as to constitute hills from 500 to more than 2000 feet in height. Some of these strata contain marine shells which are exclusively of living species, others contain a slight mixture, one or two per cent of species not known as living.
The ancient part of Vesuvius is called Somma, and consists of the remains of an older cone which appears to have been partly destroyed by explosion. In the great escarpment which this remnant of the ancient mountain presents towards the modern cone of Vesuvius, there are many dikes which are for the most part vertical, and traverse the inclined beds of lava and scoriæ which were successively superimposed during those eruptions by which the old cone was formed. They project in relief several inches, or sometimes feet, from the face of the cliff, being extremely compact, and less destructible than the intersected tuffs and porous lavas. In vertical extent they vary from a few yards to 500 feet, and in breadth from one to twelve feet. Many of them cut all the inclined beds in the escarpment of Somma from top to bottom, others stop short before they ascend above halfway. In mineral composition they scarcely differ from the lavas of Somma, the rock consisting of a base of leucite and augite, through which large crystals of augite and some of leucite are scattered.
Nothing is more remarkable than the usual parallelism of the opposite sides of the dikes, which correspond almost as regularly as the two opposite faces of a wall of masonry. This character appears at first the more inexplicable, when we consider how jagged and uneven are the rents caused by earthquakes in masses of heterogeneous composition, like those composing the cone of Somma. In explanation of this phenomenon, M. Necker refers us to Sir W. Hamilton’s account of an eruption of Vesuvius in the year 1779, who records the following fact: “The lavas, when they either boiled over the crater, or broke out from the conical parts of the volcano, constantly formed channels as regular as if they had been cut by art down the steep part of the mountain; and whilst in a state of perfect fusion, continued their course in those channels, which were sometimes full to the brim, and at other times more or less so, according to the quantity of matter in motion.
”These channels (says the same observer), I have found, upon examination after an eruption, to be in general from two to five or six feet wide, and seven or eight feet deep. They were often hid from the sight by a quantity of scoriæ that had formed a crust over them; and the lava, having been conveyed in a covered way for some yards, came out fresh again into an open channel. After an eruption, I have walked in some of those subterraneous or covered galleries, which were exceedingly curious, the sides, top, and bottom being worn perfectly smooth and even in most parts by the violence of the currents of the red-hot lavas which they had conveyed for many weeks successively.” I was able to verify this phenomenon in 1858, when a stream of lava issued from a lateral cone.[[3]] Now, the walls of a vertical fissure, through which lava has ascended in its way to a volcanic vent, must have been exposed to the same erosion as the sides of the channels before adverted to. The prolonged and uniform friction of the heavy fluid, as it is forced and made to flow upward, cannot fail to wear and smooth down the surfaces on which it rubs, and the intense heat must melt all such masses as project and obstruct the passage of the incandescent fluid.
The rock composing the dikes both in the modern and ancient part of Vesuvius is far more compact than that of ordinary lava, for the pressure of a column of melted matter in a fissure greatly exceeds that in an ordinary stream of lava; and pressure checks the expansion of those gases which give rise to vesicles in lava. There is a tendency in almost all the Vesuvian dikes to divide into horizontal prisms, a phenomenon in accordance with the formation of vertical columns in horizontal beds of lava; for in both cases the divisions which give rise to the prismatic structure are at right angles to the cooling surfaces. (See [ p. 510].)
Auvergne.—Although the latest eruptions in central France seem to have long preceded the historical era, they are so modern as to have a very intimate connection with the present superficial outline of the country and with the existing valleys and river-courses. Among a great number of cones with perfect craters, one called the Puy de Tartaret sent forth a lava-current which can be traced up to its crater, and which flowed for a distance of thirteen miles along the bottom of the present valley to the village of Nechers, covering the alluvium of the old valley in which were preserved the bones of an extinct species of horse, and of a lagomys and other quadrupeds all closely allied to recent animals, while the associated land-shells were of species now living, such as Cyclostoma elegans, Helix hortensis, H. nemoralis, H. lapicida, and Clausilia rugosa. That the current which has issued from the Puy de Tartaret may, nevertheless, be very ancient in reference to the events of human history, we may conclude, not only from the divergence of the mammiferous fauna from that of our day, but from the fact that a Roman bridge of such form and construction as continued in use only down to the fifth century, but which may be older, is now seen at a place about a mile and a half from St. Nectaire. This ancient bridge spans the river Couze with two arches, each about fourteen feet wide. These arches spring from the lava of Tartaret, on both banks, showing that a ravine precisely like that now existing had already been excavated by the river through that lava thirteen or fourteen centuries ago.
While the river Couze has in most cases, as at the site of this ancient bridge, been simply able to cut a deep channel through the lava, the lower portion of which is shown to be columnar, the same torrent has in other places, where the valley was contracted to a narrow gorge, had power to remove the entire mass of basaltic rock, causing for a short space a complete breach of continuity in the volcanic current. The work of erosion has been very slow, as the basalt is tough and hard, and one column after another must have been undermined and reduced to pebbles, and then to sand. During the time required for this operation, the perishable cone of Tartaret, occupying the lowest part of the great valley descending from Mont Dor (see [p. 542]), and damming up the river so as to cause the Lake of Chambon, has stood uninjured, proving that no great flood or deluge can have passed over this region in the interval between the eruption of Tartaret and our own times.
Puy de Côme.—The Puy de Côme and its lava-current, near Clermont, may be mentioned as another minor volcano of about the same age. This conical hill rises from the granitic platform, at an angle of between 30° and 40°, to the height of more than 900 feet. Its summit presents two distinct craters, one of them with a vertical depth of 250 feet. A stream of lava takes its rise at the western base of the hill instead of issuing from either crater, and descends the granitic slope towards the present site of the town of Pont Gibaud. Thence it pours in a broad sheet down a steep declivity into the valley of the Sioule, filling the ancient river-channel for the distance of more than a mile. The Sioule, thus dispossessed of its bed, has worked out a fresh one between the lava and the granite of its western bank; and the excavation has disclosed, in one spot, a wall of columnar basalt about fifty feet high.[[4]]
The excavation of the ravine is still in progress, every winter some columns of basalt being undermined and carried down the channel of the river, and in the course of a few miles rolled to sand and pebbles. Meanwhile the cone of Côme remains unimpaired, its loose materials being protected by a dense vegetation, and the hill standing on a ridge not commanded by any higher ground, so that no floods of rain-water can descend upon it. There is no end to the waste which the hard basalt may undergo in future, if the physical geography of the country continue unchanged—no limit to the number of years during which the heap of incoherent and transportable materials called the Puy de Côme may remain in an almost stationary condition.
Puy de Pariou.—The brim of the crater of the Puy de Pariou, near Clermont, is so sharp, and has been so little blunted by time, that it scarcely affords room to stand upon. This and other cones in an equally remarkable state of integrity have stood, I conceive, uninjured, not in spite of their loose porous nature, as might at first be naturally supposed, but in consequence of it. No rills can collect where all the rain is instantly absorbed by the sand and scoriæ, as is remarkably the case on Etna; and nothing but a water-spout breaking directly upon the Puy de Pariou could carry away a portion of the hill, so long as it is not rent or ingulfed by earthquakes.
Newer Pliocene Volcanic Rocks.—The more ancient portion of Vesuvius and Etna originated at the close of the Newer Pliocene period, when less than ten, sometimes only one, in a hundred of the shells differed from those now living. In the case of Etna, it was before stated ([p. 205]) that Post-pliocene formations occur in the neighbourhood of Catania, while the oldest lavas of the great volcano are Pliocene. These last are seen associated with sedimentary deposits at Trezza and other places on the southern and eastern flanks of the great cone (see [p. 205]).
Cyclopean Islands.—The Cyclopean Islands, called by the Sicilians Dei Faraglioni, in the sea-cliffs of which these beds of clay, tuff, and associated lava are laid open to view, are situated in the Bay of Trezza, and may be regarded as the extremity of a promontory severed from the main land. Here numerous proofs are seen of submarine eruptions, by which the argillaceous and sandy strata were invaded and cut through, and tufaceous breccias formed. Inclosed in these breccias are many angular and hardened fragments of laminated clay in different states of alteration by heat, and intermixed with volcanic sands.
The loftiest of the Cyclopean islets, or rather rocks, is about 200 feet in height, the summit being formed of a mass of stratified clay, the laminæ of which are occasionally subdivided by thin arenaceous layers. These strata dip to the N.W., and rest on a mass of columnar lava (see Fig. 599) in which the tops of the pillars are weathered, and so rounded as to be often hemispherical.
In some places in the adjoining and largest islet of the group, which lies to the north-eastward of that represented in Figure 599), the overlying clay has been greatly altered and hardened by the igneous rock, and occasionally contorted in the most extraordinary manner; yet the lamination has not been obliterated, but, on the contrary, rendered much more conspicuous, by the indurating process.
In Fig. 600 I have represented a portion of the altered rock, a few feet square, where the alternating thin laminæ of sand and clay are contorted in a manner often observed in ancient metamorphic schists. A great fissure, running from east to west, nearly divides this larger island into two parts, and lays open its internal structure. In the section thus exhibited, a dike of lava is seen, first cutting through an older mass of lava, and then penetrating the superincumbent tertiary strata. In one place the lava ramifies and terminates in thin veins, from a few feet to a few inches in thickness (see Fig. 601). The arenaceous laminæ are much hardened at the point of contact, and the clays are converted into siliceous schist. In this island the altered rocks assume a honey-comb structure on their weathered surface, singularly contrasted with the smooth and even outline which the same beds present in their usual soft and yielding state. The pores of the lava are sometimes coated, or entirely filled with carbonate of lime, and with a zeolite resembling analcime, which has been called cyclopite. The latter mineral has also been found in small fissures traversing the altered marl, showing that the same cause which introduced the minerals into the cavities of the lava, whether we suppose sublimation or aqueous infiltration, conveyed it also into the open rents of the contiguous sedimentary strata.
Dikes of Palagonia.—Dikes of vesicular and amygdaloidal lava are also seen traversing marine tuff or peperino, west of Palagonia, some of the pores of the lava being empty, while others are filled with carbonate of lime. In such cases we may suppose the tuff to have resulted from showers of volcanic sand and scoriæ, together with fragments of limestone, thrown out by a submarine explosion, similar to that which gave rise to Graham Island in 1831. When the mass was, to a certain degree, consolidated, it may have been rent open, so that the lava ascended through fissures, the walls of which were perfectly even and parallel. In one case, after the melted matter that filled the rent (Fig. 602) had cooled down, it must have been fractured and shifted horizontally by a lateral movement.
In Fig. 603, the lava has more the appearance of a vein, which forced its way through the peperino. It is highly probable that similar appearances would be seen, if we could examine the floor of the sea in that part of the Mediterranean where the waves have recently washed away the new volcanic island; for when a superincumbent mass of ejected fragments has been removed by denudation, we may expect to see sections of dikes traversing tuff, or, in other words, sections of the channels of communication by which the subterranean lavas reached the surface.
Madeira.—Although the more ancient portion of the volcanic eruptions by which the island of Madeira and the neighbouring one of Porto Santo were built up occurred, as we shall presently see, in the Upper Miocene Period, a still larger part of the island is of Pliocene date. That the latest outbreaks belonged to the Newer Pliocene Period, I infer from the close affinity to the present flora of Madeira of the fossil plants preserved in a leaf-bed in the north-eastern part of the island. These fossils, associated with some lignite in the ravine of the river San Jorge, can none of them be proved to be of extinct species, but their antiquity may be inferred from the following considerations: Firstly—The leaf-bed, discovered by Mr. Hartung and myself in 1853, at the height of 1000 feet above the level of the sea, crops out at the base of a cliff formed by the erosion of a gorge cut through alternating layers of basalt and scoriæ, the product of a vast succession of eruptions of unknown date, piled up to a thickness of 1000 feet, and which were all poured out after the plants, of which about twenty species have been recognised, flourished in Madeira. These lavas are inclined at an angle of about 15° to the north, and came down from the great central region of eruption. Their accumulation implies a long period of intermittent volcanic action, subsequently to which the ravine of San Jorge was hollowed out. Secondly—Some few of the plants, though perhaps all of living species, are supposed to be of genera not now existing in the island. They have been described by Sir Charles Bunbury and Professor Heer, and the former first pointed out that many of the leaves are of the laurel type, and analogous to those now flourishing in the modern forests of Madeira. He also recognised among them the leaves of Woodwardia radicans, and Davallia Canariensis, ferns now abundant in Madeira. Thirdly—the great age of this leaf-bed of San Jorge, which was perhaps originally formed in the crater of some ancient volcanic cone afterwards buried under lava, is proved by its belonging to a part of the eastern extremity of Madeira, which, after the close of the igneous eruptions, became covered in the adjoining district of Caniçal with blown sand in which a vast number of land-shells were buried. These fossil shells belonged to no less than 36 species, among which are many now extremely rare in the island, and others, about five per cent, extinct or unknown in any part of the world. Several of these of the genus Helix are conspicuous from the peculiarity of their forms, others from their large dimensions. The geographical configuration of the country shows that this shell-bed is considerably more modern than the leaf-bed; it must therefore be referred to the Newer Pliocene, according to the definition of this period given in a former chapter ([p. 143]).
Older Pliocene Period.—Italy.—In Tuscany, as at Radicofani, Viterbo, and Aquapendente, and in the Campagna di Roma, submarine volcanic tuffs are interstratified with the Older Pliocene strata of the Sub-apennine hills in such a manner as to leave no doubt that they were the products of eruptions which occurred when the shelly marls and sands of the Sub-appenine hills were in the course of deposition. This opinion I expressed[[5]] after my visit to Italy in 1828 and it has recently (1850) been confirmed by the argument adduced by Sir R. Murchison in favour of the submarine origin of the tertiary volcanic rocks of Italy.[[6]] These rocks are well-known to rest conformably on the Sub-apennine marls, even as far south as Monte Mario, in the suburbs of Rome. On the exact age of the deposits of Monte Mario new light has recently been thrown by a careful study of their marine fossil shells, undertaken by MM. Rayneval, Van den Hecke, and Ponzi. They have compared no less than 160 species with the shells of the Coralline Crag of Suffolk, so well described by Mr. Searles Wood; and the specific agreement between the British and Italian fossils is so great, if we make due allowance for geographical distance and the difference of latitude, that we can have little hesitation in referring both to the same period, or to the Older Pliocene of this work. It is highly probable that, between the oldest trachytes of Tuscany and the newest rocks in the neighbourhood of Naples, a series of volcanic products might be detected of every age from the Older Pliocene to the historical epoch.
Pliocene Volcanoes of the Eifel.—Some of the most perfect cones and craters in Europe, not even excepting those of the district round Vesuvius, may be seen on the left or west bank of the Rhine, near Bonn and Andernach. They exhibit characters distinct from any which I have observed elsewhere, owing to the large part which the escape of aqueous vapour has played in the eruptions and the small quantities of lava emitted. The fundamental rocks of the district are grey and red sandstones and shales, with some associated limestones, replete with fossils of the Devonian or Old Red Sandstone group. The volcanoes broke out in the midst of these inclined strata, and when the present systems of hills and valleys had already been formed. The eruptions occurred sometimes at the bottom of deep valleys, sometimes on the summit of hills, and frequently on intervening platforms. In travelling through this district we often come upon them most unexpectedly, and may find ourselves on the very edge of a crater before we had been led to suspect that we were approaching the site of any igneous outburst. Thus, for example, on arriving at the village of Gemund, immediately south of Daun, we leave the stream, which flows at the bottom of a deep valley in which strata of sandstone and shale crop out. We then climb a steep hill, on the surface of which we see the edges of the same strata dipping inward towards the mountain. When we have ascended to a considerable height, we see fragments of scoriæ sparingly scattered over the surface; until at length, on reaching the summit, we find ourselves suddenly on the edge of a tarn, or deep circular lake-basin called the Gemunder Maar. In it we recognise the ordinary form of a crater, for which we have been prepared by the occurrence of scoriæ scattered over the surface of the soil. But on examining the walls of the crater we find precipices of sandstone and shale which exhibit no signs of the action of heat; and we look in vain for those beds of lava and scoriæ, dipping outward on every side, which we have been accustomed to consider as characteristic of volcanic vents. As we proceed, however, to the opposite side of the lake, we find a considerable quantity of scoriæ and some lava, and see the whole surface of the soil sparkling with volcanic sand, and strewed with ejected fragments of half-fused shale, which preserves its laminated texture in the interior, while it has a vitrified or scoriform coating.
Other crater lakes of circular or oval form, and hollowed out of similar ancient strata, occur in the Upper Eifel, where copious aëriform discharges have taken place, throwing out vast heaps of pulverized shale into the air. I know of no other extinct volcanoes where gaseous explosions of such magnitude have been attended by the emission of so small a quantity of lava. Yet I looked in vain in the Eifel for any appearances which could lend support to the hypothesis that the sudden rushing out of such enormous volumes of gas had ever lifted up the stratified rocks immediately around the vent so as to form conical masses, having their strata dipping outward on all sides from a central axis, as is assumed in the theory of elevation craters, alluded to in the last chapter.
I have already given ([Fig. 590]) an example in the Eifel of a small stream of lava which issued from one of the craters of that district at Bertrich-Baden. It shows that when some of these volcanoes were in action the valleys had already been eroded to their present depth.
![[Fig. 590]](#5745294564006620949_fig590.jpg.id-6277100809471894032.wrap-0.html.html){kind=link}
Trass.—The tufaceous alluvium called trass, which has covered large areas in the Eifel, and choked up some valleys now partially re-excavated, is unstratified. Its base consists almost entirely of pumice, in which are included fragments of basalt and other lavas, pieces of burnt shale, slate, and sandstone, and numerous trunks and branches of trees. If, as is probable, this trass was formed during the period of volcanic eruptions, it may have originated in the manner of the moya of the Andes.
We may easily conceive that a similar mass might now be produced, if a copious evolution of gases should occur in one of the lake-basins. If a breach should be made in the side of the cone, the flood would sweep away great heaps of ejected fragments of shale and sandstone, which would be borne down into the adjoining valleys. Forests might be torn up by such a flood, and thus the occurrence of the numerous trunks of trees dispersed irregularly through the trass can be explained. The manner in which this trass conforms to the shape of the present valleys implies its comparatively modern origin, probably not dating farther back than the Pliocene Period.
[1] Caldcleugh, Phil. Trans., 1836, p. 27.
[2] See Principles, Index, “Skaptar Jokul.”
[3] Principles of Geology, vol. i, p. 626.
[4] Scrope’s Central France, p. 60, and plate.
[5] See 1st edit. of Principles of Geology, vol. iii, chaps. xiii and xiv, 1833; and former editions of this work, chap. xxxi.
[6] Quart. Geol. Journ., vol. vi, p. 281.
CHAPTER XXX.
AGE OF VOLCANIC ROCKS—continued.
Volcanic Rocks of the Upper Miocene Period. — Madeira. — Grand Canary. — Azores. — Lower Miocene Volcanic Rocks. — Isle of Mull. — Staffa and Antrim. — The Eifel. — Upper and Lower Miocene Volcanic Rocks of Auvergne. — Hill of Gergovia. — Eocene Volcanic Rocks of Monte Bolca. — Trap of Cretaceous Period. — Oolitic Period. — Triassic Period. — Permian Period. — Carboniferous Period. — Erect Trees buried in Volcanic Ash in the Island of Arran. — Old Red Sandstone Period. — Silurian Period. — Cambrian Period. — Laurentian Volcanic Rocks.
Volcanic Rocks of the Upper Miocene Period.—Madeira.—The greater part of the volcanic eruptions of Madeira, as we have already seen ([p. 532]), belong to the Pliocene Period, but the most ancient of them are of Upper Miocene date, as shown by the fossil shells included in the marine tuffs which have been upraised at San Vicente, in the northern part of the island, to the height of 1300 feet above the level of the sea. A similar marine and volcanic formation constitutes the fundamental portion of the neighbouring island of Porto Santo, forty miles distant from Madeira, and is there elevated to an equal height, and covered, as in Madeira, with lavas of supra-marine origin.
The largest number of fossils have been collected from the tuffs and conglomerates and some beds of limestone in the island of Baixo, off the southern extremity of Porto Santo. They amount in this single locality to more than sixty in number, of which about fifty are mollusca, but many of these are only casts. Some of the shells probably lived on the spot during the intervals between eruptions, and some may have been cast up into the water or air together with muddy ejections, and, falling down again, have been deposited on the bottom of the sea. The hollows in some of the fragments of vesicular lava of which the breccias and conglomerates are composed are partially filled with calc-sinter, being thus half converted into amygdaloids. Among the fossil shells common to Madeira and Porto Santo, large cones, strombs, and cowries are conspicuous among the univalves, and Cardium, Spondylus, and Lithodomus among the lamellibranchiate bivalves, and among the Echinoderms the large Clypeaster called C. altus, an extinct European Miocene fossil.
The largest list of fossils has been published by Mr. Karl Meyer, in Hartung’s “Madeira;” but in the collection made by myself, and in a still larger one formed by Mr. J. Yate Johnson, several remarkable forms not in Meyer’s list occur, as, for example, Pholadomya, and a large Terebra. Mr. Johnson also found a fine specimen of Nautilus (Atruria) ziczac ([Fig. 211]), a well-known Falunian fossil of Europe; and in the same volcanic tuff of Baixo, the Echinoderm Brissus Scillæ, a living Mediterranean species, found fossil in the Miocene strata of Malta. Mr. Meyer identifies one-third of the Madeira shells with known European Miocene (or Falunian) forms. The huge Strombus of San Vicente and Porto Santo, S. Italicus, is an extinct shell of the Sub-apennine or Older Pliocene formations. The mollusca already obtained from various localities of Madeira and Porto Santo are not less than one hundred in number, and, according to the late Dr. S. P. Woodward, rather more than a third are of species still living, but many of these are not now inhabitants of the neighbouring sea.
![[Fig. 211]](#5745294564006620949_fig207.jpg.id-2479600440623120097.wrap-0.html.html){kind=link}
It has been remarked ([p. 212]), that in the Older Pliocene and Upper Miocene deposits of Europe many forms occur of a more southern aspect than those now inhabiting the nearest sea. In like manner the fossil corals, or Zoantharia, six in number, which I obtained from Madeira, of the genera Astræa, Sarcinula, Hydnophora, were pronounced by Mr. Lonsdale to be forms foreign to the adjacent coasts, and agreeing with the fauna of a sea warmer than that now separating Madeira from the nearest part of the African coast. We learn, indeed, from the observations made in 1859, by the Reverend R. T. Lowe, that more than one-half, or fifty-three in ninety, of the marine mollusks collected by him from the sandy beach of Mogador are common British species, although Mogador is 18½ degrees south of the nearest shores of England. The living shells of Madeira and Porto Santo are in like manner those of a temperate climate, although in great part differing specifically from those of Mogador.[[1]]
Grand Canary.—In the Canaries, especially in the Grand Canary, the same marine Upper Miocene formation is found. Stratified tuffs, with intercalated conglomerates and lavas, are there seen in nearly horizontal layers in sea-cliffs about 300 feet high, near Las Palmas. Mr. Hartung and I were unable to find marine shells in these tuffs at a greater elevation than 400 feet above the sea; but as the deposit to which they belong reaches to the height of 1100 feet or more in the interior, we conceive that an upheaval of at least that amount has taken place. The Clypeaster altus, Spondylus gæderopus, Pectunculus pilosus, Cardita calyculata, and several other shells, serve to identify this formation with that of the Madeiras, and Ancillaria glandiformis, which is not rare, and some other fossils, remind us of the faluns of Touraine.
The sixty-two Miocene species which I collected in the Grand Canary were referred by the late Dr. S. P. Woodward to forty-seven genera, ten of which are no longer represented in the neighbouring sea, namely Corbis, an African form, Hinnites, now living in Oregon, Thecidium (T. Mediterranean, identical with the Miocene fossil of St. Juvat, in Brittany), Calyptræa, Hipponyx, Nerita, Erato, Oliva, Ancillaria, and Fasciolaria.
These tuffs of the southern shores of the Grand Canary, containing the Upper Miocene shells, appear to be about the same age as the most ancient volcanic rocks of the island, composed of slaty diabase, phonolite, and trachyte. Over the marine lavas and tuffs trachytic and basaltic products of subaërial volcanic origin, between 4000 and 5000 feet in thickness, have been piled, the central parts of the Grand Canary reaching the height of about 6000 feet above the level of the sea. A large portion of this mass is of Pliocene date, and some of the latest lavas have been poured out since the time when the valleys were already excavated to within a few feet of their present depth.
On the whole, the rocks of the Grand Canary, an island of a nearly circular shape, and 6½ geographical miles diameter, exhibit proofs of a long series of eruptions beginning like those of Madeira, Porto Santo, and the Azores, in the Upper Miocene period, and continued to the Post-Pliocene. The building up of the Grand Canary by subaërial eruptions, several thousand feet thick, went on simultaneously with the gradual upheaval of the earliest products of submarine eruptions, in the same manner as the Pliocene marine strata of the oldest parts of Vesuvius and Etna have been upraised during eruptions of Post-tertiary date.
In proof that movements of elevation have actually continued down to Post-tertiary times, I may remark that I found raised beaches containing shells of the Recent Period in the Grand Canary, Teneriffe, and Porto Santo. The most remarkable raised beach which I observed in the Grand Canary, in the study of which I was assisted by Don Pedro Maffiotte, is situated in the north-eastern part of the island at San Catalina, about a quarter of a mile north of Las Palmas. It intervenes between the base of the high cliff formed of the tuffs with Miocene shells and the sea-shore. From this beach, at an elevation of twenty-five feet above high-water mark, and at a distance of about 150 feet from the present shore, I obtained more than fifty species of living marine shells. Many of them, according to Dr. S. P. Woodward, are no longer inhabitants of the contiguous sea, as, for example, Strombus bubonius, which is still living on the West Coast of Africa, and Cerithium procerum, found at Mozambique; others are Mediterranean species, as Pecten Jacobæus and P. polymorphus. Some of these testacea, such as Cardita squamosa, are inhabitants of deep water, and the deposit on the whole seems to indicate a depth of water exceeding a hundred feet.
Azores.—In the island of St. Mary’s, one of the Azores, marine fossil shells have long been known. They are found on the north-east coast on a small projecting promontory called Ponta do Papagaio (or Point-Parrot), chiefly in a limestone about twenty feet thick, which rests upon, and is again covered by, basaltic lavas, scoriæ, and conglomerates. The pebbles in the conglomerate are cemented together with carbonate of lime.
Mr. Hartung, in his account of the Azores, published in 1860, describes twenty-three shells from St. Mary’s,[[2]] of which eight perhaps are identical with living species, and twelve are with more or less certainty referred to European Tertiary forms, chiefly Upper Miocene. One of the most characteristic and abundant of the new species, Cardium Hartungi, not known as fossil in Europe, is very common in Porto Santo and Baixo, and serves to connect the Miocene fauna of the Azores and the Madeiras. In some of the Azores, as well as in the Canary islands, the volcanic fires are not yet extinct, as the recorded eruptions of Lanzerote, Teneriffe, Palma, St. Michael’s, and others, attest.
Lower Miocene Volcanic Rocks.—Isle of Mull and Antrim.—I may refer the reader to the account already given ([p. 247]) of leaf-beds at Ardtun, in the Isle of Mull in the Hebrides, which bear a relation to the associated volcanic rocks of Lower Miocene date analogous to that which the Madeira leaf-bed, above described ([p. 532]), bears to the Pliocene lavas of that island. Mr. Geikie has shown that the volcanic rocks in Mull are above 3000 feet in thickness. There seems little doubt that the well-known columnar basalt of Staffa, as well as that of Antrim in Ireland, are of the same age, and not of higher antiquity, as once suspected.
The Eifel.—A large portion of the volcanic rocks of the Lower Rhine and the Eifel are coeval with the Lower Miocene deposits to which most of the “Brown-Coal” of Germany belongs. The Tertiary strata of that age are seen on both sides of the Rhine, in the neighbourhood of Bonn, resting unconformably on highly inclined and vertical strata of Silurian and Devonian rocks. The Brown-Coal formation of that region consists of beds of loose sand, sandstone, and conglomerate, clay with nodules of clay-iron-stone, and occasionally silex. Layers of light brown and sometimes black lignite are interstratified with the clays and sands, and often irregularly diffused through them. They contain numerous impressions of leaves and stems of trees, and are extensively worked for fuel, whence the name of the formation. In several places layers of trachytic tuff are interstratified, and in these tuffs are leaves of plants identical with those found in the brown-coal, showing that, during the period of the accumulation of the latter, some volcanic products were ejected. The igneous rocks of the Westerwald, and of the mountains called the Siebengebirge, consist partly of basaltic and partly of trachytic lavas, the latter being in general the more ancient of the two. There are many varieties of trachyte, some of which are highly crystalline, resembling a coarse-grained granite, with large separate crystals of feldspar. Trachytic tuff is also very abundant.
M. Von Dechen, in his work on the Siebengebirge,[[3]] has given a copious list of the animal and vegetable remains of the fresh-water strata associated with the brown-coal of that part of Germany. Plants of the genera Flabellaria, Ceanothus, and Daphnogene, including D. cinnamomifolia ([Fig. 155]), occur in these beds, with nearly 150 other plants. The fishes of the brown-coal near Bonn are found in a bituminous shale, called paper-coal, from being divisible into extremely thin leaves. The individuals are very numerous; but they appear to belong to a small number of species, some of which were referred by Agassiz to the genera Leuciscus, Aspius, and Perca. The remains of frogs also, of extinct species, have been discovered in the paper-coal; and a complete series may be seen in the museum at Bonn, from the most imperfect state of the tadpole to that of the full-grown animal. With these a salamander, scarcely distinguishable from the recent species, has been found, and the remains of many insects.
![[Fig. 155]](#5745294564006620949_fig155.jpg.id-9150200522032816137.wrap-0.html.html){kind=link}
Upper and Lower Miocene Volcanic Rocks of Auvergne.—The extinct volcanoes of Auvergne and Cantal, in central France, seem to have commenced their eruptions in the Lower Miocene period, but to have been most active during the Upper Miocene and Pliocene eras. I have already alluded to the grand succession of events of which there is evidence in Auvergne since the last retreat of the sea (see [p. 527]).
The earliest monuments of the Tertiary Period in that region are lacustrine deposits of great thickness, in the lowest conglomerates of which are rounded pebbles of quartz, mica-schist, granite, and other non-volcanic rocks, without the slightest intermixture of igneous products. To these conglomerates succeed argillaceous and calcareous marls and limestones, containing Lower Miocene shells and bones of mammalia, the higher beds of which sometimes alternate with volcanic tuff of contemporaneous origin. After the filling up or drainage of the ancient lakes, huge piles of trachytic and basaltic rocks, with volcanic breccias, accumulated to a thickness of several thousand feet, and were superimposed upon granite, or the contiguous lacustrine strata. The greater portion of these igneous rocks appear to have originated during the Upper Miocene and Pliocene periods; and extinct quadrupeds of those eras, belonging to the genera Mastodon, Rhinoceros, and others, were buried in ashes and beds of alluvial sand and gravel, which owe their preservation to overspreading sheets of lava.
In Auvergne, the most ancient and conspicuous of the volcanic masses is Mont Dor, which rests immediately on the granitic rocks standing apart from the fresh-water strata. This great mountain rises suddenly to the height of several thousand feet above the surrounding platform, and retains the shape of a flattened and somewhat irregular cone, the slope of which is gradually lost in the high plain around. This cone is composed of layers of scoriæ, pumice-stones, and their fine detritus, with interposed beds of trachyte and basalt, which descend often in uninterrupted sheets until they reach and spread themselves round the base of the mountain.[[4]] Conglomerates, also, composed of angular and rounded fragments of igneous rocks, are observed to alternate with the above; and the various masses are seen to dip off from the central axis, and to lie parallel to the sloping flanks of the mountain. The summit of Mont Dor terminates in seven or eight rocky peaks, where no regular crater can now be traced, but where we may easily imagine one to have existed, which may have been shattered by earthquakes, and have suffered degradation by aqueous agents. Originally, perhaps, like the highest crater of Etna, it may have formed an insignificant feature in the great pile, and, like it, may frequently have been destroyed and renovated.
Respecting the age of the great mass of Mont Dor, we cannot come at present to any positive decision, because no organic remains have yet been found in the tuffs, except impressions of the leaves of trees of species not yet determined. It has already been stated ([p. 234]) that the earliest eruptions must have been posterior in origin to those grits and conglomerates of the fresh-water formation of the Limagne which contain no pebbles of volcanic rocks. But there is evidence at a few points, as in the hill of Gergovia, presently to be mentioned, that some eruptions took place before the great lakes were drained, while others occurred after the desiccation of those lakes, and when deep valleys had already been excavated through fresh-water strata.
The valley in which the cone of Tartaret, above-mentioned ([p. 527]), is situated affords an impressive monument of the very different dates at which the igneous eruptions of Auvergne have happened; for while the cone itself is of Post-Pliocene date, the valley is bounded by lofty precipices composed of sheets of ancient columnar trachyte and basalt, which once flowed from the summit of Mont Dor in some part of the Miocene period. These Miocene lavas had accumulated to a thickness of nearly 1000 feet before the ravine was cut down to the level of the river Couze, a river which was at length dammed up by the modern cone and the upper part of its course transformed into a lake.
Gergovia.—It has been supposed by some observers that there is an alternation of a contemporaneous sheet of lava with fresh-water strata in the hill of Gergovia, near Clermont. But this idea has arisen from the intrusion of the dike represented in Fig. 604, which has altered the green and white marls both above and below. Nevertheless, there is a real alternation of volcanic tuff with strata containing Lower Miocene fresh-water shells, among others a Melania allied to M. inquinata ([Fig. 217]), with a Melanopsis and a Unio; there can, therefore, be no doubt that in Auvergne some volcanic explosions took place before the drainage of the lakes, and at a time when the Lower Miocene species of animals and plants still flourished.
![[Fig. 217]](#5745294564006620949_fig216.jpg.id-8853080742304759309.wrap-0.html.html){kind=link}
Eocene Volcanic Rocks.—Monte Bolca.—The fissile limestone of Monte Bolca, near Verona, has for many centuries been celebrated in Italy for the number of perfect Ichthyolites which it contains. Agassiz has described no less than 133 species of fossil fish from this single deposit, and the multitude of individuals by which many of the species are represented is attested by the variety of specimens treasured up in the principal museums of Europe. They have been all obtained from quarries worked exclusively by lovers of natural history, for the sake of the fossils. Had the lithographic stone of Solenhofen, now regarded as so rich in fossils, been in like manner quarried solely for scientific objects, it would have remained almost a sealed book to palæontologists, so sparsely are the organic remains scattered through it. When I visited Monte Bolca, in company with Sir Roderick Murchison, in 1828, we ascertained that the fish-bearing beds were of Eocene date, containing well-known species of Nummulites, and that a long series of submarine volcanic eruptions, evidently contemporaneous, had produced beds of tuff, which are cut through by dikes of basalt. There is evidence here of a long series of submarine volcanic eruptions of Eocene date, and during some of them, as Sir R. Murchison has suggested, shoals of fish were probably destroyed by the evolution of heat, noxious gases, and tufaceous mud, just as happened when Graham’s Island was thrown up between Sicily and Africa in 1831, at which time the waters of the Mediterranean were seen to be charged with red mud, and covered with dead fish over a wide area.[[5]]
Associated with the marls and limestones of Monte Bolca are beds containing lignite and shale with numerous plants, which have been described by Unger and Massalongo, and referred by them to the Eocene period. I have already cited ([p. 263]) Professor Heer’s remark, that several of the species are common to Monte Bolca and the white clay of Alum Bay, a Middle Eocene deposit; and the same botanist dwells on the tropical character of the flora of Monte Bolca and its distinctness from the sub-tropical flora of the Lower Miocene of Switzerland and Italy, in which last there is a far more considerable mixture of forms of a temperate climate, such as the willow, poplar, birch, elm, and others. That scarcely any one of the Monte Bolca fish should have been found in any other locality in Europe, is a striking illustration of the extreme imperfection of the palæontological record. We are in the habit of imagining that our insight into the geology of the Eocene period is more than usually perfect, and we are certainly acquainted with an almost unbroken succession of assemblages of shells passing one into the other from the era of the Thanet sands to that of the Bembridge beds or Paris gypsum. The general dearth, therefore, of fish in the different members of the Eocene series, Upper, Middle, and Lower, might induce a hasty reasoner to conclude that there was a poverty of ichthyic forms during this period; but when a local accident, like the volcanic eruptions of Monte Bolca, occurs, proofs are suddenly revealed to us of the richness and variety of this great class of vertebrata in the Eocene sea. The number of genera of Monte Bolca fish is, according to Agassiz, no less than seventy-five, twenty of them peculiar to that locality, and only eight common to the antecedent Cretaceous period. No less than forty-seven out of the seventy-five genera make their appearance for the first time in the Monte Bolca rocks, none of them having been met with as yet in the antecedent formations. They form a great contrast to the fish of the secondary strata, as, with the exception of the Placoids, they are all Teleosteans, only one genus, Pycnodus, belonging to the order of Ganoids, which form, as before stated, the vast majority of the ichthyolites entombed in the secondary are Mesozoic rocks.
Cretaceous Period.—M. Virlet, in his account of the geology of the Morea, p. 205, has clearly shown that certain traps in Greece are of Cretaceous date; as those, for example, which alternate conformably with cretaceous limestone and greensand between Kastri and Damala, in the Morea. They consist in great part of diallage rocks and serpentine, and of an amygdaloid with calcareous kernels, and a base of serpentine. In certain parts of the Morea, the age of these volcanic rocks is established by the following proofs: first, the lithographic limestones of the Cretaceous era are cut through by trap, and then a conglomerate occurs, at Nauplia and other places, containing in its calcareous cement many well-known fossils of the chalk and greensand, together with pebbles formed of rolled pieces of the same serpentinous trap, which appear in the dikes above alluded to.
Period of Oolite and Lias.—Although the green and serpentinous trap-rocks of the Morea belong chiefly to the Cretaceous era, as before mentioned, yet it seems that some eruptions of similar rocks began during the Oolitic period;[[6]] and it is probable that a large part of the trappean masses, called ophiolites in the Apennines, and associated with the limestone of that chain, are of corresponding age.
Trap of the New Red Sandstone Period.—In the southern part of Devonshire, trappean rocks are associated with New Red Sandstone, and, according to Sir H. De la Beche, have not been intruded subsequently into the sandstone, but were produced by contemporaneous volcanic action. Some beds of grit, mingled with ordinary red marl, resemble sands ejected from a crater; and in the stratified conglomerates occurring near Tiverton are many angular fragments of trap porphyry, some of them one or two tons in weight, intermingled with pebbles of other rocks. These angular fragments were probably thrown out from volcanic vents, and fell upon sedimentary matter then in the course of deposition.[[7]]
Trap of the Permian Period.—The recent investigations of Mr. Archibald Geikie in Ayrshire have shown that some of the volcanic rocks in that county are of Permian age, and it appears highly probable that the uppermost portion of Arthur’s Seat in the suburbs of Edinburgh marks the site of an eruption of the same era.
Trap of the Carboniferous Period.—Two classes of contemporaneous trap-rocks occur in the coal-field of the Forth, in Scotland. The newest of these, connected with the higher series of coal-measures, is well exhibited along the shores of the Forth, in Fifeshire, where they consist of basalt with olivine, amygdaloid, greenstone, wacke, and tuff. They appear to have been erupted while the sedimentary strata were in a horizontal position, and to have suffered the same dislocations which those strata have subsequently undergone. In the volcanic tuffs of this age are found not only fragments of limestone, shale, flinty slate, and sandstone, but also pieces of coal. The other or older class of carboniferous traps are traced along the south margin of Stratheden, and constitute a ridge parallel with the Ochils, and extending from Stirling to near St. Andrews. They consist almost exclusively of greenstone, becoming, in a few instances, earthy and amygdaloidal. They are regularly interstratified with the sandstone, shale, and iron-stone of the lower coal-measures, and, on the East Lomond, with Mountain Limestone. I examined these trap-rocks in 1838, in the cliffs south of St. Andrews, where they consist in great part of stratified tuffs, which are curved, vertical, and contorted, like the associated coal-measures. In the tuff I found fragments of carboniferous shale and limestone, and intersecting veins of greenstone.
Fife—Flisk Dike.—A trap dike was pointed out to me by Dr. Fleming, in the parish of Flisk, in the northern part of the county of Fife, which cuts through the grey sandstone and shale, forming the lowest part of the Old Red Sandstone, but which may probably be of carboniferous date. It may be traced for many miles, passing through the amygdaloidal and other traps of the hill called Norman’s Law in that parish. In its course it affords a good exemplification of the passage from the trappean into the Plutonic, or highly crystalline texture. Professor Gustavus Rose, to whom I submitted specimens of this dike, found it to be dolerite, and composed of greenish black augite and Labrador feldspar, the latter being the most abundant ingredient. A small quantity of magnetic iron, perhaps titaniferous, is also present. The result of this analysis is interesting, because both the ancient and modern lavas of Etna consist in like manner of augite, Labradorite, and titaniferous iron.
Erect Trees buried in Volcanic Ash at Arran.—An interesting discovery was made in 1867 by Mr. E. A. Wünsch in the carboniferous strata of the north-eastern part of the island of Arran. In the sea-cliff about five miles north of Corrie, near the village of Laggan, strata of volcanic ash occur, forming a solid rock cemented by carbonate of lime and enveloping trunks of trees, determined by Mr. Binney to belong to the genera Sigillaria and Lepidodendron. Some of these trees are at right angles to the planes of stratification, while others are prostrate and accompanied by leaves and fruits of the same genera. I visited the spot in company with Mr. Wünsch in 1870, and saw that the trees with their roots, of which about fourteen had been observed, occur at two distinct levels in volcanic tuffs parallel to each other, and inclined at an angle of about 40°, having between them beds of shale and coaly matter seven feet thick. It is evident that the trees were overwhelmed by a shower of ashes from some neighbouring volcanic vent, as Pompeii was buried by matter ejected from Vesuvius. The trunks, several of them from three to five feet in circumference, remained with their Stigmarian roots spreading through the stratum below, which had served as a soil. The trees must have continued for years in an upright position after they were killed by the shower of burning ashes, giving time for a partial decay of the interior, so as to afford hollow cylinders into which the spores of plants were wafted. These spores germinated and grew, until finally their stems were petrified by carbonate of lime like some of the remaining portions of the wood of the containing Sigillaria. Mr. Carruthers has discovered that sometimes the plants which had thus grown and become fossil in the inside of a single trunk belonged to several distinct genera. The fact that the tree-bearing deposits now dip at an angle of 40° is the more striking, as they must clearly have remained horizontal and undisturbed during a long period of intermittent and contemporaneous volcanic action.
In some of the associated carboniferous shales, ferns and calamites occur, and all the phenomena of the successive buried forests remind us of the sections in [ pp. 410 and 411] of the Nova Scotia coal-measures, with this difference only, that in the case of the South Joggins the fossilisation of the trees was effected without the eruption of volcanic matter.
Trap of the Old Red Sandstone Period.—By referring to the section explanatory of the structure of Forfarshire, already given ([p. 74]), the reader will perceive that beds of conglomerate, No. 3, occur in the middle of the Old Red Sandstone system, 1, 2, 3, 4. The pebbles in these conglomerates are sometimes composed of granitic and quartzose rocks, sometimes exclusively of different varieties of trap, which last, although purposely omitted in the section referred to, is often found either intruding itself in amorphous masses and dikes into the old fossiliferous tilestones, No. 4, or alternating with them in conformable beds. All the different divisions of the red sandstone, 1, 2, 3, 4, are occasionally intersected by dikes, but they are very rare in Nos. 1 and 2, the upper members of the group consisting of red shale and red sandstone. These phenomena, which occur at the foot of the Grampians, are repeated in the Sidlaw Hills; and it appears that in this part of Scotland volcanic eruptions were most frequent in the earlier part of the Old Red Sandstone period. The trap-rocks alluded to consist chiefly of feldspathic porphyry and amygdaloid, the kernels of the latter being sometimes calcareous, often chalcedonic, and forming beautiful agates. We meet also with claystone, greenstone, compact feldspar, and tuff. Some of these rocks look as if they had flowed as lavas over the bottom of the sea, and enveloped quartz pebbles which were lying there, so as to form conglomerates with a base of greenstone, as is seen in Lumley Den, in the Sidlaw Hills. On either side of the axis of this chain of hills (see [Fig. 55]), the beds of massive trap, and the tuffs composed of volcanic sand and ashes, dip regularly to the south-east or north-west, conformably with the shales and sandstones.
![[Fig. 55]](#5745294564006620949_fig55.jpg.id-5089162200245549702.wrap-0.html.html){kind=link}
But the geological structure of the Pentland Hills, near Edinburgh, shows that igneous rocks were there formed during the newer part of the Devonian or “Old Red” period. These hills are 1900 feet high above the sea, and consist of conglomerates and sandstones of Upper Devonian age, resting on the inclined edges of grits and slates of Lower Devonian and Upper Silurian date. The contemporaneous volcanic rocks intercalated in this Upper Old Red consist of feldspathic lavas, or feldstones, with associated tuffs or ashy beds. The lavas were some of them originally compact, others vesicular, and these last have been converted into amygdaloids. They consist chiefly of feldstone or compact feldspar. The Pentland Hills, say Messrs. Maclaren and Geikie, afford evidence that at the time of the Upper Old Red Sandstone, the district to the south-west of Edinburgh was for a long while the seat of a powerful volcano, which sent out massive streams of lava and showers of ash, and continued active until well-nigh the dawn of the Carboniferous period.[[8]]
Silurian Volcanic Rocks.—It appears from the investigations of Sir R. Murchison in Shropshire, that when the Lower Silurian strata of that country were accumulating, there were frequent volcanic eruptions beneath the sea; and the ashes and scoriæ then ejected gave rise to a peculiar kind of tufaceous sandstone or grit, dissimilar to the other rocks of the Silurian series, and only observable in places where syenitic and other trap-rocks protrude. These tuffs occur on the flanks of the Wrekin and Caer Caradoc, and contain Silurian fossils, such as casts of encrinites, trilobites, and mollusca. Although fossiliferous, the stone resembles a sandy claystone of the trap family.[[9]]
Thin layers of trap, only a few inches thick, alternate in some parts of Shropshire and Montgomeryshire with sedimentary strata of the Lower Silurian system. This trap consists of slaty porphyry and granular feldspar rock, the beds being traversed by joints like those in the associated sandstone, limestone, and shale, and having the same strike and dip.[[10]]
In Radnorshire there is an example of twelve bands of stratified trap, alternating with Silurian schists and flagstones, in a thickness of 350 feet. The bedded traps consist of feldspar porphyry, and other varieties; and the interposed Llandeilo flags are of sandstone and shale, with trilobites and graptolites.[[11]]
The Snowdonian hills in Carnarvonshire consist in great part of volcanic tuffs, the oldest of which are interstratified with the Bala and Llandeilo beds. There are some contemporaneous feldspathic lavas of this era, which, says Professor Ramsay, alter the slates on which they repose, having doubtless been poured out over them, in a melted state, whereas the slates which overlie them having been subsequently deposited after the lava had cooled and consolidated, have entirely escaped alteration. But there are greenstones associated with the same formation, which, although they are often conformable to the slates, are in reality intrusive rocks. They alter the stratified deposits both above and below them, and when traced to great distances are sometimes seen to cut through the slates, and to send off branches. Nevertheless, these greenstones appear to belong, like the lavas, to the Lower Silurian period.
Cambrian Volcanic Rocks.—The Lingula beds in North Wales have been described as 5000 feet in thickness. In the upper portion of these deposits volcanic tuffs or ashy materials are interstratified with ordinary muddy sediment, and here and there associated with thick beds of feldspathic lava. These rocks form the mountains called the Arans and the Arenigs; numerous greenstones are associated with them, which are intrusive, although they often run in the lines of bedding for a space. “Much of the ash,” says Professor Ramsay, “seems to have been subaërial. Islands, like Graham’s Island, may have sometimes raised their craters for various periods above the water, and by the waste of such islands some of the ashy matter became waterworn, whence the ashy conglomerate. Viscous matter seems also to have been shot into the air as volcanic bombs, which fell among the dust and broken crystals (that often form the ashes) before perfect cooling and consolidation had taken place.”[[12]]
Laurentian Volcanic Rocks.—The Laurentian rocks in Canada, especially in Ottawa and Argenteuil, are the oldest intrusive masses yet known. They form a set of dikes of a fine-grained dark greenstone or dolerite, composed of feldspar and pyroxene, with occasional scales of mica and grains of pyrites. Their width varies from a few feet to a hundred yards, and they have a columnar structure, the columns being truly at right angles to the plane of the dike. Some of the dikes send off branches. These dolerites are cut through by intrusive syenite, and this syenite, in its turn, is again cut and penetrated by feldspar porphyry, the base of which consists of petrosilex, or a mixture of orthoclase and quartz. All these trap-rocks appear to be of Laurentian date, as the Cambrian and Huronian rocks rest unconformably upon them.[[13]] Whether some of the various conformable crystalline rocks of the Laurentian series, such as the coarse-grained granitoid and porphyritic varieties of gneiss, exhibiting scarcely any signs of stratification, and some of the serpentines, may not also be of volcanic origin, is a point very difficult to determine in a region which has undergone so much metamorphic action.
[1] Linnean Proceedings; Zoology, 1860.
[2] Hartung, Die Azoren, 1860; also Insel Gran Canaria, Madeira und Porto Santo, 1864, Leipsig.
[3] Geognost. Beschreib. des Siebengebirges am Rhein. Bonn, 1852.
[4] Scrope’s Central France, p. 98.
[5] Principles of Geology, chap. xxvi, 9th ed., p. 432.
[6] Boblaye and Virlet, Morea, p. 23.
[7] De la Beche, Geol. Proceedings, vol. ii, p. 198.
[8] Maclaren, Geology of Fife and Lothians. Geikie, Trans. Royal Soc. Edinburgh, 1860-1861.
[9] Murchison, Silurian System, etc., p. 230.
[10] Ibid., p. 212.
[11] Murchison, Silurian System, etc., p. 325.
[12] Quart. Geol. Journ., vol. ix, p. 170, 1852.
[13] Logan, Geology of Canada, 1863.
CHAPTER XXXI.
PLUTONIC ROCKS.
General Aspect of Plutonic Rocks. — Granite and its Varieties. — Decomposing into Spherical Masses. — Rude columnar Structure. — Graphic Granite. — Mutual Penetration of Crystals of Quartz and Feldspar. — Glass Cavities in Quartz of Granite. — Porphyritic, talcose, and syenitic Granite. — Schorlrock and Eurite. — Syenite. — Connection of the Granites and Syenites with the Volcanic Rocks. — Analogy in Composition of Trachyte and Granite. — Granite Veins in Glen Tilt, Cape of Good Hope, and Cornwall. — Metalliferous Veins in Strata near their Junction with Granite. — Quartz Veins. — Exposure of Plutonic Rocks at the surface due to Denudation.
The Plutonic rocks may be treated of next in order, as they are most nearly allied to the volcanic class already considered. I have described, in the first chapter, these Plutonic rocks as the unstratified division of the crystalline or hypogene formations, and have stated that they differ from the volcanic rocks, not only by their more crystalline texture, but also by the absence of tuffs and breccias, which are the products of eruptions at the earth’s surface, whether thrown up into the air or the sea. They differ also by the absence of pores or cellular cavities, to which the expansion of the entangled gases gives rise in ordinary lava, never being scoriaceous or amygdaloidal, and never forming a porphyry with an uncrystalline base, nor alternating with tuffs.
From these and other peculiarities it has been inferred that the granites have been formed at considerable depths in the earth, and have cooled and crystallised slowly under great pressure, where the contained gases could not expand. The volcanic rocks, on the contrary, although they also have risen up from below, have cooled from a melted state more rapidly upon or near the surface. From this hypothesis of the great depth at which the granites originated, has been derived the name of “Plutonic rocks.” The beginner will easily conceive that the influence of subterranean heat may extend downward from the crater of every active volcano to a great depth below, perhaps several miles or leagues, and the effects which are produced deep in the bowels of the earth may, or rather must, be distinct; so that volcanic and Plutonic rocks, each different in texture, and sometimes even in composition, may originate simultaneously, the one at the surface, the other far beneath it. The Plutonic formations also agree with the volcanic in having veins or ramifications proceeding from central masses into the adjoining rocks, and causing alterations in these last, which will be presently described. They also resemble trap in containing no organic remains; but they differ in being more uniform in texture, whole mountain masses of indefinite extent appearing to have originated under conditions precisely similar.
The two principal members of the Plutonic family of rocks are Granite and Syenite, each of which, with their varieties, bear very much the same relation to each other as the trachytes bear to the basalts. Granite is a compound of feldspar, quartz, and mica, the feldspars being rich in silica, which forms from 60 to 70 per cent of the whole aggregate. In Syenite quartz is rare or wanting, hornblende taking the place of mica, and the proportion of silica not exceeding 50 to 60 per cent.
Granite and its Varieties.—Granite often preserves a very uniform character throughout a wide range of territory, forming hills of a peculiar rounded form, usually clad with a scanty vegetation. The surface of the rock is for the most part in a crumbling state, and the hills are often surmounted by piles of stones like the remains of a stratified mass, as in Figure 605, and sometimes like heaps of boulders, for which they have been mistaken. The exterior of these stones, originally quadrangular, acquires a rounded form by the action of air and water, for the edges and angles waste away more rapidly than the sides. A similar spherical structure has already been described as characteristic of basalt and other volcanic formations, and it must be referred to analogous causes, as yet but imperfectly understood. Although it is the general peculiarity of granite to assume no definite shapes, it is nevertheless occasionally subdivided by fissures, so as to assume a cuboidal, and even a columnar, structure. Examples of these appearances may be seen near the Land’s End, in Cornwall. (See Fig. 606.)
Feldspar, quartz, and mica are usually considered as the minerals essential to granite, the feldspar being most abundant in quantity, and the proportion of quartz exceeding that of mica. These minerals are united in what is termed a confused crystallisation; that is to say, there is no regular arrangement of the crystals in granite, as in gneiss (see [Fig. 622]), except in the variety termed graphic granite, which occurs mostly in granitic veins. This variety is a compound of feldspar and quartz, so arranged as to produce an imperfect laminar structure. The crystals of feldspar appear to have been first formed, leaving between them the space now occupied by the darker-coloured quartz. This mineral, when a section is made at right angles to the alternate plates of feldspar and quartz, presents broken lines, which have been compared to Hebrew characters. (See Fig. 608.) The variety of granite called by the French Pegmatite, which is a mixture of quartz and common feldspar, usually with some small admixture of white silvery mica, often passes into graphic granite.
![[Fig. 622]](#5745294564006620949_fig622.jpg.id-7042463040966383067.wrap-0.html.html){kind=link}
Ordinary granite, as well as syenite and eurite, usually contains two kinds of feldspar: First, the common, or orthoclase, in which potash is the prevailing alkali, and this generally occurs in large crystals of a white or flesh colour; and secondly, feldspar in smaller crystals, in which soda predominates, usually of a dead white or spotted, and striated like albite, but not the same in composition.[[1]]
As a general rule, quartz, in a compact or amorphous state, forms a vitreous mass, serving as the base in which feldspar and mica have crystallised; for although these minerals are much more fusible than silex, they have often imprinted their shapes upon the quartz. This fact, apparently so paradoxical, has given rise to much ingenious speculation. We should naturally have anticipated that, during the cooling of the mass, the flinty portion would be the first to consolidate; and that the different varieties of feldspar, as well as garnets and tourmalines, being more easily liquefied by heat, would be the last. Precisely the reverse has taken place in the passage of most granite aggregates from a fluid to a solid state, crystals of the more fusible minerals being found enveloped in hard, transparent, glassy quartz, which has often taken very faithful casts of each, so as to preserve even the microscopically minute striations on the surface of prisms of tourmaline. Various explanations of this phenomenon have been proposed by MM. de Beaumont, Fournet, and Durocher. They refer to M. Gaudin’s experiments on the fusion of quartz, which show that silex, as it cools, has the property of remaining in a viscous state, whereas alumina never does. This “gelatinous flint” is supposed to retain a considerable degree of plasticity long after the granitic mixture has acquired a low temperature. Occasionally we find the quartz and feldspar mutually imprinting their forms on each other, affording evidence of the simultaneous crystallisation of both.[[2]]
According to the experiments and observations of Gustavus Rose, the quartz of granite has the specific gravity of 2·6, which characterises silica when it is precipitated from a liquid solvent, and not that inferior density, namely, 2·3, which belongs to it when it cools in the laboratory from a state of fusion in what is called the dry way. By some it had been rashly inferred that the manner in which the consolidation of granite takes place is exceedingly different from the cooling of lavas, and that the intense heat supposed to be necessary for the production of mountain masses of Plutonic rocks might be dispensed with. But Mr. David Forbes informs me that silica can crystallise in the dry way, and he has found in quartz forming a constituent part of some trachytes, both from Guadeloupe and Iceland, glass cavities quite similar to those met with in genuine volcanic minerals.
These “glass cavities,” which with many other kindred phenomena have been carefully studied by Mr. Sorby, are those in which a liquid, on cooling, has become first viscous and then solid without crystallising or undergoing a definite change in its physical structure. Other cavities which, like those just mentioned, are frequently discernible under the microscope in the minerals composing granitic rocks, are filled, some of them with gas or vapour, others with liquid, and by the movements of the bubbles thus included the distinctness of such cavities from those filled with a glassy substance can be tested. Mr. Sorby admits that the frequent occurrence of fluid cavities in the quartz of granite implies that water was almost always present in the formation of this rock; but the same may be said of almost all lavas, and it is now more than forty years since Mr. Scrope insisted on the important part which water plays in volcanic eruptions, being so intimately mixed up with the materials of the lava that he supposed it to aid in giving mobility to the fluid mass. It is well known that steam escapes for months, sometimes for years, from the cavities of lava when it is cooling and consolidating. As to the result of Mr. Sorby’s experiments and speculations on this difficult subject, they may be stated in a few words. He concludes that the physical conditions under which the volcanic and granitic rocks originate are so far similar that in both cases they combine igneous fusion, aqueous solution, and gaseous sublimation—the proof, he says, of the operation of water in the formation of granite being quite as strong as of that of heat.[[3]]
When rocks are melted at great depths water must be present, for two reasons—First, because rainwater and seawater are always descending through fissured and porous rocks, and must at length find their way into the regions of subterranean heat; and secondly, because in a state of combination water enters largely into the composition of some of the most common minerals, especially those of the aluminous class. But the existence of water under great pressure affords no argument against our attributing an excessively high temperature to the mass with which it is mixed up. Bunsen, indeed, imagines that in Iceland water attains a white heat at a very moderate depth. To what extent some of the metamorphic rocks containing the same minerals as the granites may have been formed by hydrothermal action without the intervention of intense heat comparable to that brought into play in a volcanic eruption, will be considered when we treat of the metamorphic rocks in the thirty-third chapter.
Porphyritic Granite.—This name has been sometimes given to that variety in which large crystals of common feldspar, sometimes more than three inches in length, are scattered through an ordinary base of granite. An example of this texture may be seen in the granite of the Land’s End, in Cornwall (Fig. 609). The two larger prismatic crystals in this drawing represent feldspar, smaller crystals of which are also seen, similar in form, scattered through the base. In this base also appear black specks of mica, the crystals of which have a more or less perfect hexagonal outline. The remainder of the mass is quartz, the translucency of which is strongly contrasted to the opaqueness of the white feldspar and black mica. But neither the transparency of the quartz nor the silvery lustre of the mica can be expressed in the engraving.
The uniform mineral character of large masses of granite seems to indicate that large quantities of the component elements were thoroughly mixed up together, and then crystallised under precisely similar conditions. There are, however, many accidental, or “occasional,” minerals, as they are termed, which belong to granite. Among these black schorl or tourmaline, actinolite, zircon, garnet, and fluor spar are not uncommon; but they are too sparingly dispersed to modify the general aspect of the rock. They show, nevertheless, that the ingredients were not everywhere exactly the same; and a still greater difference may be traced in the ever-varying proportions of the feldspar, quartz, and mica.
Talcose Granite, or Protogine of the French, is a mixture of feldspar, quartz, and talc. It abounds in the Alps, and in some parts of Cornwall, producing by its decomposition the kaolin or china clay, more than 12,000 tons of which are annually exported from that country for the potteries.
Schorl-rock, and Schorly Granite.—The former of these is an aggregate of schorl, or tourmaline, and quartz. When feldspar and mica are also present, it may be called schorly granite. This kind of granite is comparatively rare.
Eurite, Feldstone.—Eurite is a rock in which the ingredients of granite are blended into a finely granular mass, mica being usually absent, and, when present, in such minute flakes as to be invisible to the naked eye. It is sometimes called Feldstone, and when the crystals of feldspar are conspicuous it becomes Feldspar porphyry. All these and other varieties of granite pass into certain kinds of trap—a circumstance which affords one of many arguments in favour of what is now the prevailing opinion, that the granites are also of igneous origin. The contrast of the most crystalline form of granite to that of the most common and earthy trap is undoubtedly great; but each member of the volcanic class is capable of becoming porphyritic, and the base of the porphyry may be more and more crystalline, until the mass passes to the kind of granite most nearly allied in mineral composition.
Syenitic Granite.—The quadruple compound of quartz, feldspar, mica, and hornblende, may be termed Syenitic Granite, and forms a passage between the granites and the syenites. This rock occurs in Scotland and in Guernsey.
Syenite.—We now come to the second division of the Plutonic rocks, or those having less than 60 per cent of silica, and which, as before stated (p. 552), are usually called syenitic. Syenite originally received its name from the celebrated ancient quarries of Syene, in Egypt. It differs from granite in having hornblende as a substitute for mica, and being without quartz. Werner at least considered syenite as a binary compound of feldspar and hornblende, and regarded quartz as merely one of its occasional minerals.
Miascite.—Miascite is one of the varieties of syenite most frequently spoken of; it is composed chiefly of orthoclase and nepheline, with hornblende and quartz as occasional accessory minerals. It derives its name from Miask, in the Ural Mountains, where it was first discovered by Gustavus Rose. Zircon-syenite is another variety closely allied to Miascite, but containing crystals of Zircon.
Connection of the Granites and Syenites with the Volcanic Rocks.—The minerals which constitute alike the Plutonic and volcanic rocks consist, almost exclusively, of seven elements, namely, silica, alumina, magnesia, lime, soda, potash, and iron (see Table [p. 449]); and these may sometimes exist in about the same proportions in a porous lava, a compact trap, and a crystalline granite. The same lava, for example, may be glassy, or scoriaceous, or stony, or porphyritic, according to the more or less rapid rate at which it cools.
It would be easy to multiply examples and authorities to prove the gradation of the Plutonic into the trap rocks. On the western side of the Fiord of Christiania, in Norway, there is a large district of trap, chiefly greenstone-porphyry and syenitic-greenstone, resting on fossiliferous strata. To this, on its southern limit, succeeds a region equally extensive of syenite, the passage from the trappean to the crystalline Plutonic rock being so gradual that it is impossible to draw a line of demarkation between them.
“The ordinary granite of Aberdeenshire,” says Dr. MacCulloch, “is the usual ternary compound of quartz, feldspar, and mica; though sometimes hornblende is substituted for the mica. But in many places a variety occurs which is composed simply of feldspar and hornblende; and in examining more minutely this duplicate compound, it is observed in some places to assume a fine grain, and at length to become undistinguishable from the greenstones of the trap family. It also passes in the same uninterrupted manner into a basalt, and at length into a soft claystone, with a schistose tendency on exposure, in no respect differing from those of the trap islands of the western coast.” The same author mentions, that in Shetland a granite composed of hornblende, mica, feldspar, and quartz graduates in an equally perfect manner into basalt.[[4]] In Hungary there are varieties of trachyte, which, geologically speaking, are of modern origin, in which crystals, not only of mica, but of quartz, are common, together with feldspar and hornblende. It is easy to conceive how such volcanic masses may, at a certain depth from the surface, pass downward into granite.
Granitic Veins.—I have already hinted at the close analogy in the forms of certain granitic and trappean veins; and it will be found that strata penetrated by Plutonic rocks have suffered changes very similar to those exhibited near the contact of volcanic dikes. Thus, in Glen Tilt, in Scotland, alternating strata of limestone and argillaceous schist come in contact with a mass of granite. The contact does not take place as might have been looked for if the granite had been formed there before the strata were deposited, in which case the section would have appeared as in Fig. 610; but the union is as represented in Fig. 611, the undulating outline of the granite intersecting different strata, and occasionally intruding itself in torturous veins into the beds of clay-slate and limestone, from which it differs so remarkably in composition. The limestone is sometimes changed in character by the proximity of the granitic mass or its veins, and acquires a more compact texture, like that of hornstone or chert, with a splintery fracture, and effervescing freely with acids.
Fig. 610 and Fig. 611: Junction of granite and arbillaceous schist in Glen Tilt. (MacCulloch.)[[5]]
The conversion of the limestone and these and many other instances into a siliceous rock, effervescing slowly with acids, would be difficult of explanation, were it not ascertained that such limestones are always impure, containing grains of quartz, mica, or feldspar disseminated through them. The elements of these minerals, when the rock has been subjected to great heat, may have been fused, and so spread more uniformly through the whole mass.
In the Plutonic, as in the volcanic rocks, there is every gradation from a tortuous vein to the most regular form of a dike, such as intersect the tuffs and lavas of Vesuvius and Etna. Dikes of granite may be seen, among other places, on the southern flank of Mount Battock, one of the Grampians, the opposite walls sometimes preserving an exact parallelism for a considerable distance. As a general rule, however, granite veins in all quarters of the globe are more sinuous in their course than those of trap. They present similar shapes at the most northern point of Scotland, and the southernmost extremity of Africa, as Figs. 612 and 613 will show.
Fig. 612: Granite veins traversing clay slate, Table Mountain, Cape of Good Hope.[[6]]
Fig. 613: Granite veins traversing gneiss, Cape Wrath.[[7]]
It is not uncommon for one set of granite veins to intersect another; and sometimes there are three sets, as in the environs of Heidelberg, where the granite on the banks of the river Necker is seen to consist of three varieties, differing in colour, grain, and various peculiarities of mineral composition. One of these, which is evidently the second in age, is seen to cut through an older granite; and another, still newer, traverses both the second and the first. In Shetland there are two kinds of granite. One of them, composed of hornblende, mica, feldspar, and quartz, is of a dark colour, and is seen underlying gneiss. The other is a red granite, which penetrates the dark variety everywhere in veins.[[8]]
Fig. 614 is a sketch of a group of granite veins in Cornwall, given by Messrs. Von Oeynhausen and Von Dechen.[[9]] The main body of the granite here is of a porphyritic appearance, with large crystals of feldspar; but in the veins it is fine-grained, and without these large crystals. The general height of the veins is from 16 to 20 feet, but some are much higher.
Granite, syenite, and those porphyries which have a granitiform structure, in short all Plutonic rocks, are frequently observed to contain metals, at or near their junction with stratified formations. On the other hand, the veins which traverse stratified rocks are, as a general law, more metalliferous near such junctions than in other positions. Hence it has been inferred that these metals may have been spread in a gaseous form through the fused mass, and that the contact of another rock, in a different state of temperature, or sometimes the existence of rents in other rocks in the vicinity, may have caused the sublimation of the metals.[[10]]
Veins of pure quartz are often found in granite as in many stratified rocks, but they are not traceable, like veins of granite or trap, to large bodies of rock of similar composition. They appear to have been cracks, into which siliceous matter was infiltered. Such segregation, as it is called, can sometimes clearly be shown to have taken place long subsequently to the original consolidation of the containing rock. Thus, for example, I observed in the gneiss of Tronstad Strand, near Drammen, in Norway, the section on the beach shown in Figure 615. It appears that the alternating strata of whitish granitiform gneiss and black hornblende-schist were first cut by a greenstone dike, about 2½ feet wide; then the crack a, b, passed through all these rocks, and was filled up with quartz. The opposite walls of the vein are in some parts incrusted with transparent crystals of quartz, the middle of the vein being filled up with common opaque white quartz.
We have seen that the volcanic formations have been called overlying, because they not only penetrate others but spread over them. M. Necker has proposed to call the granites the underlying igneous rocks, and the distinction here indicated is highly characteristic. It was, indeed, supposed by some of the earlier observers that the granite of Christiania, in Norway, was intercalated in mountain masses between the primary or palæozoic strata of that country, so as to overlie fossiliferous shale and limestone. But although the granite sends veins into these fossiliferous rocks, and is decidedly posterior in origin, its actual superposition in mass has been disproved by Professor Keilhau, whose observations on this controverted point I had opportunities, in 1837, of verifying. There are, however, on a smaller scale, certain beds of euritic porphyry, some a few feet, others many yards in thickness, which pass into granite, and deserve, perhaps, to be classed as Plutonic rather than trappean rocks, which may truly be described as interposed conformably between fossiliferous strata, as the porphyries (a, c, Fig. 616) which divide the bituminous shales and argillaceous limestones, f, f. But some of these same porphyries are partially unconformable, as b, and may lead us to suspect that the others also, notwithstanding their appearance of interstratification, have been forcibly injected. Some of the porphyritic rocks above mentioned are highly quartzose, others very feldspathic. In proportion as the masses are more voluminous, they become more granitic in their texture, less conformable, and even begin to send forth veins into contiguous strata. In a word, we have here a beautiful illustration of the intermediate gradations between volcanic and Plutonic rocks, not only in their mineralogical composition and structure, but also in their relations of position to associated formations. If the term “overlying” can in this instance be applied to a Plutonic rock, it is only in proportion as that rock begins to acquire a trappean aspect.
It has been already hinted that the heat which in every active volcano extends downward to indefinite depths must produce simultaneously very different effects near the surface and far below it; and we cannot suppose that rocks resulting from the crystallising of fused matter under a pressure of several thousand feet, much less several miles, of the earth’s crust can exactly resemble those formed at or near the surface. Hence the production at great depths of a class of rocks analogous to the volcanic, and yet differing in many particulars, might have been predicted, even had we no Plutonic formations to account for. How well these agree, both in their positive and negative characters, with the theory of their deep subterranean origin, the student will be able to judge by considering the descriptions already given.
It has, however, been objected, that if the granitic and volcanic rocks were simply different parts of one great series, we ought to find in mountain chains volcanic dikes passing upward into lava and downward into granite. But we may answer that our vertical sections are usually of small extent; and if we find in certain places a transition from trap to porous lava, and in others a passage from granite to trap, it is as much as could be expected of this evidence.
The prodigious extent of denudation which has been already demonstrated to have occurred at former periods, will reconcile the student to the belief that crystalline rocks of high antiquity, although deep in the earth’s crust when originally formed, may have become uncovered and exposed at the surface. Their actual elevation above the sea may be referred to the same causes to which we have attributed the upheaval of marine strata, even to the summits of some mountain chains.
[1] Delesse, Ann. des Mines, 1852, tome iii, p. 409, and 1848, tome xiii, p. 675.
[2] Bulletin, 2e série, iv, 1304; and D’Archiac, Hist. des Progrès de la Géol., i, 38.
[3] See Quart. Geol. Journ., vol. xiv, pp. 465, 488.
[4] Syst. of Geol., vol. i, pp. 157 and 158.
[5] Geol. Trans., First Series, vol. iii, pl. 21.
[6] Captain B. Hall, Trans. Roy. Soc. Edinburgh, vol. vii.
[7] Western Islands, pl. 31.
[8] MacCulloch, Syst. of Geol., vol. ii, p. 58.
[9] Phil. Mag. and Annals, No. 27, New Series, March, 1829.
[10] Necker, Proceedings of the Geol. Soc., No. 26, p. 392.
CHAPTER XXXII.
ON THE DIFFERENT AGES OF THE PLUTONIC ROCKS.
Difficulty in ascertaining the precise Age of a Plutonic Rock. — Test of Age by Relative Position. — Test by Intrusion and Alteration. — Test by Mineral Composition. — Test by included Fragments. — Recent and Pliocene Plutonic Rocks, why invisible. — Miocene Syenite of the Isle of Skye. — Eocene Plutonic Rocks in the Andes. — Granite altering Cretaceous Rocks. — Granite altering Lias in the Alps and in Skye. — Granite of Dartmoor altering Carboniferous Strata. — Granite of the Old Red Sandstone Period. — Syenite altering Silurian Strata in Norway. — Blending of the same with Gneiss. — Most ancient Plutonic Rocks. — Granite protruded in a solid Form.
When we adopt the igneous theory of granite, as explained in the last chapter, and believe that different Plutonic rocks have originated at successive periods beneath the surface of the planet, we must be prepared to encounter greater difficulty in ascertaining the precise age of such rocks than in the case of volcanic and fossiliferous formations. We must bear in mind that the evidence of the age of each contemporaneous volcanic rock was derived either from lavas poured out upon the ancient surface, whether in the sea or in the atmosphere, or from tuffs and conglomerates, also deposited at the surface, and either containing organic remains themselves or intercalated between strata containing fossils. But the same tests entirely fail, or are only applicable in a modified degree, when we endeavour to fix the chronology of a rock which has crystallised from a state of fusion in the bowels of the earth. In that case we are reduced to the tests of relative position, intrusion, alteration of the rocks in contact, included fragments, and mineral character; but all these may yield at best a somewhat ambiguous result.
Test of Age by Relative Position.—Unaltered fossiliferous strata of every age are met with reposing immediately on Plutonic rocks; as at Christiania, in Norway, where the Post-pliocene deposits rest on granite; in Auvergne, where the fresh-water Miocene strata, and at Heidelberg, on the Rhine, where the New Red sandstone occupy a similar place. In all these, and similar instances, inferiority in position is connected with the superior antiquity of granite. The crystalline rock was solid before the sedimentary beds were superimposed, and the latter usually contain in them rounded pebbles of the subjacent granite.
Test by Intrusion and Alteration.—But when Plutonic rocks send veins into strata, and alter them near the point of contact, in the manner before described ([p. 559]), it is clear that, like intrusive traps, they are newer than the strata which they invade and alter. Examples of the application of this test will be given in the sequel.
Test by Mineral Composition.—Notwithstanding a general uniformity in the aspect of Plutonic rocks, we have seen in the last chapter that there are many varieties, such as syenite, talcose granite, and others. One of these varieties is sometimes found exclusively prevailing throughout an extensive region, where it preserves a homogeneous character; so that, having ascertained its relative age in one place, we can recognise its identity in others, and thus determine from a single section the chronological relations of large mountain masses. Having observed, for example, that the syenitic granite of Norway, in which the mineral called zircon abounds, has altered the Silurian strata wherever it is in contact, we do not hesitate to refer other masses of the same zircon-syenite in the south of Norway to a post-Silurian date. Some have imagined that the age of different granites might, to a great extent, be determined by their mineral characters alone; syenite, for instance, or granite with hornblende, being more modern than common or micaceous granite. But modern investigations have proved these generalisations to have been premature.
Test by Included Fragments.—This criterion can rarely be of much importance, because the fragments involved in granite are usually so much altered that they cannot be referred with certainty to the rocks whence they were derived. In the White Mountains, in North America, according to Professor Hubbard, a granite vein, traversing granite, contains fragments of slate and trap which must have fallen into the fissure when the fused materials of the vein were injected from below,[[1]] and thus the granite is shown to be newer than those slaty and trappean formations from which the fragments were derived.
Recent and Pliocene Plutonic Rocks, why invisible.—The explanations already given in the 28th and in the last chapter of the probable relation of the Plutonic to the volcanic formations, will naturally lead the reader to infer that rocks of the one class can never be produced at or near the surface without some members of the other being formed below. It is not uncommon for lava-streams to require more than ten years to cool in the open air; and where they are of great
depth, a much longer period. The melted matter poured from Jorullo, in Mexico, in the year 1759, which accumulated in some places to the height of 550 feet, was found to retain a high temperature half a century after the eruption.[[2]] We may conceive, therefore, that great masses of subterranean lava may remain in a red-hot or incandescent state in the volcanic foci for immense periods, and the process of refrigeration may be extremely gradual. Sometimes, indeed, this process may be retarded for an indefinite period by the accession of fresh supplies of heat; for we find that the lava in the crater of Stromboli, one of the Lipari Islands, has been in a state of constant ebullition for the last two thousand years; and we may suppose this fluid mass to communicate with some caldron or reservoir of fused matter below. In the Isle of Bourbon, also, where there has been an emission of lava once in every two years for a long period, the lava below can scarcely fail to have been permanently in a state of liquefaction. If then it be a reasonable conjecture, that about 2000 volcanic eruptions occur in the course of every century, either above the waters of the sea or beneath them,[[3]] it will follow that the quantity of Plutonic rock generated or in progress during the Recent epoch must already have been considerable.
But as the Plutonic rocks originate at some depth in the earth’s crust, they can only be rendered accessible to human observation by subsequent upheaval and denudation. Between the period when a Plutonic rock crystallises in the subterranean regions and the era of its protrusion at any single point of the surface, one or two geological periods must usually intervene. Hence, we must not expect to find the Recent or even the Pliocene granites laid open to view, unless we are prepared to assume that sufficient time has elapsed since the commencement of the Pliocene period for great upheaval and denudation. A Plutonic rock, therefore, must, in general, be of considerable antiquity relatively to the fossiliferous and volcanic formations, before it becomes extensively visible. As we know that the upheaval of land has been sometimes accompanied in South America by volcanic eruptions and the emission of lava, we may conceive the more ancient Plutonic rocks to be forced upward to the surface by the newer rocks of the same class formed successively below—subterposition in the Plutonic, like superposition in the sedimentary rocks, being usually characteristic of a newer origin.
In Fig. 617 an attempt is made to show the inverted order in which sedimentary and Plutonic formations may occur in the earth’s crust. The oldest Plutonic rock, No. I, has been upheaved at successive periods until it has become exposed to view in a mountain-chain. This protrusion of No. I has been caused by the igneous agency which produced the newer Plutonic rocks Nos. II, III and IV. Part of the primary fossiliferous strata, No. I, have also been raised to the surface by the same gradual process. It will be observed that the Recent strata No. 4 and the Recent granite or Plutonic rock No. IV are the most remote from each other in position, although of contemporaneous date. According to this hypothesis, the convulsions of many periods will be required before Recent or Post-tertiary granite will be upraised so as to form the highest ridges and central axes of mountain-chains. During that time the recent strata No. 4 might be covered by a great many newer sedimentary formations.
Miocene Plutonic Rocks.—A considerable mass of syenite, in the Isle of Skye, is described by Dr. MacCulloch as intersecting limestone and shale, which are of the age of the lias. The limestone, which at a greater distance from the granite contains shells, exhibits no traces of them near its junction, where it has been converted into a pure crystalline marble.[[4]] MacCulloch pointed out that the syenite here, as in Raasay, was newer than the secondary rocks, and Mr. Geikie has since shown that there is a strong probability that this Plutonic rock may be of Miocene age, because a similar Syenite having a true granitic character in its crystallisation has modified the Tertiary volcanic rocks of Ben More, in Mull, some of which have undergone considerable metamorphism.
Eocene Plutonic Rocks.—In a former part of this volume (Chapter 16), the great nummulitic formation of the Alps and Pyrenees was referred to the Eocene period, and it follows that vast movements which have raised those fossiliferous rocks from the level of the sea to the height of more than 10,000 feet above its level have taken place since the commencement of the Tertiary epoch. Here, therefore, if anywhere, we might expect to find hypogene formations of Eocene date breaking out in the central axis or most disturbed region of the loftiest chain in Europe. Accordingly, in the Swiss Alps, even the flysch, or upper portion of the nummulitic series, has been occasionally invaded by Plutonic rocks, and converted into crystalline schists of the hypogene class. There can be little doubt that even the talcose granite or gneiss of Mont Blanc itself has been in a fused or pasty state since the flysch was deposited at the bottom of the sea; and the question as to its age is not so much whether it be a secondary or tertiary granite or gneiss, as whether it should be assigned to the Eocene or Miocene epoch.
Great upheaving movements have been experienced in the region of the Andes, during the Post-tertiary period. In some part, therefore, of this chain, we may expect to discover tertiary Plutonic rocks laid open to view; and Mr. Darwin’s account of the Chilian Andes, to which the reader may refer, fully realises this expectation: for he shows that we have strong ground to presume that Plutonic rocks there exposed on a large scale are of later date than certain Secondary and Tertiary formations.
But the theory adopted in this work of the subterranean origin of the hypogene formations would be untenable, if the supposed fact here alluded to, of the appearance of tertiary granite at the surface, was not a rare exception to the general rule. A considerable lapse of time must intervene between the formation of Plutonic and metamorphic rocks in the nether regions and their emergence at the surface. For a long series of subterranean movements must occur before such rocks can be uplifted into the atmosphere or the ocean; and, before they can be rendered visible to man, some strata which previously covered them must have been stripped off by denudation.
We know that in the Bay of Baiæ in 1538, in Cutch in 1819, and on several occasions in Peru and Chili, since the commencement of the present century, the permanent upheaval or subsidence of land has been accompanied by the simultaneous emission of lava at one or more points in the same volcanic region. From these and other examples it may be inferred that the rising or sinking of the earth’s crust, operations by which sea is converted into land, and land into sea, are a part only of the consequences of subterranean igneous action. It can scarcely be doubted that this action consists, in a great degree, of the baking, and occasionally the liquefaction, of rocks, causing them to assume, in some cases a larger, in others a smaller volume than before the application of heat. It consists also in the generation of gases, and their expansion by heat, and the injection of liquid matter into rents formed in superincumbent rocks. The prodigious scale on which these subterranean causes have operated in Sicily since the deposition of the Newer Pliocene strata will be appreciated when we remember that throughout half the surface of that island such strata are met with, raised to the height of from 50 to that of 2000 and even 3000 feet above the level of the sea. In the same island also the older rocks which are contiguous to these marine tertiary strata must have undergone, within the same period, a similar amount of upheaval.
The like observations may be extended to nearly the whole of Europe, for, since the commencement of the Eocene Period, the entire European area, including some of the central and very lofty portions of the Alps themselves, as I have elsewhere shown,[[5]] has, with the exception of a few districts, emerged from the deep to its present altitude. There must, therefore, have been at great depths in the earth’s crust, within the same period, an amount of subterranean change corresponding to this vast alteration of level affecting a whole continent.
The principal effect of subterranean movements during the Tertiary Period seems to have consisted in the upheaval of hypogene formations of an age anterior to the Carboniferous. The repetition of another series of movements, of equal violence, might upraise the Plutonic and metamorphic rocks of many secondary periods; and, if the same force should still continue to act, the next convulsions might bring up to the day the tertiary and recent hypogene rocks. In the course of such changes many of the existing sedimentary strata would suffer greatly by denudation, others might assume a metamorphic structure, or become melted down into Plutonic and volcanic rocks. Meanwhile the deposition of a great thickness of new strata would not fail to take place during the upheaval and partial destruction of the older rocks. But I must refer the reader to the last chapter but one of this volume for a fuller explanation of these views.
Plutonic Rocks of Cretaceous Period.—It will be shown in the next chapter that chalk, as well as lias, has been altered by granite in the eastern Pyrenees. Whether such granite be cretaceous or tertiary, cannot easily be decided. Suppose b, c, d, Fig. 618, to be three members of the Cretaceous series, the lowest of which, b, has been altered by the granite A, the modifying influence not having extended so far as c, or having but slightly affected its lowest beds. Now it can rarely be possible for the geologist to decide whether the beds d existed at the time of the intrusion of A, and alteration of b and c, or whether they were subsequently thrown down upon c. But as some Cretaceous and even Tertiary rocks have been raised to the height of more than 9000 feet in the Pyrenees, we must not assume that plutonic formations of the same periods may not have been brought up and exposed by denudation, at the height of 2000 or 3000 feet on the flanks of that chain.
Plutonic Rocks of the Oolite and Lias.—In the Department of the Hautes Alpes, in France, M. Élie de Beaumont traced a black argillaceous limestone, charged with belemnites, to within a few yards of a mass of granite. Here the limestone begins to put on a granular texture, but is extremely fine-grained. When nearer the junction it becomes grey, and has a saccharoid structure. In another locality, near Champoleon, a granite composed of quartz, black mica, and rose-coloured feldspar is observed partly to overlie the secondary rocks, producing an alteration which extends for about 30 feet downward, diminishing in the beds which lie farthest from the granite. (See Fig. 619.) In the altered mass the argillaceous beds are hardened, the limestone is saccharoid, the grits quartzose, and in the midst of them is a thin layer of an imperfect granite. It is also an important circumstance that near the point of contact, both the granite and the secondary rocks become metalliferous, and contain nests and small veins of blende, galena, iron, and copper pyrites. The stratified rocks become harder and more crystalline, but the granite, on the contrary, softer and less perfectly crystallised near the junction.[[6]] Although the granite is incumbent in the section (Fig. 619), we cannot assume that it overflowed the strata, for the disturbances of the rocks are so great in this part of the Alps that their original position is often inverted.
At Predazzo, in the Tyrol, secondary strata, some of which are limestones of the Oolitic period, have been traversed and altered by Plutonic rocks, one portion of which is an augitic porphyry, which passes insensibly into granite. The limestone is changed into granular marble, with a band of serpentine at the junction.[[7]]
Plutonic Rocks of Carboniferous Period.—The granite of Dartmoor, in Devonshire, was formerly supposed to be one of the most ancient of the Plutonic rocks, but is now ascertained to be posterior in date to the culm-measures of that county, which from their position, and, as containing true coal-plants, are now known to be members of the true Carboniferous series. This granite, like the syenitic granite of Christiania, has broken through the stratified formations, on the north-west side of Dartmoor, the successive members of the culm-measures abutting against the granite, and becoming metamorphic as they approach. These strata are also penetrated by granite veins, and Plutonic dikes, called “elvans.”[[8]] The granite of Cornwall is probably of the same date, and, therefore, as modern as the Carboniferous strata, if not newer.
Plutonic Rocks of Silurian Period.—It has long been known that a very ancient granite near Christiania, in Norway, is posterior in date to the Lower Silurian strata of that region, although its exact position in the Palæozoic series cannot be defined. Von Buch first announced, in 1813, that it was of newer origin than certain limestones containing orthocerata and trilobites. The proofs consist in the penetration of granite veins into the shale and limestone, and the alteration of the strata, for a considerable distance from the point of contact, both of these veins and the central mass from which they emanate. (See [p. 562]) Von Buch supposed that the Plutonic rock alternated with the fossiliferous strata, and that large masses of granite were sometimes incumbent upon the strata; but this idea was erroneous, and arose from the fact that the beds of shale and limestone often dip towards the granite up to the point of contact, appearing as if they would pass under it in mass, as at a, Fig. 620, and then again on the opposite side of the same mountain, as at b, dip away from the same granite. When the junctions, however, are carefully examined, it is found that the Plutonic rock intrudes itself in veins, and nowhere covers the fossiliferous strata in large overlying masses, as is so commonly the case with trappean formations.[[9]]
Now this granite, which is more modern than the Silurian strata of Norway, also sends veins in the same country into an ancient formation of gneiss; and the relations of the Plutonic rock and the gneiss, at their junction, are full of interest when we duly consider the wide difference of epoch which must have separated their origin.
The length of this interval of time is attested by the following facts: The fossiliferous, or Silurian, beds rest unconformably upon the truncated edges of the gneiss, the inclined strata of which had been denuded before the sedimentary beds were superimposed (see Figure 621). The signs of denudation are twofold; first, the surface of the gneiss is seen occasionally, on the removal of the newer beds containing organic remains, to be worn and smoothed; secondly, pebbles of gneiss have been found in some of these Silurian strata. Between the origin, therefore, of the gneiss and the granite there intervened, first, the period when the strata of gneiss were denuded; secondly, the period of the deposition of the Silurian deposits upon the denuded and inclined gneiss, a. Yet the granite produced after this long interval is often so intimately blended with the ancient gneiss, at the point of junction, that it is impossible to draw any other than an arbitrary line of separation between them; and where this is not the case, tortuous veins of granite pass freely through gneiss, ending sometimes in threads, as if the older rock had offered no resistance to their passage. These appearances may probably be due to hydrothermal action (see [p. 584]). I shall merely observe in this place that had such junctions alone been visible, and had we not learnt, from other sections, how long a period elapsed between the consolidation of the gneiss and the injection of this granite, we might have suspected that the gneiss was scarcely solidified, or had not yet assumed its complete metamorphic character when invaded by the Plutonic rock. From this example we may learn how impossible it is to conjecture whether certain granites in Scotland, and other countries, which send veins into gneiss and other metamorphic rocks, are primary, or whether they may not belong to some secondary or tertiary period.
Oldest Granites.—It is not half a century since the doctrine was very general that all granitic rocks were primitive, that is to say, that they originated before the deposition of the first sedimentary strata, and before the creation of organic beings (see [p. 34]). But so greatly are our views now changed, that we find it no easy task to point out a single mass of granite demonstrably more ancient than known fossiliferous deposits. Could we discover some Laurentian strata resting immediately on granite, there being no alterations at the point of contact, nor any intersecting granitic veins, we might then affirm the Plutonic rock to have originated before the oldest known fossiliferous strata. Still it would be presumptuous, as we have already pointed out ([p. 464]), to suppose that when a small part only of the globe has been investigated, we are acquainted with the oldest fossiliferous strata in the crust of our planet. Even when these are found, we cannot assume that there never were any antecedent strata containing organic remains, which may have become metamorphic. If we find pebbles of granite in a conglomerate of the Lower Laurentian system, we may then feel assured that the parent granite was formed before the Laurentian formation. But if the incumbent strata be merely Cambrian or Silurian, the fundamental granite, although of high antiquity, may be posterior in date to known fossiliferous formations.
Protrusion of Solid Granite.—In part of Sutherlandshire, near Brora, common granite, composed of feldspar, quartz, and mica is in immediate contact with Oolitic strata, and has clearly been elevated to the surface at a period subsequent to the deposition of those strata.[[10]] Professor Sedgwick and Sir R. Murchison conceive that this granite has been upheaved in a solid form; and that in breaking through the submarine deposits, with which it was not perhaps originally in contact, it has fractured them so as to form a breccia along the line of junction. This breccia consists of fragments of shale, sandstone, and limestone, with fossils of the oolite, all united together by a calcareous cement. The secondary strata at some distance from the granite are but slightly disturbed, but in proportion to their proximity the amount of dislocation becomes greater.
Mr. T. McKenney Hughes has suggested to me in explanation of these phenomena that they may be the effect of the association of more pliant strata with hard unyielding rocks, the whole of which were subjected simultaneously to great movements, whether of elevation or subsidence, and of lateral pressure, during which the more solid granite, being incapable of compression, was forced through the softer beds of shale, sandstone, and limestone. He remarks that similar breccias with slickensides are observed on a minor scale where rocks of different composition and rigidity are contorted together. Such protrusion may have been brought about by degrees by innumerable shocks of earthquakes repeated after long intervals of time along the same tract of country. The opening of new fissures in the hardest rocks is a frequent accompaniment of such convulsions, and during the consequent vibrations, breccias must often be caused. But these catastrophes, as we well know, do not imply that the land or sea of the disturbed region are rendered uninhabitable by living beings, and by no means indicate a state of things different from that witnessed in the ordinary course of nature.
[1] Silliman’s Journ., No. 69, p. 123.
[2] See “Principles,” Index, “Jorullo.”
[3] Ibid., “Volcanic Eruptions.”
[4] “Western Islands,” vol. i, p. 330.
[5] See map of Europe, and explanation, in Principles, book i.
[6] Élie de Beaumont sur les Montagnes de l’Oisans, etc. Mém. de la Soc. d’Hist. Nat. de Paris, tome v.
[7] Von Buch, Annales de Chimie, etc.
[8] Proceed. Geol. Soc., vol. ii, p. 562; and Trans., 2nd series, vol. v, p. 686.
[9] See the Gæa Norvegica and other works of Keilhau, with whom I examined this country.
[10] Murchison, Geol. Trans., 2nd series, vol. ii, p. 307.
CHAPTER XXXIII.
METAMORPHIC ROCKS.
General Character of Metamorphic Rocks. — Gneiss. — Hornblende-schist. — Serpentine. — Mica-schist. — Clay-slate. — Quartzite. — Chlorite-schist. — Metamorphic Limestone. — Origin of the metamorphic Strata. — Their Stratification. — Fossiliferous Strata near intrusive Masses of Granite converted into Rocks identical with different Members of the metamorphic Series. — Arguments hence derived as to the Nature of Plutonic Action. — Hydrothermal Action, or the Influence of Steam and Gases in producing Metamorphism. — Objections to the metamorphic Theory considered.
We have now considered three distinct classes of rocks: first, the aqueous, or fossiliferous; secondly, the volcanic; and, thirdly, the Plutonic; and it remains for us to examine those crystalline (or hypogene) strata to which the name of metamorphic has been assigned. The last-mentioned term expresses, as before explained, a theoretical opinion that such strata, after having been deposited from water, acquired, by the influence of heat and other causes, a highly crystalline texture. They who still question this opinion may call the rocks under consideration the stratified hypogene formations or crystalline schists.
These rocks, when in their characteristic or normal state, are wholly devoid of organic remains, and contain no distinct fragments of other rocks, whether rounded or angular. They sometimes break out in the central parts of mountain chains, but in other cases extend over areas of vast dimensions, occupying, for example, nearly the whole of Norway and Sweden, where, as in Brazil, they appear alike in the lower and higher grounds. However crystalline these rocks may become in certain regions, they never, like granite or trap, send veins into contiguous formations. In Great Britain, those members of the series which approach most nearly to granite in their composition, as gneiss, mica-schist, and hornblende-schist, are confined to the country north of the rivers Forth and Clyde.
Many attempts have been made to trace a general order of succession or superposition in the members of this family; clay-slate, for example, having been often supposed to hold invariably a higher geological position than mica-schist, and mica-schist to overlie gneiss. But although such an order may prevail throughout limited districts, it is by no means universal. To this subject, however, I shall again revert, in Chapter XXXV, where the chronological relations of the metamorphic rocks are pointed out.
Principal Metamorphic Rocks.—The following may be enumerated as the principal members of the metamorphic class:—gneiss, mica-schist, hornblende-schist, clay-slate, chlorite-schist, hypogene or metamorphic limestone, and certain kinds of quartz-rock or quartzite.
Gneiss.—The first of these, gneiss, may be called stratified—or by those who object to that term, foliated—granite, being formed of the same materials as granite, namely, feldspar, quartz, and mica. In the specimen in Fig. 622, the white layers consist almost exclusively of granular feldspar, with here and there a speck of mica and grain of quartz. The dark layers are composed of grey quartz and black mica, with occasionally a grain of feldspar intermixed. The rock splits most easily in the plane of these darker layers, and the surface thus exposed is almost entirely covered with shining spangles of mica. The accompanying quartz, however, greatly predominates in quantity, but the most ready cleavage is determined by the abundance of mica in certain parts of the dark layer. Instead of consisting of these thin laminæ, gneiss is sometimes simply divided into thick beds, in which the mica has only a slight degree of parallelism to the planes of stratification.
Hand specimens may often be obtained from such gneiss which are undistinguishable from granite, affording an argument to which we shall allude in the concluding part of this chapter, in favour of those who regard all granite and syenite not as igneous rocks, but as aqueous formations so altered as to have lost all signs of their original stratified arrangement. Gneiss in geology is commonly used to designate not merely stratified and foliated rocks having the same component materials as granite or syenite, but also in a wider sense to embrace the formation with which other members of the metamorphic series, such as hornblende-schist, may alternate, and which are then considered subordinate to the true gneiss.
The different varieties of rock allied to gneiss, into which feldspar enters as an essential ingredient, will be understood by referring to what was said of granite. Thus, for example, hornblende may be superadded to mica, quartz, and feldspar, forming a hornblendic or syenitic gneiss; or talc may be substituted for mica, constituting talcose gneiss (called stratified protogine by the French), a rock composed of feldspar, quartz, and talc, in distinct crystals or grains.
Eurite, which has already been mentioned as a Plutonic rock, occurs also with precisely the same composition in beds subordinate to gneiss or mica-slate.
Hornblende-schist is usually black, and composed principally of hornblende, with a variable quantity of feldspar, and sometimes grains of quartz. When the hornblende and feldspar are in nearly equal quantities, and the rock is not slaty, it corresponds in character with the greenstones of the trap family, and has been called “primitive greenstone.” It may be termed hornblende rock, or amphibolite. Some of these hornblendic masses may really have been volcanic rocks, which have since assumed a more crystalline or metamorphic texture.
Serpentine is a greenish rock, a silicate of magnesia, in which there is sometimes from 30 to 40 per cent of magnesia. It enters largely into the composition of a trap dike cutting through Old Red Sandstone in Forfarshire, and in that case is probably an altered basaltic dike which had contained much olivine. The theory of its having been originally a volcanic product subsequently altered by metamorphism may at first sight seem inconsistent with its occurrence in large and regularly stratified masses in the metamorphic series in Scotland, as in Aberdeenshire. But it has been suggested in explanation that such serpentine may have been originally regularly-bedded trap tuff, and volcanic breccia, with much olivine, which would still retain a stratified appearance after their conversion into a metamorphic rock.
Actinolite Schist is a slaty foliated rock, composed chiefly of actinolite, an emerald-green mineral, allied to hornblende, with some admixture of garnet, mica, and quartz.
Mica-schist or Micaceous Schist is, next to gneiss, one of the most abundant rocks of the metamorphic series. It is slaty, essentially composed of mica and quartz, the mica sometimes appearing to constitute the whole mass. Beds of pure quartz also occur in this formation. In some districts, garnets in regular twelve-sided crystals form an integrant part of mica-schist. This rock passes by insensible gradations into clay-slate.
Clay-slate—Argillaceous Schist—Argillite.—This rock sometimes resembles an indurated clay or shale. It is for the most part extremely fissile, often affording good roofing-slate. Occasionally it derives a shining and silky lustre from the minute particles of mica or talc which it contains. It varies from greenish or bluish-grey to a lead colour; and it may be said of this, more than of any other schist, that it is common to the metamorphic and fossiliferous series, for some clay-slates taken from each division would not be distinguishable by mineral characters alone. It is not uncommon to meet with an argillaceous rock having the same composition, without the slaty cleavage, which may be called argillite.
Chlorite Schist is a green slaty rock, in which chlorite is abundant in foliated plates, usually blended with minute grains of quartz, or sometimes with feldspar or mica; often associated with, and graduating into, gneiss and clay-slate.
Quartzite, or Quartz Rock, is an aggregate of grains of quartz which are either in minute crystals, or in many cases slightly rounded, occurring in regular strata, associated with gneiss or other metamorphic rocks. Compact quartz, like that so frequently found in veins, is also found together with granular quartzite. Both of these alternate with gneiss or mica-schist, or pass into those rocks by the addition of mica, or of feldspar and mica.
Crystalline, or Metamorphic Limestone.—This hypogene rock, called by the earlier geologists primary limestone, is sometimes a white crystalline granular marble, which when in thick beds can be used in sculpture; but more frequently it occurs in thin beds, forming a foliated schist much resembling in colour and arrangement certain varieties of gneiss and mica-schist. When it alternates with these rocks, it often contains some crystals of mica, and occasionally quartz, feldspar, hornblende, talc, chlorite, garnet, and other minerals. It enters sparingly into the structure of the hypogene districts of Norway, Sweden, and Scotland, but is largely developed in the Alps.
Origin of the Metamorphic Strata.—Having said thus much of the mineral composition of the metamorphic rocks, I may combine what remains to be said of their structure and history with an account of the opinions entertained of their probable origin. At the same time, it may be well to forewarn the reader that we are here entering upon ground of controversy, and soon reach the limits where positive induction ends, and beyond which we can only indulge in speculations. It was once a favourite doctrine, and is still maintained by many, that these rocks owe their crystalline texture, their want of all signs of a mechanical origin, or of fossil contents, to a peculiar and nascent condition of the planet at the period of their formation. The arguments in refutation of this hypothesis will be more fully considered when I show, in Chapter XXXV, to how many different ages the metamorphic formations are referable, and how gneiss, mica-schist, clay-slate, and hypogene limestone (that of Carrara, for example) have been formed, not only since the first introduction of organic beings into this planet, but even long after many distinct races of plants and animals had flourished and passed away in succession.
The doctrine respecting the crystalline strata implied in the name metamorphic may properly be treated of in this place; and we must first inquire whether these rocks are really entitled to be called stratified in the strict sense of having been originally deposited as sediment from water. The general adoption by geologists of the term stratified, as applied to these rocks, sufficiently attests their division into beds very analogous, at least in form, to ordinary fossiliferous strata. This resemblance is by no means confined to the existence in both occasionally of a laminated structure, but extends to every kind of arrangement which is compatible with the absence of fossils, and of sand, pebbles, ripple-mark, and other characters which the metamorphic theory supposes to have been obliterated by Plutonic action. Thus, for example, we behold alike in the crystalline and fossiliferous formations an alternation of beds varying greatly in composition, colour, and thickness. We observe, for instance, gneiss alternating with layers of black hornblende-schist or of green chlorite-schist, or with granular quartz or limestone; and the interchange of these different strata may be repeated for an indefinite number of times. In the like manner, mica-schist alternates with chlorite-schist, and with beds of pure quartz or of granular limestone. We have already seen that, near the immediate contact of granitic veins and volcanic dikes, very extraordinary alterations in rocks have taken place, more especially in the neighbourhood of granite. It will be useful here to add other illustrations, showing that a texture undistinguishable from that which characterises the more crystalline metamorphic formations has actually been superinduced in strata once fossiliferous.
Fossiliferous Strata rendered metamorphic by intrusive Masses of Granite.—In the southern extremity of Norway there is a large district, on the west side of the fiord of Christiania, which I visited in 1837 with the late Professor Keilhau, in which syenitic granite protrudes in mountain masses through fossiliferous strata, and usually sends veins into them at the point of contact. The stratified rocks, replete with shells and zoophytes, consist chiefly of shale, limestone, and some sandstone, and all these are invariably altered near the granite for a distance of from 50 to 400 yards. The aluminous shales are hardened, and have become flinty. Sometimes they resemble jasper. Ribboned jasper is produced by the hardening of alternate layers of green and chocolate-coloured schist, each stripe faithfully representing the original lines of stratification. Nearer the granite the schist often contains crystals of hornblende, which are even met with in some places for a distance of several hundred yards from the junction; and this black hornblende is so abundant that eminent geologists, when passing through the country, have confounded it with the ancient hornblende-schist, subordinate to the great gneiss formation of Norway. Frequently, between the granite and the hornblende-slate above-mentioned, grains of mica and crystalline feldspar appear in the schist, so that rocks resembling gneiss and mica-schist are produced. Fossils can rarely be detected in these schists, and they are more completely effaced in proportion to the more crystalline texture of the beds, and their vicinity to the granite.
In some places the siliceous matter of the schist becomes a granular quartz; and when hornblende and mica are added, the altered rock loses its stratification, and passes into a kind of granite. The limestone, which at points remote from the granite is of an earthy texture and blue colour, and often abounds in corals, becomes a white granular marble near the granite, sometimes siliceous, the granular structure extending occasionally upward of 400 yards from the junction; the corals being for the most part obliterated, though sometimes preserved, even in the white marble. Both the altered limestone and hardened slate contain garnets in many places, also ores of iron, lead, and copper, with some silver. These alterations occur equally whether the granite invades the strata in a line parallel to the general strike of the fossiliferous beds, or in a line at right angles to their strike, both of which modes of junction will be seen by the ground-plan in Fig. 623.[[1]]
The granite of Cornwall sends forth veins into a coarse argillaceous-schist, provincially termed killas. This killas is converted into hornblende-schist near the contact with the veins. These appearances are well seen at the junction of the granite and killas, in St. Michael’s Mount, a small island nearly 300 feet high, situated in the bay, at a distance of about three miles from Penzance. The granite of Dartmoor, in Devonshire, says Sir H. De la Beche, has intruded itself into the Carboniferous slate and slaty sandstone, twisting and contorting the strata, and sending veins into them. Hence some of the slate rocks have become “micaceous; others more indurated, and with the characters of mica-slate and gneiss; while others again appear converted into a hard zoned rock strongly impregnated with feldspar.”[[2]]
We learn from the investigation of M. Dufrenoy that in the eastern Pyrenees there are mountain masses of granite posterior in date to the formations called lias and chalk of that district, and that these fossiliferous rocks are greatly altered in texture, and often charged with iron-ore, in the neighbourhood of the granite. Thus in the environs of St. Martin, near St. Paul de Fenouillet, the chalky limestone becomes more crystalline and saccharoid as it approaches the granite, and loses all trace of the fossils which it previously contained in abundance. At some points, also, it becomes dolomitic, and filled with small veins of carbonate of iron, and spots of red iron-ore. At Rancie the lias nearest the granite is not only filled with iron-ore, but charged with pyrites, tremolite, garnet, and a new mineral somewhat allied to feldspar, called, from the place in the Pyrenees where it occurs, “couzeranite.”
“Hornblende-schist,” says Dr. MacCulloch, “may at first have been mere clay; for clay or shale is found altered by trap into Lydian stone, a substance differing from hornblende-schist almost solely in compactness and uniformity of texture.”[[3]] “In Shetland,” remarks the same author, “argillaceous-schist (or clay-slate), when in contact with granite, is sometimes converted into hornblende-schist, the schist becoming first siliceous, and ultimately, at the contact, hornblende-schist.” In like manner gneiss and mica-schist may be nothing more than altered micaceous and argillaceous sandstones, granular quartz may have been derived from siliceous sandstone, and compact quartz from the same materials. Clay-slate may be altered shale, and granular marble may have originated in the form of ordinary limestone, replete with shells and corals, which have since been obliterated; and, lastly, calcareous sands and marls may have been changed into impure crystalline limestones.
The anthracite and plumbago associated with hypogene rocks may have been coal; for not only is coal converted into anthracite in the vicinity of some trap dikes, but we have seen that a like change has taken place generally even far from the contact of igneous rocks, in the disturbed region of the Appalachians. At Worcester, in the State of Massachusetts, 45 miles due west of Boston, a bed of plumbago and impure anthracite occurs, interstratified with mica-schist. It is about two feet in thickness, and has been made use of both as fuel, and in the manufacture of lead pencils. At the distance of 30 miles from the plumbago, there occurs, on the borders of Rhode Island, an impure anthracite in slates containing impressions of coal-plants of the genera Pecopteris, Neuropteris, Calamites, etc. This anthracite is intermediate in character between that of Pennsylvania and the plumbago of Worcester, in which last the gaseous or volatile matter (hydrogen, oxygen, and nitrogen) is to the carbon only in the proportion of three per cent. After traversing the country in various directions, I came to the conclusion that the carboniferous shales or slates with anthracite and plants, which in Rhode Island often pass into mica-schists, have at Worcester assumed a perfectly crystalline and metamorphic texture; the anthracite having been nearly transmuted into that state of pure carbon which is called plumbago or graphite.[[4]]
Now the alterations above described as superinduced in rocks by volcanic dikes and granite veins prove incontestably that powers exist in nature capable of transforming fossiliferous into crystalline strata, a very few simple elements constituting the component materials common to both classes of rocks. These elements, which are enumerated in the table at [p. 499], may be made to form new combinations by what has been termed Plutonic action, or those chemical changes which are no doubt connected with the passage of heat, unusually heated steam and waters, through the strata.
Hydrothermal Action, or the Influence of Steam and Gases in producing Metamorphism.—The experiments of Gregory Watt, in fusing rocks in the laboratory, and allowing them to consolidate by slow cooling, prove distinctly that a rock need not be perfectly melted in order that a re-arrangement of its component particles should take place, and a partial crystallisation ensue.[[5]] We may easily suppose, therefore, that all traces of shells and other organic remains may be destroyed, and that new chemical combinations may arise, without the mass being so fused as that the lines of stratification should be wholly obliterated. We must not, however, imagine that heat alone, such as may be applied to a stone in the open air, can constitute all that is comprised in Plutonic action. We know that volcanoes in eruption not only emit fluid lava, but give off steam and other heated gases, which rush out in enormous volume, for days, weeks, or years continuously, and are even disengaged from lava during its consolidation.
We also know that long after volcanoes have spent their force, hot springs continue for ages to flow out at various points in the same area. In regions, also, subject to violent earthquakes such springs are frequently observed issuing from rents, usually along lines of fault or displacement of the rocks. These thermal waters are most commonly charged with a variety of mineral ingredients, and they retain a remarkable uniformity of temperature from century to century. A like uniformity is also persistent in the nature of the earthy, metallic, and gaseous substances with which they are impregnated. It is well ascertained that springs, whether hot or cold, charged with carbonic acid, especially with hydrofluoric acid, which is often present in small quantities, are powerful causes of decomposition and chemical reaction in rocks through which they percolate.
The changes which Daubrée has shown to have been produced by the alkaline waters of Plombières in the Vosges, are more especially instructive.[[6]] These waters have a heat of 160° F., or an excess of 109° above the average temperature of ordinary springs in that district. They were conveyed by the Romans to baths through long conduits or aqueducts. The foundations of some of their works consisted of a bed of concrete made of lime, fragments of brick, and sandstone. Through this and other masonry the hot waters have been percolating for centuries, and have given rise to various zeolites—apophyllite and chabazite among others; also to calcareous spar, arragonite, and fluor spar, together with siliceous minerals, such as opal—all found in the inter-spaces of the bricks and mortar, or constituting part of their re-arranged materials. The quantity of heat brought into action in this instance in the course of 2000 years has, no doubt, been enormous, but the intensity of it developed at any one moment has been always inconsiderable.
From these facts and from the experiments and observations of Sénarmont, Daubrée, Delesse, Scheerer, Sorby, Sterry Hunt, and others, we are led to infer that when in the bowels of the earth there are large volumes of matter containing water and various acids intensely heated under enormous pressure, these subterranean fluid masses will gradually part with their heat by the escape of steam and various gases through fissures, producing hot springs; or by the passage of the same through the pores of the overlying and injected rocks. Even the most compact rocks may be regarded, before they have been exposed to the air and dried, in the light of sponges filled with water. According to the experiments of Henry, water, under a hydrostatic pressure of 96 feet, will absorb three times as much carbonic acid gas as it can under the ordinary pressure of the atmosphere. There are other gases, as well as the carbonic acid, which water absorbs, and more rapidly in proportion to the amount of pressure. Although the gaseous matter first absorbed would soon be condensed, and part with its heat, yet the continual arrival of fresh supplies from below might, in the course of ages, cause the temperature of the water, and with it that of the containing rock, to be materially raised; the water acts not only as a vehicle of heat, but also by its affinity for various silicates, which, when some of the materials of the invaded rocks are decomposed, form quartz, feldspar, mica, and other minerals. As for quartz, it can be produced under the influence of heat by water holding alkaline silicates in solution, as in the case of the Plombières springs. The quantity of water required, according to Daubrée, to produce great transformations in the mineral structure of rocks, is very small. As to the heat required, silicates may be produced in the moist way at about incipient red heat, whereas to form the same in the dry way would require a much higher temperature.
M. Fournet, in his description of the metalliferous gneiss near Clermont, in Auvergne, states that all the minute fissures of the rock are quite saturated with free carbonic acid gas; which gas rises plentifully from the soil there and in many parts of the surrounding country. The various elements of the gneiss, with the exception of the quartz, are all softened; and new combinations of the acid with lime, iron, and manganese are continually in progress.[[7]]
The power of subterranean gases is well illustrated by the stufas of St. Calogero in the Lipari Islands, where the horizontal strata of tuffs, forming cliffs 200 feet high, have been discoloured in places by the jets of steam often above the boiling point, called “stufas,” issuing from the fissures; and similar instances are recorded by M. Virlet of corrosion of rocks near Corinth, and by Dr. Daubeny of decomposition of trachytic rocks by sulphureted hydrogen and muriatic acid gases in the Solfatara, near Naples. In all these instances it is clear that the gaseous fluids must have made their way through vast thicknesses of porous or fissured rocks, and their modifying influence may spread through the crust for thousands of yards in thickness.
It has been urged as an argument against the metamorphic theory, that rocks have a small power of conducting heat, and it is true that when dry, and in the air, they differ remarkably from metals in this respect. The syenite of Norway, as we have seen ([p. 558]), has sometimes altered fossiliferous strata both in the direction of their dip and strike for a distance of a quarter of a mile, but the theory of gneiss and mica-schist above proposed requires us to imagine that the same influence has extended through strata miles in thickness. Professor Bischof has shown what changes may be superinduced, on black marble and other rocks, by the steam of a hot spring having a temperature of no more than 133° to 167° Fahrenheit, and we are becoming more and more acquainted with the prominent part which water is playing in distributing the heat of the interior through mountain masses of incumbent strata, and of introducing into them various mineral elements in a fluid or gaseous state. Such facts may induce us to consider whether many granites and other rocks of that class may not sometimes represent merely the extreme of a similar slow metamorphism. But, on the other hand, the heat of lava in a volcanic crater when it is white and glowing like the sun must convince us that the temperature of a column of such a fluid at the depth of many miles exceeds any heat which can ever be witnessed at the surface. That large portions of the Plutonic rocks had been formed under the influence of such intense heat is in perfect accordance with their great volume, uniform composition, and absence of stratification. The forcing also of veins into contiguous stratified or schistose rocks is a natural consequence of the hydrostatic pressure to which columns of molten matter many miles in height must give rise.
Objections to the Metamorphic Theory considered.—It has been objected to the metamorphic theory that the crystalline schists contain a considerable proportion of potash and soda, whilst the sedimentary strata out of which they are supposed to have been formed are usually wanting in alkaline matter. But this reasoning proceeds on mistaken data, for clay, marl, shale, and slate often contain a considerable proportion of alkali, so much so as to make them frequently unfit to be burnt into bricks or pottery, and the Old Red Sandstone in Forfarshire and other parts of Scotland, derived from disintegration of granite, contains much triturated feldspar rich in potash. In the common salt by which strata are often largely impregnated, as in Patagonia, much soda is present, and potash enters largely into the composition of fossil sea-weeds, and recent analysis has also shown that the carboniferous strata in England, the Upper and Lower Silurian in East Canada, and the oldest clay-slates in Norway, all contain as much alkali as is generally present in metamorphic rocks.
Another objection has been derived from the alternation of highly crystalline strata with others less crystalline. The heat, it is said, in its ascent from below, must have traversed the less altered schists before it reached a higher and more crystalline bed. In answer to this, it may be observed, that if a number of strata differing greatly in composition from each other be subjected to equal quantities of heat, or hydrothermal action, there is every probability that some will be much more fusible or soluble than others. Some, for example, will contain soda, potash, lime, or some other ingredient capable of acting as a flux or solvent; while others may be destitute of the same elements, and so refractory as to be very slightly affected by the same causes. Nor should it be forgotten that, as a general rule, the less crystalline rocks do really occur in the upper, and the more crystalline in the lower part of each metamorphic series.
[1] Keilhau, Gæa Norvegica, pp. 61-63.
[2] Geol. Manual, p. 479.
[3] Syst. of Geol., vol. i, pp. 210, 211.
[4] See Lyell, Quart. Geol. Journ., vol. i, p. 199.
[5] Phil. Trans., 1804.
[6] Daubrée, Sur le Métamorphisme. Paris, 1860.
[7] See Principles, Index, “Carbonated Springs,” etc.
CHAPTER XXXIV.
METAMORPHIC ROCKS—continued.
Definition of slaty Cleavage and Joints. — Supposed Causes of these Structures. — Crystalline Theory of Cleavage. — Mechanical Theory of Cleavage. — Condensation and Elongation of slate Rocks by lateral Pressure. — Lamination of some volcanic Rocks due to Motion. — Whether the Foliation of the crystalline Schists be usually parallel with the original Planes of Stratification. — Examples in Norway and Scotland. — Causes of Irregularity in the Planes of Foliation.
We have already seen that chemical forces of great intensity have frequently acted upon sedimentary and fossiliferous strata long subsequently to their consolidation, and we may next inquire whether the component minerals of the altered rocks usually arrange themselves in planes parallel to the original planes of stratification, or whether, after crystallisation, they more commonly take up a different position.
In order to estimate fairly the merits of this question, we must first define what is meant by the terms cleavage and foliation. There are four distinct forms of structure exhibited in rocks, namely, stratification, joints, slaty cleavage, and foliation; and all these must have different names, even though there be cases where it is impossible, after carefully studying the appearances, to decide upon the class to which they belong.
Slaty Cleavage.—Professor Sedgwick, whose essay “On the Structure of large Mineral Masses” first cleared the way towards a better understanding of this difficult subject, observes, that joints are distinguishable from lines of slaty cleavage in this, that the rock intervening between two joints has no tendency to cleave in a direction parallel to the planes of the joints, whereas a rock is capable of indefinite subdivision in the direction of its slaty cleavage. In cases where the strata are curved, the planes of cleavage are still perfectly parallel. This has been observed in the slate rocks of part of Wales (see Fig. 624), which consists of a hard greenish slate. The true bedding is there indicated by a number of parallel stripes, some of a lighter and some of a darker colour than the general mass. Such stripes are found to be parallel to the true planes of stratification, wherever these are manifested by ripple-mark or by beds containing peculiar organic remains. Some of the contorted strata are of a coarse mechanical structure, alternating with fine-grained crystalline chloritic slates, in which case the same slaty cleavage extends through the coarser and finer beds, though it is brought out in greater perfection in proportion as the materials of the rock are fine and homogeneous. It is only when these are very coarse that the cleavage planes entirely vanish. In the Welsh hills these planes are usually inclined at a very considerable angle to the planes of the strata, the average angle being as much as from 30° to 40°. Sometimes the cleavage planes dip towards the same point of the compass as those of stratification, but often to opposite points.[[1]] The cleavage, as represented in Fig. 624, is generally constant over the whole of any area affected by one great set of disturbances, as if the same lateral pressure which caused the crumpling up of the rock along parallel, anticlinal, and synclinal axes caused also the cleavage.
Mr. T. McK. Hughes remarks, that where a rough cleavage cuts flag-stones at a considerable angle to the planes of stratification, the rock often splits into large slabs, across which the lines of bedding are frequently seen, but when the cleavage planes approach within about 15° of stratification, the rock is apt to split along the lines of bedding. He has also called my attention to the fact that subsequent movements in a cleaved rock sometimes drag and bend the cleavage planes along the junction of the beds in the manner indicated in Fig. 625.
Jointed Structure.—In regard to joints, they are natural fissures which often traverse rocks in straight and well-determined lines. They afford to the quarryman, as Sir R. Murchison observes, when speaking of the phenomenon, as exhibited in Shropshire and the neighbouring counties, the greatest aid in the extraction of blocks of stone; and, if a sufficient number cross each other, the whole mass of rock is split into symmetrical blocks. The faces of the joints are for the most part smoother and more regular than the surfaces of true strata. The joints are straight-cut chinks, sometimes slightly open, and often passing, not only through layers of successive deposition, but also through balls of limestone or other matter which have been formed by concretionary action since the original accumulation of the strata. Such joints, therefore, must often have resulted from one of the last changes superinduced upon sedimentary deposits.[[2]]
In Fig. 626 the flat-surfaces of rock, A, B, C, represent exposed faces of joints, to which the walls of other joints, J J, are parallel. S S are the lines of stratification; D D are lines of slaty cleavage, which intersect the rock at a considerable angle to the planes of stratification.
In the Swiss and Savoy Alps, as Mr. Bakewell has remarked, enormous masses of limestone are cut through so regularly by nearly vertical partings, and these joints are often so much more conspicuous than the seams of stratification, that an inexperienced observer will almost inevitably confound them, and suppose the strata to be perpendicular in places where in fact they are almost horizontal.[[3]]
Now such joints are supposed to be analogous to the partings which separate volcanic and Plutonic rocks into cuboidal and prismatic masses. On a small scale we see clay and starch when dry split into similar shapes; this is often caused by simple contraction, whether the shrinking be due to the evaporation of water, or to a change of temperature. It is well known that many sandstones and other rocks expand by the application of moderate degrees of heat, and then contract again on cooling; and there can be no doubt that large portions of the earth’s crust have, in the course of past ages, been subjected again and again to very different degrees of heat and cold. These alternations of temperature have probably contributed largely to the production of joints in rocks.
In many countries where masses of basalt rest on sandstone, the aqueous rock has, for the distance of several feet from the point of junction, assumed a columnar structure similar to that of the trap. In like manner some hearth-stones, after exposure to the heat of a furnace without being melted, have become prismatic. Certain crystals also acquire by the application of heat a new internal arrangement, so as to break in a new direction, their external form remaining unaltered.
Crystalline Theory of Cleavage.—Professor Sedgwick, speaking of the planes of slaty cleavage, where they are decidedly distinct from those of sedimentary deposition, declared, in the essay before alluded to, his opinion that no retreat of parts, no contraction in the dimensions of rocks in passing to a solid state, can account for the phenomenon. He accordingly referred it to crystalline or polar forces acting simultaneously, and somewhat uniformly, in given directions, on large masses having a homogeneous composition.
Sir John Herschel, in allusion to slaty cleavage, has suggested that “if rocks have been so heated as to allow a commencement of crystallisation—that is to say, if they have been heated to a point at which the particles can begin to move among themselves, or at least on their own axes, some general law must then determine the position in which these particles will rest on cooling. Probably, that position will have some relation to the direction in which the heat escapes. Now, when all, or a majority of particles of the same nature have a general tendency to one position, that must of course determine a cleavage-plane. Thus we see the infinitesimal crystals of fresh-precipitated sulphate of barytes, and some other such bodies, arrange themselves alike in the fluid in which they float; so as, when stirred, all to glance with one light, and give the appearance of silky filaments. Some sorts of soap, in which insoluble margarates[[4]] exist, exhibit the same phenomenon when mixed with water; and what occurs in our experiments on a minute scale may occur in nature on a great one.”[[5]]
Mechanical Theory of Cleavage.—Professor Phillips has remarked that in some slaty rocks the form of the outline of fossil shells and trilobites has been much changed by distortion, which has taken place in a longitudinal, transverse, or oblique direction. This change, he adds, seems to be the result of a “creeping movement” of the particles of the rock along the planes of cleavage, its direction being always uniform over the same tract of country, and its amount in space being sometimes measurable, and being as much as a quarter or even half an inch. The hard shells are not affected, but only those which are thin.[[6]] Mr. D. Sharpe, following up the same line of inquiry, came to the conclusion that the present distorted forms of the shells in certain British slate rocks may be accounted for by supposing that the rocks in which they are imbedded have undergone compression in a direction perpendicular to the planes of cleavage, and a corresponding expansion in the direction of the dip of the cleavage.[[7]]
Subsequently (1853) Mr. Sorby demonstrated the great extent to which this mechanical theory is applicable to the slate rocks of North Wales and Devonshire,[[8]] districts where the amount of change in dimensions can be tested and measured by comparing the different effects exerted by lateral pressure on alternating beds of finer and coarser materials. Thus, for example, in Fig. 627 it will be seen that the sandy bed d f, which has offered greater resistance, has been sharply contorted, while the fine-grained strata, a, b, c, have remained comparatively unbent. The points d and f in the stratum d f must have been originally four times as far apart as they are now. They have been forced so much nearer to each other, partly by bending, and partly by becoming elongated in the direction of what may be called the longer axes of their contortions, and lastly, to a certain small amount, by condensation. The chief result has obviously been due to the bending; but, in proof of elongation, it will be observed that the thickness of the bed d f is now about four times greater in those parts lying in the main direction of the flexures than in a plane perpendicular to them; and the same bed exhibits cleavage planes in the direction of the greatest movement, although they are much fewer than in the slaty strata above and below.
Above the sandy bed d f, the stratum c is somewhat disturbed, while the next bed, b, is much less so, and a not at all; yet all these beds, c, b, and a, must have undergone an equal amount of pressure with d, the points a and g having approximated as much towards each other as have d and f. The same phenomena are also repeated in the beds below d, and might have been shown, had the section been extended downward. Hence it appears that the finer beds have been squeezed into a fourth of the space they previously occupied, partly by condensation, or the closer packing of their ultimate particles (which has given rise to the great specific gravity of such slates), and partly by elongation in the line of the dip of the cleavage, of which the general direction is perpendicular to that of the pressure. “These and numerous other cases in North Devon are analogous,” says Mr. Sorby, “to what would occur if a strip of paper were included in a mass of some soft plastic material which would readily change its dimensions. If the whole were then compressed in the direction of the length of the strip of paper, it would be bent and puckered up into contortions, while the plastic material would readily change its dimensions without undergoing such contortions; and the difference in distance of the ends of the paper, as measured in a direct line or along it, would indicate the change in the dimensions of the plastic material.”
By microscopic examination of minute crystals, and by other observations, Mr. Sorby has come to the conclusion that the absolute condensation of the slate rocks amounts upon an average to about one half their original volume. Most of the scales of mica occurring in certain slates examined by Mr. Sorby lie in the plane of cleavage; whereas in a similar rock not exhibiting cleavage they lie with their longer axes in all directions. May not their position in the slates have been determined by the movement of elongation before alluded to? To illustrate this theory some scales of oxide of iron were mixed with soft pipe-clay in such a manner that they inclined in all directions. The dimensions of the mass were then changed artificially to a similar extent to what has occurred in slate rocks, and the pipe-clay was then dried and baked. When it was afterwards rubbed to a flat surface perpendicular to the pressure and in the line of elongation, or in a plane corresponding to that of the dip of cleavage, the particles were found to have become arranged in the same manner as in natural slates, and the mass admitted of easy fracture into thin flat pieces in the plane alluded to, whereas it would not yield in that perpendicular to the cleavage.[[9]]
Dr. Tyndall, when commenting in 1856 on Mr. Sorby’s experiments, observed that pressure alone is sufficient to produce cleavage, and that the intervention of plates of mica or scales of oxide of iron, or any other substances having flat surfaces, is quite unnecessary. In proof of this he showed experimentally that a mass of “pure white wax, after having been submitted to great pressure, exhibited a cleavage more clean than that of any slate-rock, splitting into laminæ of surpassing tenuity.”[[10]] He remarks that every mass of clay or mud is divided and subdivided by surfaces among which the cohesion is comparatively small. On being subjected to pressure, such masses yield and spread out in the direction of least resistance, small nodules become converted into laminæ separated from each other by surfaces of weak cohesion, and the result is that the mass cleaves at right angles to the line in which the pressure is exerted. In further illustration of this, Mr. Hughes remarks that “concretions which in the undisturbed beds have their longer axes parallel to the bedding are, where the rock is much cleaved, frequently found flattened laterally, so as to have their longer axes parallel to the cleavage planes, and at a considerable angle, even right angles, to their former position.”
Mr. Darwin attributes the lamination and fissile structure of volcanic rocks of the trachytic series, including some obsidians in Ascension, Mexico, and elsewhere, to their having moved when liquid in the direction of the laminæ. The zones consist sometimes of layers of air-cells drawn out and lengthened in the supposed direction of the moving mass.[[11]]
Foliation of Crystalline Schists.—After studying, in 1835, the crystalline rocks of South America, Mr. Darwin proposed the term foliation for the laminæ or plates into which gneiss, mica-schist, and other crystalline rocks are divided. Cleavage, he observes, may be applied to those divisional planes which render a rock fissile, although it may appear to the eye quite or nearly homogeneous. Foliation may be used for those alternating layers or plates of different mineralogical nature of which gneiss and other metamorphic schists are composed.
That the planes of foliation of the crystalline schists in Norway accord very generally with those of original stratification is a conclusion long since espoused by Keilhau.[[12]] Numerous observations made by Mr. David Forbes in the same country (the best probably in Europe for studying such phenomena on a grand scale) confirm Keilhau’s opinion. In Scotland, also, Mr. D. Forbes has pointed out a striking case where the foliation is identical with the lines of stratification in rocks well seen near Crianlorich on the road to Tyndrum, about eight miles from Inverarnon, in Perthshire. There is in that locality a blue limestone foliated by the intercalation of small plates of white mica, so that the rock is often scarcely distinguishable in aspect from gneiss or mica-schist. The stratification is shown by the large beds and coloured bands of limestone all dipping, like the folia, at an angle of 32° N.E.[[13]] In stratified formations of every age we see layers of siliceous sand with or without mica, alternating with clay, with fragments of shells or corals, or with seams of vegetable matter, and we should expect the mutual attraction of like particles to favour the crystallisation of the quartz, or mica, or feldspar, or carbonate of lime, along the planes of original deposition, rather than in planes placed at angles of 20 or 40 degrees to those of stratification.
We have seen how much the original planes of stratification may be interfered with or even obliterated by concretionary action in deposits still retaining their fossils, as in the case of the magnesian limestone (see [p. 63]). Hence we must expect to be frequently baffled when we attempt to decide whether the foliation does or does not accord with that arrangement which gravitation, combined with current-action, imparted to a deposit from water. Moreover, when we look for stratification in crystalline rocks, we must be on our guard not to expect too much regularity. The occurrence of wedge-shaped masses, such as belong to coarse sand and pebbles—diagonal lamination (p. 42)—ripple-marked, unconformable stratification,—the fantastic folds produced by lateral pressure—faults of various width—intrusive dikes of trap—organic bodies of diversified shapes, and other causes of unevenness in the planes of deposition, both on the small and on the large scale, will interfere with parallelism. If complex and enigmatical appearances did not present themselves, it would be a serious objection to the metamorphic theory. Mr. Sorby has shown that the peculiar structure belonging to ripple-marked sands, or that which is generated when ripples are formed during the deposition of the materials, is distinctly recognisable in many varieties of mica-schists in Scotland.[[14]]
In Fig. 628 I have represented carefully the lamination of a coarse argillaceous schist which I examined in 1830 in the Pyrenees. In part it approaches in character to a green and blue roofing-slate, while part is extremely quartzose, the whole mass passing downward into micaceous schist. The vertical section here exhibited is about three feet in height, and the layers are sometimes so thin that fifty may be counted in the thickness of an inch. Some of them consist of pure quartz. There is a resemblance in such cases to the diagonal lamination which we see in sedimentary rocks, even though the layers of quartz and of mica, or of feldspar and other minerals, may be more distinct in alternating folia than they were originally.
[1] Geol. Trans., 2nd series, vol. iii, p. 461.
[2] Silurian System, p. 246.
[3] Introduction to Geology, chap. iv.
[4] Margaric acid is an oleaginous acid, formed from different animal and vegetable fatty substances. A margarate is a compound of this acid with soda, potash, or some other base, and is so named from its pearly lustre.
[5] Letter to the author, dated Cape of Good Hope, Feb. 20, 1836.
[6] Report, Brit. Assoc., Cork, 1843, Sect. p. 60.
[7] Quart. Geol. Journ., vol. iii, p. 87, 1847.
[8] On the Origin of Slaty Cleavage, by H. C. Sorby, Edin. New Phil. Journ., 1853, vol. lv, p. 137.
[9] Sorby, as cited above, p. 741, note.
[10] Tyndall, View of the Cleavage of Crystals and Slate rocks.
[11] Darwin, Volcanic Islands, pp. 69, 70.
[12] Norske Mag. Naturvidsk., vol. i, p. 71.
[13] Memoir read before the Geol. Soc. London, Jan. 31, 1855.
[14] H. C. Sorby, Quart. Geol. Journ., vol. xix., p. 401.
CHAPTER XXXV.
ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS.
Difficulty of ascertaining the Age of metamorphic Strata. — Metamorphic Strata of Eocene date in the Alps of Switzerland and Savoy. — Limestone and Shale of Carrara. — Metamorphic Strata of older date than the Silurian and Cambrian Rocks. — Order of Succession in metamorphic Rocks. — Uniformity of mineral Character. — Supposed Azoic Period. — Connection between the Absence of Organic Remains and the Scarcity of calcareous Matter in metamorphic Rocks.
According to the theory adopted in the last chapter, the metamorphic strata have been deposited at one period, and have become crystalline at another. We can rarely hope to define with exactness the date of both these periods, the fossils having been destroyed by Plutonic action, and the mineral characters being the same, whatever the age. Superposition itself is an ambiguous test, especially when we desire to determine the period of crystallisation. Suppose, for example, we are convinced that certain metamorphic strata in the Alps, which are covered by cretaceous beds, are altered lias; this lias may have assumed its crystalline texture in the cretaceous or in some tertiary period, the Eocene for example.
When discussing the ages of the Plutonic rocks, we have seen that examples occur of various primary, secondary, and tertiary deposits converted into metamorphic strata near their contact with granite. There can be no doubt in these cases that strata once composed of mud, sand, and gravel, or of clay, marl, and shelly limestone, have for the distance of several yards, and in some instances several hundred feet, been turned into gneiss, mica-schist, hornblende-schist, chlorite-schist, quartz rock, statuary marble, and the rest. (See the two preceding chapters.) It may be easy to prove the identity of two different parts of the same stratum; one, where the rock has been in contact with a volcanic or Plutonic mass, and has been changed into marble or hornblende-schist, and another not far distant, where the same bed remains unaltered and fossiliferous; but when hydrothermal action, as described in Chapter XXXIII, has operated gradually on a more extensive scale, it may have finally destroyed all monuments of the date of its development throughout a whole mountain chain, and all the labour and skill of the most practised observers are required, and may sometimes be at fault. I shall mention one or two examples of alteration on a grand scale, in order to explain to the student the kind of reasoning by which we are led to infer that dense masses of fossiliferous strata have been converted into crystalline rocks.
Eocene Strata rendered metamorphic in the Alps.—In the eastern part of the Alps, some of the Palæozoic strata, as well as the older Mesozoic formations, including the oolitic and cretaceous rocks, are distinctly recognisable. Tertiary deposits also appear in a less elevated position on the flanks of the Eastern Alps; but in the Central or Swiss Alps, the Palæozoic and older Mesozoic formations disappear, and the Cretaceous, Oolitic, Liassic, and at some points even the Eocene strata, graduate insensibly into metamorphic rocks, consisting of granular limestone, talc-schist, talcose-gneiss, micaceous schist, and other varieties.
As an illustration of the partial conversion into gneiss of portions of a highly inclined set of beds, I may cite Sir R. Murchison’s memoir on the structure of the Alps. Slates provincially termed “flysch” (see [p. 278]), overlying the nummulite limestone of Eocene date, and comprising some arenaceous and some calcareous layers, are seen to alternate several times with bands of granitoid rock, answering in character to gneiss. In this case heat, vapour, or water at a high temperature may have traversed the more permeable beds, and altered them so far as to admit of an internal movement and re-arrangement of the molecules, while the adjoining strata did not give passage to the same heated gases or water, or, if so, remained unchanged because they were composed of less fusible or decomposable materials. Whatever hypothesis we adopt, the phenomena establish beyond a doubt the possibility of the development of the metamorphic structure in a tertiary deposit in planes parallel to those of stratification. The strata appear clearly to have been affected, though in a less intense degree, by that same Plutonic action which has entirely altered and rendered metamorphic so many of the subjacent formations; for in the Alps this action has by no means been confined to the immediate vicinity of granite. Granite, indeed, and other Plutonic rocks, rarely make their appearance at the surface, notwithstanding the deep ravines which lay open to view the internal structure of these mountains. That they exist below at no great depth we cannot doubt, for at some points, as in the Valorsine, near Mont Blanc, granite and granitic veins are observable, piercing through talcose gneiss, which passes insensibly upward into secondary strata.
It is certainly in the Alps of Switzerland and Savoy, more than in any other district in Europe, that the geologist is prepared to meet with the signs of an intense development of Plutonic action; for here strata thousands of feet thick have been bent, folded, and overturned, and marine secondary formations of a comparatively modern date, such as the Oolitic and Cretaceous, have been upheaved to the height of 12,000, and some Eocene strata to elevations of 10,000 feet above the level of the sea; and even deposits of the Miocene era have been raised 4000 or 5000 feet, so as to rival in height the loftiest mountains in Great Britain. In one of the sections described by M. Studer in the highest of the Bernese Alps, namely in the Roththal, a valley bordering the line of perpetual snow on the northern side of the Jungfrau, there occurs a mass of gneiss 1000 feet thick, and 15,000 feet long, which I examined, not only resting upon, but also again covered by strata containing oolitic fossils. These anomalous appearances may partly be explained by supposing great solid wedges of intrusive gneiss to have been forced in laterally between strata to which I found them to be in many sections unconformable. The superposition, also, of the gneiss to the oolite may, in some cases, be due to a reversal of the original position of the beds in a region where the convulsions have been on so stupendous a scale.
Northern Apennines.—Carrara.—The celebrated marble of Carrara, used in sculpture, was once regarded as a type of primitive limestone. It abounds in the mountains of Massa Carrara, or the “Apuan Alps,” as they have been called, the highest peaks of which are nearly 6000 feet high. Its great antiquity was inferred from its mineral texture, from the absence of fossils, and its passage downward into talc-schist and garnetiferous mica-schist; these rocks again graduating downward into gneiss, which is penetrated, at Forno, by granite veins. But the researches of MM. Savi, Boué, Pareto, Guidoni, De la Beche, Hoffman, and Pilla demonstrated that this marble, once supposed to be formed before the existence of organic beings, is, in fact, an altered limestone of the Oolitic period, and the underlying crystalline schists are secondary sandstones and shales, modified by Plutonic action. In order to establish these conclusions it was first pointed out that the calcareous rocks bordering the Gulf of Spezia, and abounding in Oolitic fossils, assume a texture like that of Carrara marble, in proportion as they are more and more invaded by certain trappean and Plutonic rocks, such as diorite, serpentine, and granite, occurring in the same country.
It was then observed that, in places where the secondary formations are unaltered, the uppermost consist of common Apennine limestone with nodules of flint, below which are shales, and at the base of all, argillaceous and siliceous sandstones. In the limestone fossils are frequent, but very rare in the underlying shale and sandstone. Then a gradation was traced laterally from these rocks into another and corresponding series, which is completely metamorphic; for at the top of this we find a white granular marble, wholly devoid of fossils, and almost without stratification, in which there are no nodules of flint, but in its place siliceous matter disseminated through the mass in the form of prisms of quartz. Below this, and in place of the shales, are talc-schists, jasper, and hornstone; and at the bottom, instead of the siliceous and argillaceous sandstones, are quartzite and gneiss.[[1]] Had these secondary strata of the Apennines undergone universally as great an amount of transmutation, it would have been impossible to form a conjecture respecting their true age; and then, according to the method of classification adopted by the earlier geologists, they would have ranked as primary rocks. In that case the date of their origin would have been thrown back to an era antecedent to the deposition of the Lower Silurian or Cambrian strata, although in reality they were formed in the Oolitic period, and altered at some subsequent and perhaps much later epoch.
Metamorphic Strata of older date than the Silurian and Cambrian Rocks.—It was remarked ([Fig. 617]) that as the hypogene rocks, both stratified and unstratified, crystallise originally at a certain depth beneath the surface, they must always, before they are upraised and exposed at the surface, be of considerable antiquity, relatively to a large portion of the fossiliferous and volcanic rocks. They may be forming at all periods; but before any of them can become visible, they must be raised above the level of the sea, and some of the rocks which previously concealed them must have been removed by denudation.
![[Fig. 617]](#5745294564006620949_fig617.jpg.id-338624773112595473.wrap-0.html.html){kind=link}
In Canada, as we have seen ([p. 491]), the Lower Laurentian gneiss, quartzite, and limestone may be regarded as metamorphic, because, among other reasons, organic remains (Eozoon Canadense) have been detected in a part of one of the calcareous masses. The Upper Laurentian or Labrador series lies unconformably upon the Lower, and differs from it chiefly in having as yet yielded no fossils. It consists of gneiss with Labrador-feldspar and feldstones, in all 10,000 feet thick, and both its composition and structure lead us to suppose that, like the Lower Laurentian, it was originally of sedimentary origin and owes its crystalline condition to metamorphic action. The remote date of the period when some of these old Laurentian strata of Canada were converted into gneiss may be inferred from the fact that pebbles of that rock are found in the overlying Huronian formation, which is probably of Cambrian age ([p. 490]).
The oldest stratified rock of Scotland is the hornblendic gneiss of Lewis, in the Hebrides, and that of the north-west coast of Ross-shire, represented at the base of the section given at [Fig. 82]. It is the same as that intersected by numerous granite veins which forms the cliffs of Cape Wrath, in Sutherlandshire (see [ Fig. 613]), and is conjectured to be of Laurentian age. Above it, as shown in the section ([Fig. 82]), lie unconformable beds of a reddish or purple sandstone and conglomerate, nearly horizontal, and between 3000 and 4000 feet thick. In these ancient grits no fossils have been found, but they are supposed to be of Cambrian date, for Sir R. Murchison found Lower Silurian strata resting unconformably upon them. These strata consist of quartzite with annelid burrows already alluded to ([p. 112]), and limestone in which Mr. Charles Peach was the first to find, in 1854, three or four species of Orthoceras, also the genera Cyrtoceras and Lituites, two species of Murchisonia, a Pleurotomaria, a species of Maclurea, one of Euomphalus, and an Orthis. Several of the species are believed by Mr. Salter to be identical with Lower Silurian fossils of Canada and the United States.
![[Fig. 82]](#5745294564006620949_fig82.jpg.id-2108604346708091135.wrap-0.html.html){kind=link}
![[ Fig. 613]](#5745294564006620949_fig613.jpg.id-3610590110285031402.wrap-0.html.html){kind=link}
![[Fig. 82]](#5745294564006620949_fig82.jpg.id-9112332131363290555.wrap-0.html.html){kind=link}
The discovery of the true age of these fossiliferous rocks was one of the most important steps made of late years in the progress of British Geology, for it led to the unexpected conclusion that all the Scotch crystalline strata to the eastward, once called primitive, which overlie the limestone and quartzite in question, are referable to some part of the Silurian series.
These Scotch metamorphic strata are of gneiss, mica-schist, and clay-slate of vast thickness, and having a strike from north-east to south-west almost at right angles to that of the older Laurentian gneiss before mentioned. The newer crystalline series, comprising the crystalline rocks of Aberdeenshire, Perthshire, and Forfarshire, were inferred by Sir R. Murchison to be altered Silurian strata; and his opinion has been since confirmed by the observations of three able geologists, Messrs. Ramsay, Harkness, and Geikie. The newest of the series is a clay-slate, on which, along the southern borders of the Grampians, the Lower Old Red, containing Cephalaspis Lyelli, Pterygotus Anglicus, and Parka decipiens, rests unconformably.
Order of Succession in Metamorphic Rocks.—There is no universal and invariable order of superposition in metamorphic rocks, although a particular arrangement may prevail throughout countries of great extent, for the same reason that it is traceable in those sedimentary formations from which crystalline strata are derived. Thus, for example, we have seen that in the Apennines, near Carrara, the descending series, where it is metamorphic, consists of, first, saccharine marble; second, talcose-schist; and third, of quartz-rock and gneiss: where unaltered, of, first, fossiliferous limestone; second, shale; and third, sandstone.
But if we investigate different mountain chains, we find gneiss, mica-schist, hornblende-schist, chlorite-schist, hypogene limestone, and other rocks, succeeding each other, and alternating with each other in every possible order. It is, indeed, more common to meet with some variety of clay-slate forming the uppermost member of a metamorphic series than any other rock; but this fact by no means implies, as some have imagined, that all clay-slates were formed at the close of an imaginary period when the deposition of the crystalline strata gave way to that of ordinary sedimentary deposits. Such clay-slates, in fact, are variable in composition, and sometimes alternate with fossiliferous strata, so that they may be said to belong almost equally to the sedimentary and metamorphic order of rocks. It is probable that, had they been subjected to more intense Plutonic action, they would have been transformed into hornblende-schist, foliated chlorite-schist, scaly talcose-schist, mica-schist, or other more perfectly crystalline rocks, such as are usually associated with gneiss.
Uniformity of Mineral Character in Hypogene Rocks.—It is true, as Humboldt has happily remarked, that when we pass to another hemisphere, we see new forms of animals and plants, and even new constellations in the heavens; but in the rocks we still recognise our old acquaintances—the same granite, the same gneiss, the same micaceous schist, quartz-rock, and the rest. There is certainly a great and striking general resemblance in the principal kinds of hypogene rocks in all countries, however different their ages; but each of them, as we have seen, must be regarded as geological families of rocks, and not as definite mineral compounds. They are more uniform in aspect than sedimentary strata, because these last are often composed of fragments varying greatly in form, size, and colour, and contain fossils of different shapes and mineral composition, and acquire a variety of tints from the mixture of various kinds of sediment. The materials of such strata, if they underwent metamorphism, would be subject to chemical laws, simple and uniform in their action, the same in every climate, and wholly undisturbed by mechanical and organic causes. It would, however, be a great error to assume, as some have done, that the hypogene rocks, considered as aggregates of simple minerals, are really more homogeneous in their composition than the several members of the sedimentary series. Not only do the proportional quantities of feldspar, quartz, mica, hornblende, and other minerals, vary in hypogene rocks bearing the same name; but what is still more important, the ingredients, as we have seen, of the same simple mineral are not always constant (see [ p. 503] and table, [p. 499]).
Supposed Azoic Period.—The total absence of any trace of fossils has inclined many geologists to attribute the origin of the most ancient strata to an azoic period, or one antecedent to the existence of organic beings. Admitting, they say, the obliteration, in some cases, of fossils by Plutonic action, we might still expect that traces of them would oftener be found in certain ancient systems of slate which can scarcely be said to have assumed a crystalline structure. But in urging this argument it seems to have been forgotten that there are stratified formations of enormous thickness, and of various ages, some of them even of Tertiary date, and which we know were formed after the earth had become the abode of living creatures, which are, nevertheless, in some districts, entirely destitute of all vestiges of organic bodies. In some, the traces of fossils may have been effaced by water and acids, at many successive periods; indeed the removal of the calcareous matter of fossil shells is proved by the fact of such organic remains being often replaced by silex or other minerals, and sometimes by the space once occupied by the fossil being left empty, or only marked by a faint impression.
Those who believed the hypogene rocks to have originated antecedently to the creation of organic beings, imputed the absence of lime, so remarkable in metamorphic strata, to the non-existence of those mollusca and zoophytes by which shells and corals are secreted; but when we ascribe the crystalline formations to Plutonic action, it is natural to inquire whether this action itself may not tend to expel carbonic acid and lime from the materials which it reduces to fusion or semi-fusion. Not only carbonate of lime, but also free carbonic acid gas, is given off plentifully from the soil and crevices of rocks in regions of active and spent volcanoes, as near Naples and in Auvergne. By this process, fossil shells or corals may often lose their carbonic acid, and the residual lime may enter into the composition of augite, hornblende, garnet, and other hypogene minerals. Although we cannot descend into the subterranean regions where volcanic heat is developed, we can observe in regions of extinct volcanoes, such as Auvergne and Tuscany, hundreds of springs, both cold and thermal, flowing out from granite and other rocks, and having their waters plentifully charged with carbonate of lime.
If all the calcareous matter transferred in the course of ages by these and thousands of other springs from the lower part of the earth’s crust to the atmosphere could be presented to us in a solid form, we should find that its volume was comparable to that of many a chain of hills. Calcareous matter is poured into lakes and the ocean by a thousand springs and rivers; so that part of almost every new calcareous rock chemically precipitated, and of many reefs of shelly and coralline stone, must be derived from mineral matter subtracted by Plutonic agency, and driven up by gas and steam from fused and heated rocks in the bowels of the earth.
The scarcity of limestone in many extensive regions of metamorphic rocks, as in the Eastern and Southern Grampians of Scotland, may have been the result of some action of this kind; and if the limestones of the Lower Laurentian in Canada afford a remarkable exception to the general rule, we must not forget that it is precisely in this most ancient formation that the Eozoon Canadense has been found. The fact that some distinct bands of limestone from 700 to 1500 feet thick occur here, may be connected with the escape from destruction of some few traces of organic life, even in a rock in which metamorphic action has gone so far as to produce serpentine, augite, and other minerals found largely intermixed with the carbonate of lime.
[1] See notices of Savi, Hoffman, and others, referred to by Boué, Bull. de la Soc. Géol. de France, tome v, p. 317 and tome iii, p. 44; also Pilla, cited by Murchison, Quart. Geol. Journ., vol. v, p. 266.
CHAPTER XXXVI.
MINERAL VEINS.
Different Kinds of mineral Veins. — Ordinary metalliferous Veins or Lodes. — Their frequent Coincidence with Faults. — Proofs that they originated in Fissures in solid Rock. — Veins shifting other Veins. — Polishing of their Walls or “Slicken sides.” Shells and Pebbles in Lodes. — Evidence of the successive Enlargement and Reopening of veins. — Examples in Cornwall and in Auvergne. — Dimensions of Veins. — Why some alternately swell out and contract. — Filling of Lodes by Sublimation from below. — Supposed relative Age of the precious Metals. — Copper and lead Veins in Ireland older than Cornish Tin. — Lead Vein in Lias, Glamorganshire. — Gold in Russia, California, and Australia. — Connection of hot Springs and mineral Veins.
The manner in which metallic substances are distributed through the earth’s crust, and more especially the phenomena of those more or less connected masses of ore called mineral veins, from which the larger part of the precious metals used by man are obtained, are subjects of the highest practical importance to the miner, and of no less theoretical interest to the geologist.
On different Kinds of Mineral Veins.—The mineral veins with which we are most familiarly acquainted are those of quartz and carbonate of lime, which are often observed to form lenticular masses of limited extent traversing both hypogene strata and fossiliferous rocks. Such veins appear to have once been chinks or small cavities, caused, like cracks in clay, by the shrinking of the mass, during desiccation, or in passing from a higher to a lower temperature. Siliceous, calcareous, and occasionally metallic matters have sometimes found their way simultaneously into such empty spaces, by infiltration from the surrounding rocks. Mixed with hot water and steam, metallic ores may have permeated the mass until they reached those receptacles formed by shrinkage, and thus gave rise to that irregular assemblage of veins, called by the Germans a “stockwerk,” in allusion to the different floors on which the mining operations are in such cases carried on.
The more ordinary or regular veins are usually worked in vertical shafts, and have evidently been fissures produced by mechanical violence. They traverse all kinds of rocks, both hypogene and fossiliferous, and extend downward to indefinite or unknown depths. We may assume that they correspond with such rents as we see caused from time to time by the shock of an earthquake. Metalliferous veins referable to such agency are occasionally a few inches wide, but more commonly three or four feet. They hold their course continuously in a certain prevailing direction for miles or leagues, passing through rocks varying in mineral composition.
That Metalliferous Veins were Fissures.—As some intelligent miners, after an attentive study of metalliferous veins, have been unable to reconcile many of their characteristics with the hypothesis of fissures, I shall begin by stating the evidence in its favour. The most striking fact, perhaps, which can be adduced in its support is, the coincidence of a considerable proportion of mineral veins with faults, or those dislocations of rocks which are indisputably due to mechanical force, as above explained ([p. 87]). There are even proofs in almost every mining district of a succession of faults, by which the opposite walls of rents, now the receptacles of metallic substances, have suffered displacement. Thus, for example, suppose a a, Fig. 629, to be a tin lode in Cornwall, the term lode being applied to veins containing metallic ores. This lode, running east and west, is a yard wide, and is shifted by a copper lode (b b) of similar width. The first fissure (a a) has been filled with various materials, partly of chemical origin, such as quartz, fluor-spar, peroxide of tin, sulphuret of copper, arsenical pyrites, bismuth, and sulphuret of nickel, and partly of mechanical origin, comprising clay and angular fragments or detritus of the intersected rocks. The plates of quartz and the ores are, in some places, parallel to the vertical sides or walls of the vein, being divided from each other by alternating layers of clay or other earthy matter. Occasionally the metallic ores are disseminated in detached masses among the vein-stones.
It is clear that, after the gradual introduction of the tin and other substances, the second rent (b b) was produced by another fracture accompanied by a displacement of the rocks along the plane of b b. This new opening was then filled with minerals, some of them resembling those in a a, as fluor-spar (or fluate of lime) and quartz; others different, the copper being plentiful and the tin wanting or very scarce. We must next suppose a third movement to occur, breaking asunder all the rocks along the line c c, Fig. 630; the fissure, in this instance, being only six inches wide, and simply filled with clay, derived, probably, from the friction of the walls of the rent, or partly, perhaps, washed in from above. This new movement has displaced the rock in such a manner as to interrupt the continuity of the copper vein (b b), and, at the same time, to shift or heave laterally in the same direction a portion of the tin vein which had not previously been broken.
Again, in Fig. 631 we see evidence of a fourth fissure (d d), also filled with clay, which has cut through the tin vein (a a), and has lifted it slightly upward towards the south. The various changes here represented are not ideal, but are exhibited in a section obtained in working an old Cornish mine, long since abandoned, in the parish of Redruth, called Huel Peever, and described both by Mr. Williams and Mr. Carne.[[1]] The principal movement here referred to, or that of c c, Fig. 631, extends through a space of no less than 84 feet; but in this, as in the case of the other three, it will be seen that the outline of the country above, d, c, b, a, etc., or the geographical features of Cornwall, are not affected by any of the dislocations, a powerful denuding force having clearly been exerted subsequently to all the faults. (See [p. 93].) It is commonly said in Cornwall, that there are eight distinct systems of veins, which can in like manner be referred to as many successive movements or fractures; and the German miners of the Hartz Mountains speak also of eight systems of veins, referable to as many periods.
Besides the proofs of mechanical action already explained, the opposite walls of veins are often beautifully polished, as if glazed, and are not unfrequently striated or scored with parallel furrows and ridges, such as would be produced by the continued rubbing together of surfaces of unequal hardness. These smoothed surfaces resemble the rocky floor over which a glacier has passed (see [Fig. 106]). They are common even in cases where there has been no shift, and occur equally in non-metalliferous fissures. They are called by miners “slicken-sides,” from the German schlichten, to plane, and seite, side. It is supposed that the lines of the striæ indicate the direction in which the rocks were moved.
![[Fig. 106]](#5745294564006620949_fig106.jpg.id-3193720498996761840.wrap-0.html.html){kind=link}
In some of the veins in the mountain limestone of Derbyshire, containing lead, the vein-stuff, which is nearly compact, is occasionally traversed by what may be called a vertical crack passing down the middle of the vein. The two faces in contact are slicken-sides, well polished and fluted, and sometimes covered by a thin coating of lead-ore. When one side of the vein-stuff is removed, the other side cracks, especially if small holes be made in it, and fragments fly off with loud explosions, and continue to do so for some days. The miner, availing himself of this circumstance, makes with his pick small holes about six inches apart, and four inches deep, and on his return in a few hours finds every part ready broken to his hand.[[2]]
That a great many veins communicated originally with the surface of the country above, or with the bed of the sea, is proved by the occurrence in them of well-rounded pebbles, agreeing with those in superficial alluviums, as in Auvergne and Saxony. Marine fossil shells, also, have been found at great depths, having probably been ingulfed during submarine earthquakes. Thus, a gryphæa is stated by M. Virlet to have been met with in a lead-mine near Semur, in France, and a madrepore in a compact vein of cinnabar in Hungary.[[3]] In Bohemia, similar pebbles have been met with at the depth of 180 fathoms; and in Cornwall, Mr. Carne mentions true pebbles of quartz and slate in a tin lode of the Relistran Mine, at the depth of 600 feet below the surface. They were cemented by oxide of tin and bisulphuret of copper, and were traced over a space more than twelve feet long and as many wide.[[4]] When different sets or systems of veins occur in the same country, those which are supposed to be of contemporaneous origin, and which are filled with the same kind of metals, often maintain a general parallelism of direction. Thus, for example, both the tin and copper veins in Cornwall run nearly east and west, while the lead veins run north and south; but there is no general law of direction common to different mining districts. The parallelism of the veins is another reason for regarding them as ordinary fissures, for we observe that faults and trap dikes, admitted by all to be masses of melted matter which have filled rents, are often parallel.
Fracture, Re-opening and Successive Formation of Veins.—Assuming, then, that veins are simply fissures in which chemical and mechanical deposits have accumulated, we may next consider the proofs of their having been filled gradually and often during successive enlargements.
Werner observed, in a vein near Gersdorff, in Saxony, no less than thirteen beds of different minerals, arranged with the utmost regularity on each side of the central layer. This layer was formed of two plates of calcareous spar, which had evidently lined the opposite walls of a vertical cavity. The thirteen beds followed each other in corresponding order, consisting of fluor-spar, heavy spar, galena, etc. In these cases the central mass has been last formed, and the two plates which coat the walls of the rent on each side are the oldest of all. If they consist of crystalline precipitates, they may be explained by supposing the fissure to have remained unaltered in its dimensions, while a series of changes occurred in the nature of the solutions which rose up from below: but such a mode of deposition, in the case of many successive and parallel layers, appears to be exceptional.
If a vein-stone consist of crystalline matter, the points of the crystals are always turned inward, or towards the centre of the vein; in other words, they point in the direction where there was space for the development of the crystals. Thus each new layer receives the impression of the crystals of the preceding layer, and imprints its crystals on the one which follows, until at length the whole of the vein is filled: the two layers which meet dovetail the points of their crystals the one into the other. But in Cornwall, some lodes occur where the vertical plates, or combs, as they are there called, exhibit crystals so dovetailed as to prove that the same fissure has been often enlarged. Sir H. De la Beche gives the following curious and instructive example (Fig. 632), from a copper-mine in granite, near Redruth.[[5]] Each of the plates or combs (a, b, c, d, e, f) is double, having the points of their crystals turned inward along the axis of the comb. The sides or walls (2, 3, 4, 5 and 6) are parted by a thin covering of ochreous clay, so that each comb is readily separable from another by a moderate blow of the hammer. The breadth of each represents the whole width of the fissure at six successive periods, and the outer walls of the vein, where the first narrow rent was formed, consisted of the granitic surfaces 1 and 7.
A somewhat analogous interpretation is applicable to many other cases, where clay, sand, or angular detritus, alternate with ores and vein-stones. Thus, we may imagine the sides of a fissure to be incrusted with siliceous matter, as Von Buch observed, in Lancerote, the walls of a volcanic crater formed in 1731 to be traversed by an open rent in which hot vapours had deposited hydrate of silica, the incrustation nearly extending to the middle.[[6]] Such a vein may then be filled with clay or sand, and afterwards re-opened, the new rent dividing the argillaceous deposit, and allowing a quantity of rubbish to fall down. Various metals and spars may then be precipitated from aqueous solutions among the interstices of this heterogeneous mass.
That such changes have repeatedly occurred, is demonstrated by occasional cross-veins, implying the oblique fracture of previously formed chemical and mechanical deposits. Thus, for example, M. Fournet, in his description of some mines in Auvergne worked under his superintendence, observes that the granite of that country was first penetrated by veins of granite, and then dislocated, so that open rents crossed both the granite and the granitic veins. Into such openings, quartz, accompanied by sulphurets of iron and arsenical pyrites, was introduced. Another convulsion then burst open the rocks along the old line of fracture, and the first set of deposits were cracked and often shattered, so that the new rent was filled, not only with angular fragments of the adjoining rocks, but with pieces of the older vein-stones. Polished and striated surfaces on the sides or in the contents of the vein also attest the reality of these movements. A new period of repose then ensued, during which various sulphurets were introduced, together with hornstone quartz, by which angular fragments of the older quartz before mentioned were cemented into a breccia. This period was followed by other dilatations of the same veins, and the introduction of other sets of mineral deposits, as well as of pebbles of the basaltic lavas of Auvergne, derived from superficial alluviums, probably of Miocene or even Older Pliocene date. Such repeated enlargement and re-opening of veins might have been anticipated, if we adopt the theory of fissures, and reflect how few of them have ever been sealed up entirely, and that a country with fissures only partially filled must naturally offer much feebler resistance along the old lines of fracture than anywhere else.
Cause of alternate Contraction and Swelling of Veins.—A large proportion of metalliferous veins have their opposite walls nearly parallel, and sometimes over a wide extent of country. There is a fine example of this in the celebrated vein of Andreasburg in the Hartz, which has been worked for a depth of 500 yards perpendicularly, and 200 horizontally, retaining almost everywhere a width of three feet. But many lodes in Cornwall and elsewhere are extremely variable in size, being one or two inches in one part, and then eight or ten feet in another, at the distance of a few fathoms, and then again narrowing as before. Such alternate swelling and contraction is so often characteristic as to require explanation. The walls of fissures in general, observes Sir H. De la Beche, are rarely perfect planes throughout their entire course, nor could we well expect them to be so, since they commonly pass through rocks of unequal hardness and different mineral composition. If, therefore, the opposite sides of such irregular fissures slide upon each other, that is to say, if there be a fault, as in the case of so many mineral veins, the parallelism of the opposite walls is at once entirely destroyed, as will be readily seen by studying Figs. 633 to 635.
Let a b, Fig. 633, be a line of fracture traversing a rock, and let a b, Fig. 634, represent the same line. Now, if we cut in two a piece of paper representing this line, and then move the lower portion of this cut paper sideways from a to a′, taking care that the two pieces of paper still touch each other at the points 1, 2, 3, 4, 5, we obtain an irregular aperture at c, and isolated cavities at d, d, d, and when we compare such figures with nature we find that, with certain modifications, they represent the interior of faults and mineral veins. If, instead of sliding the cut paper to the right hand, we move the lower part towards the left, about the same distance that it was previously slid to the right, we obtain considerable variation in the cavities so produced, two long irregular open spaces, f, f, Fig. 635, being then formed. This will serve to show to what slight circumstances considerable variations in the character of the openings between unevenly fractured surfaces may be due, such surfaces being moved upon each other, so as to have numerous points of contact.
Most lodes are perpendicular to the horizon, or nearly so; but some of them have a considerable inclination or “hade,” as it is termed, the angles of dip being very various. The course of a vein is frequently very straight; but if tortuous, it is found to be choked up with clay, stones, and pebbles, at points where it departs most widely from verticality. Hence at places, such as a, Fig. 636, the miner complains that the ores are “nipped,” or greatly reduced in quantity, the space for their free deposition having been interfered with in consequence of the pre-occupancy of the lode by earthy materials. When lodes are many fathoms wide, they are usually filled for the most part with earthy matter, and fragments of rock, through which the ores are disseminated. The metallic substances frequently coat or encircle detached pieces of rock, which our miners call “horses” or “riders.” That we should find some mineral veins which split into branches is also natural, for we observe the same in regard to open fissures.
Chemical Deposits in Veins.—If we now turn from the mechanical to the chemical agencies which have been instrumental in the production of mineral veins, it may be remarked that those parts of fissures which were choked up with the ruins of fractured rocks must always have been filled with water; and almost every vein has probably been the channel by which hot springs, so common in countries of volcanoes and earthquakes, have made their way to the surface. For we know that the rents in which ores abound extend downward to vast depths, where the temperature of the interior of the earth is more elevated. We also know that mineral veins are most metalliferous near the contact of Plutonic and stratified formations, especially where the former send veins into the latter, a circumstance which indicates an original proximity of veins at their inferior extremity to igneous and heated rocks. It is moreover acknowledged that even those mineral and thermal springs which, in the present state of the globe, are far from volcanoes, are nevertheless observed to burst out along great lines of upheaval and dislocation of rocks.[[7]] It is also ascertained that all the substances with which hot springs are impregnated agree with those discharged in a gaseous form from volcanoes. Many of these bodies occur as vein-stones; such as silex, carbonate of lime, sulphur, fluor-spar, sulphate of barytes, magnesia, oxide of iron, and others. I may add that, if veins have been filled with gaseous emanations from masses of melted matter, slowly cooling in the subterranean regions, the contraction of such masses as they pass from a plastic to a solid state would, according to the experiments of Deville on granite (a rock which may be taken as a standard), produce a reduction in volume amounting to 10 per cent. The slow crystallisation, therefore, of such Plutonic rocks supplies us with a force not only capable of rending open the incumbent rocks by causing a failure of support, but also of giving rise to faults whenever one portion of the earth’s crust subsides slowly while another contiguous to it happens to rest on a different foundation, so as to remain unmoved.
Although we are led to infer, from the foregoing reasoning, that there has often been an intimate connection between metalliferous veins and hot springs holding mineral matter in solution, yet we must not on that account expect that the contents of hot springs and mineral veins would be identical. On the contrary, M. E. de Beaumont has judiciously observed that we ought to find in veins those substances which, being least soluble, are not discharged by hot springs—or that class of simple and compound bodies which the thermal waters ascending from below would first precipitate on the walls of a fissure, as soon as their temperature began slightly to diminish. The higher they mount towards the surface, the more will they cool, till they acquire the average temperature of springs, being in that case chiefly charged with the most soluble substances, such as the alkalies, soda and potash. These are not met with in veins, although they enter so largely into the composition of granitic rocks.[[8]]
To a certain extent, therefore, the arrangement and distribution of metallic matter in veins may be referred to ordinary chemical action, or to those variations in temperature which waters holding the ores in solution must undergo, as they rise upward from great depths in the earth. But there are other phenomena which do not admit of the same simple explanation. Thus, for example, in Derbyshire, veins containing ores of lead, zinc, and copper, but chiefly lead, traverse alternate beds of limestone and greenstone. The ore is plentiful where the walls of the rent consist of limestone, but is reduced to a mere string when they are formed of greenstone, or “toad-stone,” as it is called provincially. Not that the original fissure is narrower where the greenstone occurs, but because more of the space is there filled with vein-stones, and the waters at such points have not parted so freely with their metallic contents.
“Lodes in Cornwall,” says Mr. Robert W. Fox, “are very much influenced in their metallic riches by the nature of the rock which they traverse, and they often change in this respect very suddenly, in passing from one rock to another. Thus many lodes which yield abundance of ore in granite, are unproductive in clay-slate, or killas and vice versa.
Supposed relative Age of the different Metals.—After duly reflecting on the facts above described, we cannot doubt that mineral veins, like eruptions of granite or trap, are referable to many distinct periods of the earth’s history, although it may be more difficult to determine the precise age of veins; because they have often remained open for ages, and because, as we have seen, the same fissure, after having been once filled, has frequently been re-opened or enlarged. But besides this diversity of age, it has been supposed by some geologists that certain metals have been produced exclusively in earlier, others in more modern times; that tin, for example, is of higher antiquity than copper, copper than lead or silver, and all of them more ancient than gold. I shall first point out that the facts once relied upon in support of some of these views are contradicted by later experience, and then consider how far any chronological order of arrangement can be recognised in the position of the precious and other metals in the earth’s crust.
In the first place, it is not true that veins in which tin abounds are the oldest lodes worked in Great Britain. The government survey of Ireland has demonstrated that in Wexford veins of copper and lead (the latter as usual being argentiferous) are much older than the tin of Cornwall. In each of the two countries a very similar series of geological changes has occurred at two distinct epochs—in Wexford, before the Devonian strata were deposited; in Cornwall, after the Carboniferous epoch. To begin with the Irish mining district: We have granite in Wexford traversed by granite veins, which veins also intrude themselves into the Silurian strata, the same Silurian rocks as well as the veins having been denuded before the Devonian beds were superimposed. Next we find, in the same county, that elvans, or straight dikes of porphyritic granite, have cut through the granite and the veins before mentioned, but have not penetrated the Devonian rocks. Subsequently to these elvans, veins of copper and lead were produced, being of a date certainly posterior to the Silurian, and anterior to the Devonian; for they do not enter the latter, and, what is still more decisive, streaks or layers of derivative copper have been found near Wexford in the Devonian, not far from points where mines of copper are worked in the Silurian strata.
Although the precise age of such copper lodes cannot be defined, we may safely affirm that they were either filled at the close of the Silurian or commencement of the Devonian period. Besides copper, lead, and silver, there is some gold in these ancient or primary metalliferous veins. A few fragments also of tin found in Wicklow in the drift are supposed to have been derived from veins of the same age.[[9]]
Next, if we turn to Cornwall, we find there also the monuments of a very analogous sequence of events. First, the granite was formed; then, about the same period, veins of fine-grained granite, often tortuous (see [Fig. 614]), penetrating both the outer crust of granite and the adjoining fossiliferous or primary rocks, including the coal-measures; thirdly, elvans, holding their course straight through granite, granitic veins, and fossiliferous slates; fourthly, veins of tin also containing copper, the first of those eight systems of fissures of different ages already alluded to, p. 607. Here, then, the tin lodes are newer than the elvans. It has, indeed, been stated by some Cornish miners that the elvans are in some instances posterior to the oldest tin-bearing lodes, but the observations of Sir H. de la Beche during the survey led him to an opposite conclusion, and he has shown how the cases referred to in corroboration can be otherwise interpreted.[[10]] We may, therefore, assert that the most ancient Cornish lodes are younger than the coal-measures of that part of England, and it follows that they are of a much later date than the Irish copper and lead of Wexford and some adjoining counties. How much later, it is not so easy to declare, although probably they are not newer than the beginning of the Permian period, as no tin lodes have been discovered in any red sandstone which overlies the coal in the south-west of England.
![[Fig. 614]](#5745294564006620949_fig614.jpg.id-2428354163904719727.wrap-0.html.html){kind=link}
There are lead veins in Glamorganshire which enter the lias, and others near Frome, in Somersetshire, which have been traced into the Inferior Oolite. In Bohemia, the rich veins of silver of Joachimsthal cut through basalt containing olivine, which overlies tertiary lignite, in which are leaves of dicotyledonous trees. This silver, therefore, is decidedly a tertiary formation. In regard to the age of the gold of the Ural mountains, in Russia, which, like that of California, is obtained chiefly from auriferous alluvium, it occurs in veins of quartz in the schistose and granitic rocks of that chain, and is supposed by Sir R. Murchison, MM. Deverneuil and Keyserling to be newer than the syenitic granite of the Ural—perhaps of tertiary date. They observe that no gold has yet been found in the Permian conglomerates which lie at the base of the Ural Mountains, although large quantities of iron and copper detritus are mixed with the pebbles of those Permian strata. Hence it seems that the Uralian quartz veins, containing gold and platinum, were not formed, or certainly not exposed to aqueous denudation, during the Permian era.
In the auriferous alluvium of Russia, California, and Australia, the bones of extinct land-quadrupeds have been met with, those of the mammoth being common in the gravel at the foot of the Ural Mountains, while in Australia they consist of huge marsupials, some of them of the size of the rhinoceros and allied to the living wombat. They belong to the genera Diprotodon and Nototherium of Professor Owen. The gold of Northern Chili is associated in the mines of Los Hornos with copper pyrites, in veins traversing the cretaceo-oolitic formations, so-called because its fossils have the character partly of the cretaceous and partly of the oolitic fauna of Europe.[[11]] The gold found in the United States, in the mountainous parts of Virginia, North and South Carolina, and Georgia, occurs in metamorphic Silurian strata, as well as in auriferous gravel derived from the same.
Gold has now been detected in almost every kind of rock, in slate, quartzite, sandstone, limestone, granite, and serpentine, both in veins and in the rocks themselves at short distances from the veins. In Australia it has been worked successfully not only in alluvium, but in vein-stones in the native rock, generally consisting of Silurian shales and slates. It has been traced on that continent over more than nine degrees of latitude (between the parallels of 30° and 39° S.), and over twelve of longitude, and yielded in 1853 an annual supply equal, if not superior, to that of California; nor is there any apparent prospect of this supply diminishing, still less of the exhaustion of the gold-fields.
Origin of Gold in California.—Mr. J. Arthur Phillips,[[12]] in his treatise “On the Gold Fields of California,” has shown that the ore in the gold workings is derived from drifts, or gravel clay, and sand, of two distinct geological ages, both comparatively modern, but belonging to different river-systems, the older of which is so ancient as to be capped by a thick sheet of lava divided by basaltic columns. The auriferous quartz of these drifts is derived from veins apparently due to hydrothermal agency, proceeding from granite and penetrating strata supposed to be of Jurassic and Triassic date. The fossil wood of the drift is sometimes beautifully silicified, and occasionally the trunks of trees are replaced by iron pyrites, but gold seems not to have been found as in the pyrites of similarly petrified trees in the drift of Australia.
The formation of recent metalliferous veins is now going on, according to Mr. Phillips, in various parts of the Pacific coast. Thus, for example, there are fissures at the foot of the eastern declivity of the Sierra Nevada in the state of that name, from which boiling water and steam escape, forming siliceous incrustations on the sides of the fissures. In one case, where the fissure is partially filled up with silica inclosing iron and copper pyrites, gold has also been found in the vein-stone.
It has been remarked by M. de Beaumont, that lead and some other metals are found in dikes of basalt and greenstone, as well as in mineral veins connected with trap-rock, whereas tin is met with in granite and in veins associated with the Plutonic series. If this rule hold true generally, the geological position of tin accessible to the miner will belong, for the most part, to rocks older than those bearing lead. The tin veins will be of higher relative antiquity for the same reason that the “underlying” igneous formations or granites which are visible to man are older, on the whole, than the overlying or trappean formations.
If different sets of fissures, originating simultaneously at different levels in the earth’s crust, and communicating, some of them with volcanic, others with heated Plutonic masses, be filled with different metals, it will follow that those formed farthest from the surface will usually require the longest time before they can be exposed superficially. In order to bring them into view, or within reach of the miner, a greater amount of upheaval and denudation must take place in proportion as they have lain deeper when first formed and filled. A considerable series of geological revolutions must intervene before any part of the fissure which has been for ages in the proximity of the Plutonic rock, so as to receive the gases discharged from it when it was cooling, can emerge into the atmosphere. But I need not enlarge on this subject, as the reader will remember what was said in the 30th, 32nd, and 35th chapters on the chronology of the volcanic and hypogene formations.
[1] Geol. Trans., vol. iv, p. 139; Trans. Royal Geol. Society, Cornwall, vol. ii, p. 90
[2] Conybeare and Phil. Geol., p. 401, and Farey’s Derbyshire, p. 243.
[3] Fournet, Études sur les Dépôts Métallifères.
[4] Carne, Trans. Geol. Soc., Cornwall, vol. iii, p. 238.
[5] Geol. Rep. on Cornwall, p. 340.
[6] Principles, chap. xxvii, 8th edit., p. 422.
[7] See Dr. Daubeny’s Volcanoes.
[8] Bulletin, iv, p. 1278.
[9] Sir H. De la Beche, MS. Notes on Irish Survey.
[10] Report on Geology of Cornwall, p. 310.
[11] Darwin’s South America, p. 209, etc.
[12] Proc. Royal Soc., 1868, p. 294.
INDEX.
——::——
The Fossils, the names of which appear in Italics, are figured in the Text.
ABBEVILLE, flint tools of, [152]
Aberdeenshire, granite of, [558]
Abich, M., on trachytic rocks, [504]
Acer trilobatum, Miocene, [220], [221]
Acrodus nobilis, Lias, [359]
Acrogens, term explained, [303]
Acrolepis Sedgwickii, Permian, [390]
Actæon acutus, Great Oolite, [345]
Actinocyclas, in Atlantic mud, [288]
Actinolite, [499], [502]
—— schist, [578]
Æchmodus Leachii, Lias, [358]
Adiantites Hibernica, Old Red, [441]
Agassiz on fish of Sheppey, [267]
—— on fish of the Brown-Coal, [540]
—— on fish of Monte Bolca, [544]
—— on Old Red fossil fish, [443], [447]
—— on Silurian fish, [460]
Age of metamorphic rocks, [597]
—— of Plutonic rocks, [564]
—— of strata, tests of, [123]
—— of volcanic rocks, [520]
Agglomerate described, [509]
Agnostus integer. A. Rex, [488]
Air-breathers of the Coal, [413]
Aix-la-Chapelle, Cretaceous flora of, [302]
Alabaster defined, [39]
Alberti on Keuper, [376]
Albite, [499], [500]
Aldeby and Chillesford beds, [192]
Alkali, present in the Palæozoic strata, [587]
Alpine blocks on the Jura, [169]
Alps, age of metamorphic rocks in, [599]
——, nummulitic limestone and flysch of, [77]
Alum schists of Norway and Sweden, [489]
Alluvial deposits, Recent and Post-pliocene, [151]
Alluvium, term explained, [99]
—— in Auvergne, [100]
Alternations of marine and fresh-water strata, [72]
Alum Bay beds, plants of the, [262]
Amblyrhynchus cristatus, a living marine saurian, [362]
America. See United States, Canada, Nova Scotia.
——, North, Glacial formations of, [182]
——, South, gradual rise of land in, [72]
——, Silurian strata of, [478]
American character of Lower Miocene flora, [238]
—— forms in Swiss Miocene flora, [223]
Amiens, flint tools of, [152]
Ammonites bifrons, Lias, [356]
—— Braikenridgii, Oolite, [351]
—— Bucklandi, Lias, [356]
—— Deshayesii, Neocomian, [311]
—— Humphresianus, Inferior Oolite, [351]
—— Jason, Oxford Clay, [340]
—— Noricus, Speeton, [312]
—— macrocephalus, Oolite, [352]
—— margaritatus, Lias, [357]
—— planorbis, Lias, [356]
—— rhotomagensis, Chalk marl, [298]
Amphibole group of minerals, [499], [502]
Amphistegina Hauerina, Vienna basin, [225]
Amphitherium Broderipii, in Stonesfield, [348]
—— Prevostii, Stonesfield slate, [347]
Ampullaria glauca, [56]
Amygdaloid, [507]
Analcime, [500]
Anamesite, a variety of basalt, [504]
Ananchytes ovatus, White chalk, [293]
——, with crania attached, [49]
Ancillaria subulata, Eocene, [57]
Ancyloceras gigas, [309]
—— spinigerum, Gault, [301]
—— Duvallei, Neocomian, [312]
Ancylus velletia (A. elegans), [55]
Andalusite, [500]
Andes, Plutonic rocks of the, [569]
Andreasburg, metalliferous vein of, [611]
Angelin, on Cambrian of Sweden, [489]
Angiosperms, [303]
—— of the Coal, [429]
Anglesea, dike cutting through shale in, [514]
Anodonta Cordierii, [54]
—— Jukesii, Upper Old Red, [441]
—— latimarginata, [54]
Anoplotherium commune, Binstead, [254]
—— gracile, Paris basin, [271]
Anorthite, [499], [501]
Annularia sphenophylloides, Coal, [425]
Antholithes, coal-measures, [429]
Anthracite, conversion of coal into, [408]
Anticlinal and synclinal curves, [74], [85]
Antrim, Chalk altered by a dike in, [516]
——, Lower Miocene, volcanic rocks of, [539]
Antwerp Crag, [204]
Apateon pedestris, a carboniferous reptile, [406]
Apatite, [500]
Apennines, Northern, metamorphic rocks of, [599]
Apes, fossil of the Upper Miocene, [215]
Apiocrinites rotundus, Bradford, [343]
Appalachians, long lines of flexures in, [92], [93]
——, vast thickness of successive strata in, [110]
Aptychus, part of ammonite, [336]
Aqueous rocks defined, [27], [35]
Araucaria sphærocarpa, Inferior Oolite, [348]
Arbroath, section of Old Red at, [74]
Archæopteryx macrura, Solenhofen, [338]
Archegosaurus minor and A. medius, coal measures, [406], [407]
Archiac, M. de, on nummulites, [277]
——, on chalk of France, [306]
Arctic Miocene Flora, [239]
Area of the Wealden, [319]
Areas, permanence of continental, [117]
Arenaceous rocks described, [35]
Arenicolites linearis, Arenig beds, [475]
Arenig or Stiper-Stones group, [474]
——, volcanic formations of, [549]
Argile plastique, [276]
Argillaceous rocks described, [36]
Argillite, Argillaceous schist, [579]
Argyll, Duke of, on Isle of Mull leaf-beds, [247]
Armagh, bone-beds in Mountain Limestone at, [437]
Arran, amygdaloid filled with spar near, [518]
——, erect trees in volcanic ash of, [546]
——, Greenstone dike in, [514]
Arthur’s seat, trap rocks of, [545]
Arvicola, tooth of, [165]
Asaphus caudatus, Silurian, [467]
—— tyrannus, A. Buchii, [474]
Ascension, lamination of volcanic rocks in, [595]
Ash, Mr., on fossils of Tremadoc beds, [483]
Ashby-de-la-Zouch, fault in coal field of, [91]
Aspidura loricata, Muschelkalk, [379]
Astarte borealis (=A. arctica=A. compressa), [176]
—— Omalii, Crag, [199]
Asterophyllites foliosus, Coal, [425]
Astrangia lineata (Anthophyllum lineatum), [229]
Astræa basaltiforme, Carboniferous, [432]
Astropecten crispatus, London clay, [266]
Atherfield clay, [309]
Atlantic mud, composition of, [287]
Atrypa reticularis, Aymestry, [462]
Aturia ziczac (Nautilus ziczac), [266]
Augite, [499], [502]
Auricula, recent, [55]
Austen, Mr. Godwin, on marine deposit of Selsea Bill, [182]
——, on boulders in chalk, [292]
Australian cave breccias, [158]
Australia, auriferous gravel of, [617]
Auvergne, alluvium in, [100]
——, chain of extinct volcanoes in, [495]
——, granite veins in, [610]
——, Lower Miocene of, [233]
——, Miocene volcanic rocks of, [540]
——, Post-pliocene volcanic eruptions in, [527]
——, springs from spent volcanoes in, [604]
Aveline Mr., on Tarannon shales, [468]
Avicula contorta, Rhætic beds, [366]
—— cygnipes, Lias, [355]
—— inæquivalvis, Lias, [355]
—— socialis, Muschelkalk, [379]
Aviculopecten papyraceus, coal measures, [405]
—— sublobatus, mountain limestone, [434]
Aymestry Limestone, [461]
Azoic period, supposed, [603]
Azores, Miocene lavas with shells, [539]
BACILLARIA paradoxa, [51]
Baculites anceps, Lower Chalk, [298]
—— Fauiasii, chalk, [286]
Baffin’s Bay, formation of drift in, [171], [173]
Bagshot sands, [258], [259], [262]
Baiæ, Bay of, subterranean igneous action in, [569]
Bakewell, Mr., on cleavage in Swiss Alps, [590]
Bala and Caradoc beds, [470]
Balistidæ, defensive spine of, [261]
Bangor, or Longmynd group, [485]
Banksia, seed and fruit of, Lower Miocene, [238]
Barmouth sandstones, [486]
Barnes, Mr. J., on insects in American coal, [416]
Barnstaple, Upper Devonian of, [450]
Barrande, M. Joachim, his “Primordial Zone,” [471], [482], [487]
——, on metamorphosis of trilobites, [471]
Barrett, Mr., on bird in Blackdown beds, [299]
Barton series sands and clays, [258]
—— shells, percentage of, common to London clay, [258]
Basalt, columnar, [511]
——, composition of, [504]
Basaltic rocks, poor in silica, [504]
——, specific gravity of minerals in, [504]
Basilosaurus, Eocene, United States, [280]
Basset, term explained, [83]
Basterot, M. de, on Bordeaux tertiary strata, [141]
Bath Oolite, [342]
Batrachian reptiles in coal, [406]
Bay of Fundy, denudation in coalfield in, [418]
Bean, Mr., on Yorkshire Oolite, [350]
Bear Island carboniferous flora, [441]
Beaumont, M. E. de, on island in Cretaceous sea, [305]
——, on mineral veins, [613]
——, on Jurassic plutonic rocks, [571]
——, on formation of granite, [553]
Beckles, Mr. S. H., on footprints in Hastings sands, [315], [330]
—— on Mammalia of Purbeck, [326]
Belemnitella mucronata, Chalk, [283]
Belemnites hastatus, Oxford clay, [340]
—— Puzosianus, Oxford clay, [341]
Belgium, Lower Miocene of, [241]
Bellerophon costatus, Mountain Limestone, [436]
Belosepia sepioidea, Sheppey, [266]
Belt, Mr., on subdivision of Lingula Flags, [484]
Bembridge beds, Yarmouth, [252]
Berger, Dr., on rocks altered by dikes, [515]
Berlin, Miocene strata near, [242]
Bernese Alps, gneiss in the, [599]
Berthier on isomorphism, [502]
Bertrich-Baden, columnar basalt of, [512]
Beyrich on term Oligocene for Lower Miocene, [244]
Billings, Mr., on trilobites, [471]
Binney, Mr., on Sigillariæ in volcanic ash, [546]
——, on Stigmaria, the root of Sigillaria, [426]
Biotite, [499], [501]
Bird in argile plastique, [276]
Bischoff, Professor, on Nile and Rhine mud, [154]
——, on conversion of coal into anthracite, [403]
——, on hydrothermal action, [586]
Blackdown beds, [301]
Blacklead of Borrowdale, [65]
Bog-iron-ore, [52]
Bohemia, Cambrian rocks of, [487]
——, silver veins in, [616]
Bolderberg, in Belgium, Upper Miocene of, [224]
Bone-bed of fish remains, Armagh, [437]
—— of Upper Ludlow, [450]
—— of the Trias, [367]
Boom, Lower Miocene of, [241]
Bordeaux, Upper Miocene of, [214]
Borrowdale, blacklead of, [65]
Bosquet, M. on chalk fossils, [283]
——, on Maestricht beds, [283]
Botanical nomenclature, [303]
Boucher de Perthes on Abbeville alluvium, [152]
Boulder-clay, whether formed by icebergs or land-ice, [166-73], [178]
Boulder-clay of Canada, [182]
—— fauna of, [176], [189]
Boulders and pebbles in chalk, [292]
Bournemouth beds (Lower Bagshot), [262]
Bovey Tracey, lignites and clays of, [246]
Bowerbank, Mr., on fossil fruits of London Clay, [265]
——, on fossil fruits of Sheppey, [265]
Bowman, Mr., on uniting of distinct coal-seams, [401]
Brachiopoda, preponderance of, in older rocks, [470]
——, mode of recognising shells of, [471]
Bracklesham beds and Bagshot Sands, [259]
Bradford encrinites, [342]
Breccias of Lower Permian, [391]
Brick-earth or fluviatile loam, [153]
Bridlington drift, [189]
Bristol, dolomitic conglomerate of, [373]
Bristow, Mr., on volcanic minerals, [500]
Brixham cave near Torquay, [158]
Brocchi on Italian tertiary strata, [141]
—— on subapennine strata, [208]
Brockenhurst, corals and shells of, [257]
Brodie, Rev. P. B., on Lias insects, [363]
Brodie, Mr. W. R., on Purbeck mammalia, [326]
Brongniart, M. Adolphe, on botanical nomenclature, [303]
——, on Lias plants, [364]
——, on flora of the Bunter, [380]
——, on flora of the coal, [420]
——, on fruit of Lepidodendron, [424]
——, M. Alex., on Tertiary series, [141]
Bronteus flabellifer, Devonian, [453]
Brora, oolitic coal formation of, [350]
Brown, Mr. Richard, on Stigmaria, [426]
——, on carboniferous rain-prints, [416]
Brown, Robert, on Eocene protaceous fruit, [264]
Brown, Reverend T., on marine shells in Scotch drift, [177]
Brown-coal of Germany, [540]
Bryce, Mr., on Scotch till, [176]
Bryozoa of Mountain Limestone, [433]
—— and polyzoa, terms explained, [197]
Buch, von. See Von Buch.
Buckland, Dr., on Kirkdale cave, [158]
——, on violent death of saurians, [362]
——, on spines of fish, [359]
——, on Eocene oysters, [268]
——, on pot-stones in chalk, [291]
Buddle, Mr., on creeps in coal-mines, [78]
Bulimus ellipticus, Bembridge, [253]
—— lubricus, Loess, [56]
Bullock, Capt., R.N., on Atlantic mud, [287]
Bunbury, Sir C., on leaf-bed of Madeira, [532]
——, on ferns of the Maryland coal, [421]
Bunter of Germany, [380]
—— or Lower Trias of England, [372]
Buprestis? Elytron of, Stonesfield, [346]
Burmeister on trilobites, [471]
CAINOZOIC, term defined, [123]
Caithness, fish beds of, [443]
Calamite, root of, [425]
Calamites Sucowii, coal, and restored stem, [424]
Calamophyllia radiata, Bath Oolite, [342]
Calcaire de la Beauce, age of the, [230]
—— grossier, fossils of the, [274]
—— siliceux of France, [273]
Calcareous matter poured out by springs, [604]
—— rocks described, [36]
—— nodules in Lias, [63]
Calcarina rarispina, Eocene, [275]
Calceola sandalina, Devonian, [453]
——, schiefer of Germany, [453]
California, aurifrous gravel of, [617]
——, gold in petrified wood of age of alluvium, [601]
Calymene Blumenbachii, Silurian, [466]
Cambrian Group, classification of the, [481]
Cambrian, Upper, [482]
——, Lower, [484]
——, of Sweden and Norway, [489]
——, strata of Bohemia, [487]
——, of North America, [489]
——, volcanic rocks, [549]
Campophyllum flexuosum, [431]
Canada, Cambrian of, [489]
——, Devonian of, [455]
——, trap-rocks of, [549]
Canadian drift, [182]
Canary, Grand, shelly tuffs of, [538]
Cantal, Lower Miocene of the, [231]
Cape Breton, rain-prints in coal-measures of, [416]
Cape Wrath, granite veins in gneiss at, [560]
Caradoc and Bala beds, [470]
Carbonate of lime in rocks, how tested, [37]
Carboniferous Group, subdivisions of the, [394]
—— flora, [420-30]
—— limestone, thickness of, [396]
——, marine fauna of the, [432]
—— Period, trap-rocks of, [545]
—— plutonic rocks, [572]
—— reptiles, [406]
—— insects, [405]
Carcharodon angustidens, Bracklesham, [262]
Cardiganshire, section of slaty cleavage in, [589]
Cardiocarpon Ottonis, Permian, [393]
Cardita (Venericardia) planicosta, [260]
—— sulcata, Barton, [259]
Cardium dissimile, Portland Stone, [336]
—— rhæticum, Rhætic Beds, [366]
—— striatulum, Kimmeridge clay, [336]
Carne, Mr. N., on Cornish lodes, [607]
Carpenter, Dr., on Atlantic mud, [288]
——, on Eozoon Canadense, [491]
Carrara, marble of, [599]
Carruthers, Mr., on Eocene proteaceous fruit, [265]
——, on cycads of the Purbeck, [332]
——, on leaves of calamite, [425]
——, on spores of carboniferous Lycopodiaceæ, [422]
——, on structure of sigillaria, [426]
——, on trees in volcanic ash, [547]
Cashmere, recent formations in, [146]
Cassian, St., Triassic strata of, [376]
Castrogiovanni, curved strata near, [86]
Catania, laterite formed in, [510]
——, Tertiary beds in, [206]
Catillus Lamarckii, White Chalk, [295]
Caucasus, absence of lakes in the, [187]
Caulopteris primæva, Coal, [421]
Cave-breccias of Australia, [158]
Cavern deposits with human and animal remains, [156]
Caves of Kirkdale and Brixham, [157]
Celts described, [152]
Cementing of strata, [61]
Cephalaspis Lyelli, Old Red, [446]
Ceratites nodosus, Muschelkalk, [379]
Cerithium concavum, Headon, [256]
—— elegans, Hempstead beds, [245]
—— (Terebra) Portlandicum, [335]
—— plicatum, Hempstead beds, [245]
—— melanoides, [268]
Cervus alces, tooth of, [164]
Cestracion Phillippi, Recent, [297]
Chabasite, [500]
Chalk, composition, extent, and origin of, [286]
—— of Faxoe, [286]
—— flints, origin of, [290]
—— fossils of the White, [293-6]
——, iceborne boulders in the, [292]
—— of North and South Europe, [305]
——, Lower White, without flints, [298]
—— marl, fossils of the, [298]
—— Period, popular error concerning, [288]
Chalk-pit with pot-stones, view of, [291]
Chama squamosa, Barton, [258]
Champoleon, junction of granite with Jurassic strata near, [571]
Chara elastica, C. medicaginula, [58]
—— tuberculata, Bembridge, [253]
Charpentier, M., on Alpine glaciers, [170]
——, on depression of Alps in Glacial Period, [185]
Chatham coal-field, [383]
Cheirotherium, footprints of, [372]
Chemical deposits in veins, [612]
—— and mechanical deposits, [60]
Chiapa, fall of volcanic dust at, [523]
Chichester, erratics near, [181]
Chili, copper pyrites with gold in, [616]
——, walls cracked by earthquake in, [87]
Chillesford and Aldeby beds, [192]
Chimæra monstrosa, Lias, [359]
Chlorite-schist, [579]
Chloritic series, or Upper Greensand, [298]
Christiania, Euritic porphyry at, [562]
——, granite veins in Silurian strata of, [572]
——, quartz vein in gneiss at, [561]
Chronological groups of formations, [129]
Chronology, test of, in rocks, [121]
Cinder-bed of the Purbeck, [325]
Cinnamomum polymorphum, Miocene, [219]
—— Rossmässleri, Miocene, [239]
Claiborne beds, Eocene fossils of, [279]
Clarke County, United States, Zeuglodon of, [279]
Classification of Tertiary formations, [137], [143]
——, value of shells in, [142]
Clausilia bidens, Loess, [56]
Clay defined, [36]
—— iron-stone defined, [404]
——, plastic, [267]
—— slate, [579]
——, Weald, [313]
Cleavage explained, [502]
——, crystalline theory of, [591]
——, mechanical theory of, [592]
—— of metamorphic rocks, [588]
Cleidotheca operculata, [483]
Clermont, metalliferous gneiss near, [586]
Climate of the Crags, [200]
—— of the Coal, [430]
—— of the Miocene in the Arctic regions, [240]
—— of the Post-pliocene period, [161]
Clinkstone, [506]
Clinton group, fossils of the, [479]
Clyde, buried canoes in estuary of, [146]
——, arctic marine shells in drifts of, [176]
Clymenia linearis, Devonian, [451]
Clymenien-Kalk of Germany, [450]
Coal, conversion into anthracite of, [403]
—— a land and swamp formation, [397]
——, cause of the purity of, [402]
——, conversion of lignite into, [403]
——, erect trees in, [411]
——, structure of the, [412]
——, vegetation of the, [420]
——, air-breathers in the, [405], [413]
Coal Period, climate of the, [430]
—— field of Virginia, [382]
—— measures of Nova Scotia, [408]
—— measures, thickness of, in Wales, [397]
—— pipes, danger of, [390]
——, rainprints in, [416]
—— seams, uniting of, [400]
Coalbrook-Dale, faults in, [88]
Cochliodus contortus, [437]
Cockfield Fell rocks, altered by dikes, [516]
Coelacanthus granulatus, Permian, [390]
Coleoptera of Œningen beds, [223]
Collyrites ringens, Inferior Oolite, [351]
Columnar structure of volcanic rocks, [510]
—— basalt in the Vicentin, [511]
Compact feldspar, [501]
Concretionary structure, [63]
Cone of Tartaret, [527], [542]
—— of Côme, [28]
Cones and craters described, [495]
——, absence of, in England, [30]
Conformable stratification, [39]
Conglomerate or pudding-stone, [36]
——, Dolomitic, of Bristol, [373]
Coniferæ of the coal-measures, [427]
Connecticut Valley, New Red Sandstone of, [381]
Conocephalus striatus, [488]
Conocoryphe striata, [488]
Conrad, Mr., on age of American cretaceous rocks, [307]
Consolidation of strata, [61]
Continents and oceans, permanence of, [117]
Contorted strata, in drift, [178]
Conularia ornata, Devonian, [453]
Conulus priscus, Coal, [415]
Conus deperditus, Bracklesham, [262]
Conybeare and Phillips on ninety-fathom dike, [90]
Conybeare, Mr., on reptiles of the Lias, [360]
Copper lode near Redruth, [607]
Coprolite bed of Chloritic Series, [299]
—— beds of Red and Coralline crags, [197], [198]
Coprolites of fish from the chalk, [298]
Coral Rag, fossils of the, [339]
Coralline of White Crag, [197]
Corals of the Devonian, [451]
—— of the Mountain Limestone, [433]
——, Neozoic type of, [431]
——, Palæozoic type of, [431]
Corbicella (Cyrena) fluminalis, [54]
Corbula pisum, Hempstead beds, [245]
Corinth, corrosion of rocks by gases near, [586]
Cornbrash or Forest Marble, [341]
Cornwall, granite veins in, [561], [582]
——, lodes in, [615]
——, mass of granite in, [552]
——, vertical sections of veins in mine, [607]
Cosequina volcano, burying of organic remains by, [523]
Crag, term defined, [192]
—— of Antwerp, [204]
——, fauna of, its relation to that of present seas, [201]
——, Norwich, [193]
——, Coralline or White, [197]
——, Red, [194]
——, tables of marine testacea in, [202]
—— deposits, climate of, [200]
Crania attached to a sea-urchin, [49]
—— Parisiensis, White Chalk, [294]
Crassatella sulcata, Barton, [259]
Craters and cones described, [495]
——, Theory of Elevation, [496]
Craven fault, [90]
Creeps in coal-mines, [78]
Cretaceous rocks of United States, [307]
—— Period, error as to continuity of, [288]
——, flora of the Upper, [302]
—— volcanic rocks, [544]
—— plutonic rocks, [570]
—— Period, distinct mineral character of rocks in, [292]
—— rocks, classification of, [282]
—— strata, connection between Upper and Lower, [301]
Crinoidea of Mountain Limestone, [433]
Croatia, Lower Miocene beds of, [242]
Croll, Mr., on amount of subaërial denudation, [114]
Cromer forest-bed, [191]
Crop out, term explained, [83]
Crossopterygidæ, or fringe-finned fish, [443]
Crowfoot, Mr., on shells of Aldeby beds, [192]
Crust of the earth defined, [26]
Crustaceans of Old Red Sandstone, [446]
Cryptodon angulatum, London Clay, [266]
Crystalline Limestone, [579]
—— rocks defined, [32]
—— schists, much alkali in the, [587]
—— theory of cleavage, [591]
Cup and Star corals, [431]
Curved strata, [73-76]
Cutch, salt-layers in the Runn of, [375]
Cuvier, M., on fauna of the Paris basin, [271]
——, on Mammalia of Paris gypsum, [231]
——, on Tertiary series, [141]
Cyathocrinus caryocrinoides, [433]
—— planus, [433]
Cyathophyllum cæspitosum, [451]
Cyclopean isles, beds of tuff and clay in, [529]
——, contorted strata in, [530]
Cyclopteris Hibernica, Old Red, [441]
Cyclostigma (Lepidodendron), Old Red, [441]
Cyclostoma elegans, Loess, [56]
Cylindrites acutus, Great Oolite, [345]
Cypress swamps of the Mississippi, [402]
Cyprides in the Weald Clay, [315]
Cypridina serrato-striata, [451]
Cypris in fresh-water deposits, [57]
—— gibbosa, C. tuberculata, C. leguminella, [324]
—— striato-punctata, C. fasciculata, C. granulata, [325]
—— Purbeckensis, Cypris punctata, [331]
—— spinigera, Weald Clay, [315]
Cyrena (Corbicella) fluminalis, [54]
—— cuneiformis, Woolwich Clays, [268]
—— obovata, [54]
—— semistriata, Hempstead beds, [245]
Cystideæ of Silurian rocks, [466]
Cythere inflata, coal-measures, [405]
DADOXYLON, fragment of coniferous wood, [428]
Dana, on volcanic minerals, [500]
Danish kitchen-middens, [146]
Dapedius monilifer, Lias, [358]
Darbishire on shells of Moel Tryfaen, [180]
Dartmoor, post-carboniferous granite of, [572]
—— intrusive granite at, [572]
Darwin, Mr., on foliation and lamination, [595]
——, on mammalia of South America, [160]
——, on marine saurian, [362]
——, on rise of part of South America, [72]
——, on sinking of coral reefs, [72]
——, on plutonic rocks of the Andes, [569]
——, on relationship of extinct to living types, [160]
Dates of discovery of fossil vertebrata, [464]
Daubeny, Dr., on decomposition of trachytic rocks, [586]
Daubrée, on formation of zeolites, [521]
——, on alkaline waters of Plombières, [584]
Davidson, Mr., on Spiriferina, [355]
Davis, Mr. E., on fossils of Lingula Flags, [484]
Dawkins, Mr. Boyd, on Hyæna spelæa, [158]
——, on mammalia of Cromer Forest-bed, [191]
——, on Triassic mammifer, [369]
Dawson, Dr., on Devonian flora and insects, [456], [457]
——, on Eozoon Canadense, [491]
——, on Nova Scotia coal-measures, [409]
——, on Nova Scotia coal-plants, [410], [412]
——, on Pupa vetusta, [415]
——, on reptiles and shells in Nova Scotia coal, [413]
——, on structure of calamite, [425]
——, on structure of sigillaria, [426]
Deane, Dr., on footprints in Trias, [382]
Debey, Dr., on flora and fauna of Aix, [302-04]
Dechen, M. von, on organic remains of the brown coal, [540]
——, on Cornish granite veins, [560]
De la Beche, Sir H., on granite of Dartmoor, [582]
——, on Carrara marble, [599]
——, on mineral veins, [616]
——, on Redruth copper-mine, [610]
——, on saurians of the Lias, [362]
——, on trap-rocks of New Red, [545]
——, on Welsh coal-measures, [397]
Delesse, on action of water in metamorphism, [585]
Deltas, strata accumulated in, [28]
Dendrerpeton in Coal, [413]
Denudation defined, [96]
——, subaërial, [97]
——, littoral, [102]
——, submarine, [105]
——, average annual amount of subaërial, [113]
—— of carboniferous strata, [396]
—— counteracting upheaval, [106-15], [108-15]
—— a means of exposing crystalline rocks, [563]
——, trap-dikes cut off by, [518]
—— and volcanic force antagonistic powers, [115]
Deposition, rate of, shown by fossils, [47]
Derbyshire, veins in Mountain Limestone, [608]
Derivative shells of the Red Crag, [195-203]
Desnoyers, M., on age of Faluns, [142]
——, on Eocene fossil footprints, [272]
Desor, M., on Celtic coins in lake-dwellings, [149]
Devonian Period, Upper, [450]
Middle, [450]
Lower, [453]
—— fossils of the Eifel, [534]
—— of Russia, [454]
—— of United States and Canada, [455]
—— insects of Canada, [457]
—— strata, classification of, [439-50]
Devonshire, cleavage of slate rocks in, [593]
Diabase, [505]
Diagonal, or cross-stratification, [42]
Diagram of fossiliferous rocks, [137]
—— of plutonic and sedimentary formations, [567]
Diallage, [500], [502]
Diastopora diluviana, Bath Oolite, [343]
Diatomaceæ forming tripoli, [51]
Diceras Lonsdalii, Neocomian, [310]
Didelphys Azaræ, Recent, [347]
Didymograpsus geminus, [476]
—— Murchisonii, [473]
Dike cutting through shale, Anglesea, [515]
—— cutting through chalk, Antrim, [515], [516]
Dikelocephalus Minnesotensis, [490]
Dikes defined, [30]
—— of Monte Somma, [526]
—— in Palagonia, ground-plan of, [532]
——, volcanic or trap, [513-7]
Diluvium, origin of term, [167]
Dinornis Palapteryx, of New Zealand, [160]
Dinotherium giganteum, [212]
Diorite, [505]
Dip and strike, terms explained, [80]
Diplograpsus folium, Llandeilo Flags, [474]
—— pristis, Llandeilo beds, [473]
Dirt-bed of the Purbeck, [331]
Dogger-bank described, [105]
Dolerite, a variety of basalt, [504]
Dolomite defined, [38]
Dolomitic conglomerate of Bristol, [373]
Downs, escarpments of North and South, [104]
Downton Sandstone, [459]
Dowson, Mr., on shells of Aldeby beds, [192]
Drew, Mr., on Hastings Sands, [316]
Drift of Ireland, [190]
—— of Norfolk cliffs, [190]
—— of Scandinavia, [174]
—— of Bridlington, [189]
—— carried by icebergs, [172]
—— shells in Canada, [183]
——, contorted strata in, [178]
——, marine shells in Scotch, [175]
Dudley Limestone, [465]
Dufrenoy, M., on granite of Pyrenees, [582]
Dumont, Professor, on Belgian Lower Eocene, [282]
Duncan, Dr., on Neozoic corals passing down to Devonian, [432]
Dundry Hill, near Bristol, section of, [130]
Dunker, Dr., on wealden of Germany, [319]
Dura Den, yellow sandstone of, [440]
EARTH’S crust defined, [26]
Echinoderms of Suffolk Crag, [200]
Echinosphæronites balticus, [472]
Egerton, Sir P., on fish of Headon series, [256]
——, on fish of the Permian, [389]
——, on fish of Penarth beds, [366]
Ehrenberg, Professor, on term Bryozoum, [197]
——, on Silurian foraminifera, [478]
——, on infusoria, [51]
Eifel Limestone, [453]
——, Lake-craters of, [534]
—— Miocene, volcanic rocks of, [539]
—— Pliocene, volcanoes of the, [534]
——, trass of the, [535]
Elephas antiquus, molar of, [163]
—— meridionalis, molar of, [163]
—— primigenius, molar of, [162]
Elevation craters, theory of, [496]
Elvans, term explained, [572]
—— of Ireland and Cornwall, [615]
Elytron of Buprestis? Stonesfield, [346]
Emmons, Professor, on jaws of Triassic quadruped, [383]
——, on Dromatherium, [383]
Encrinites of Bradford, [342]
Encrinus liliiformis, Muschelkalk, [379]
Endogens, term explained, [303]
Engihoul cave, human and animal remains in, [157]
England and Wales, glaciation of, [180]
Enstatite, [501]
Eocene areas of Europe, map of, [250]
—— foraminifera, [274]
—— formations of France, [270-6]
—— of England, [252]
—— period, volcanic rocks of, [543]
——, plutonic rocks of the, [568]
——, metamorphic rocks of the, [598]
—— of France, footprints in, [272]
—— and Miocene, line between the, [230], [250]
——, term defined, [143]
—— of the United States, [278]
Eozoon Canadense, oldest known fossil, [492]
Epidote, [500]
Eppelsheim, Dinotherium of, [225]
Equisetaceæ of the Coal, [424]
Equisetites columnaris, Keuper, [376]
Equus caballus, tooth of, [164]
Erratic blocks, nature of, [167]
—— of Greenland, [171]
—— near Chichester, [181]
—— in the Red Crag, [201]
Erratics, Alpine, [169]
Escarpments explained, [104]
Eschara disticha, White Chalk, [296]
Escharina oceani, White Chalk, [296]
Estheria minuta, Trias, [370]
—— ovata, Richmond, Virginia, [383]
Ethridge, Mr., on Atlantic mud, [288]
——, on Devonian series, in Devon, [450]
——, on Devonian fauna, [451], [454]
——, on mollusca of Bracklesham, [260]
——, on St. Cassian fossils, [377]
Etna, built up since Newer Pliocene, [204]
——, Pliocene lavas of, [529]
Ettingshausen on Sheppey Eocene fruit, [265]
Eunomia radiata, Bath Oolite, [342]
Eunotia bidens, Atlantic mud, [288]
Euomphalus pentangulatus, [435]
Eurite, [557], [578]
Euritic porphyry of Norway, [562]
Evans, Mr., on Archæopteryx, [337]
Exogens, [297]
Exogyra virgula, Kimmeridge Clay, [336]
Extracrinus (Pentacrinus) Briareus, Lias, [357]
FALCONER, Dr., on Miocene fauna of Siwalik Hills, [226]
——, on Brixham Cave flint knives, [157]
——, on Purbeck mammalia, [326]
Faluns of Loire, recent shells in, [214]
—— of Touraine, [211]
Farnham, phosphate of lime near, [299]
Fascicularia aurantium, Coralline crag, [199]
Faults in coal-measures of Coalbrook Dale, [88]
—— described, [87-92]
—— often the result of repeated movements, [90]
Fauna of the crag, its relation to that of our present seas, [201]
—— of the Mountain Limestone, [430]
—— of the Paris basin, [271]
Favosites cervicornis, Devonian, [451]
—— Gothlandica, Silurian, [465]
Favre, M. E., on glaciers and moraines of the Caucasus, [187]
Faxoe, chalk of, [285]
Feldspar-porphyry, [557]
Feldspar, varieties of, [499], [500]
Feldstone, [557]
Felis tigris, tooth of, [166]
Fenestella retiformis, Magnesian Limestone, [388]
Ferns of the coal, [421]
Fife, trap-dike in, [543]
Fish, fossil of the Carboniferous, [436]
——, Eocene of Monte Bolca, [544]
——, oldest known fossil, [463]
——, number of living, [445]
——, fresh-water and marine, [58]
—— of the Upper Ludlow, [459]
—— of the Old Red Sandstone, [443-5]
—— of the Permian marl slate, [389]
—— of the brown coal, [540]
—— of the Lias, [358]
Fisherton, Greenland lemming in drift of, [161]
Fissures, filled with metallic matter, [606]
Fitton, Dr., on the Neocomian strata, [314]
Fleming, Dr., on Parka decipiens, [448]
——, on trap-dike in Fife, [546]
Flints in the Chalk, [290]
Flisk dike of Fife, [546]
Flora of the Carboniferous, [420]
——, Devonian, compared to Carboniferous, [457]
—— of the Subapennines, [208]
——, Lower Miocene of Switzerland, [235]
——, Miocene of the Arctic Regions, [239]
——, Older Pliocene of Italy, [208]
—— of the Permian, [392]
—— of the Upper Cretaceous, [302]
——, Upper Miocene of Switzerland, [215-22]
—— of the Wealden, [320]
Fluvio-marine or Norwich Crag, [193]
Flysch of the Alps, [278]
——, plutonic rocks invading, [568]
Folding and denudation of Nova Scotia Carboniferous rocks, [417]
Folds of parallel strata, arrangement and direction of, [93]
Foliation of crystalline rocks, [595]
——, irregularities in, [596]
Folkestone and Hythe beds, [308]
Fontainebleau, Gres de, [230]
Footprints in Potsdam sandstone, [490]
—— of reptiles in Coal-measures, [408]
——, fossil in New red, [381]
—— in Paris gypsum, [272]
Foraminifera, Eocene, [275]
—— of Mountain Limestone, [437]
—— of the Chalk, [287]
Forbes, Mr. David, on glass cavities in quartz, [555]
——, on planes of foliation, [595]
——, on specific gravity of quartz, [500]
——, on volcanic minerals, [498]
Forbes, Professor E., on fossils of Bembridge beds, [252]
——, on Hempstead beds, [244]
——, on shells of the crag, [200]
——, on sphæronites, [472]
——, on subdivisions of the Purbeck, [333]
——, on testacea of the Faluns, [212]
——, on thickness of Upper Neocomian, [309]
Forest-bed at Cromer, [191]
—— marble or cornbrash, [341]
——, submerged, [103], [104]
——, fossil in Coal, [400]
——, fossil of Isle of Portland, [332]
Forfarshire, Cephalaspis beds of, [446]
——, contorted strata in, [178]
Formation, term defined, [27]
Fossil, term defined, [29]
—— trees erect in coal, [410]
—— Fish of Old Red Sandstone, [442]
Fossiliferous groups, table of succession of, [131]
Fossils, arrangement of, in strata, [47]
——, destruction of, in older formations, [139]
——, fresh-water and marine, [52]
—— obliterated by metamorphic action, [603]
——, recent, and Post-pliocene, [154-65]
—— of the drift, [176], [180], [192]
—— of the Crags, [193-203]
——, Upper Miocene, [214-29]
——, Lower Miocene of Switzerland, [236]
—— of the Hempstead Beds, [244]
——, Eocene, [253]
—— of the Barton Clay, [259]
—— of the White Chalk, [293]
—— of the Neocomian, [309]
—— of the Oolite, [324]
—— of the Stonesfield Slate, [347]
—— of the Lias, [354]
—— of the Trias, [370]
—— of the Magnesian Limestone, [387]
—— of the Coal, [405]
—— plants of the Coal, [421]
—— of the Mountain Limestone, [430]
——, Devonian, [449]
——, Silurian, [460]
——, Cambrian, [484]
—— Laurentian, [492]
Fournet, M. on metalliferous gneiss, [586]
——, on veins in granite, [610]
Fox, Rev. D., on Isle of Wight Eocene fossils, [254]
Fox, Mr. R., on lodes in Cornwall, [614]
Fractures of strata, and faults, [87]
Fragments, included, a test of age of plutonic rocks, [565]
——, included, a test of age of strata, [129]
—— a test of age in volcanic rocks, [524]
France, Eocene formations of, [270-6]
——, Lower Miocene of, [231]
——, Upper Miocene of, [211]
Freshfield, Mr., on absence of lakes in the Caucasus, [187]
Fresh-water strata, how distinguished from marine, [53-9]
—— formation of Auvergne, [233]
Fucoid sandstones of Sweden, [489]
Fulgur canaliculatus, Maryland, [228]
Fuller’s earth, fossils of the, [348]
Fundy, Bay of, fossil trees exposed in cliffs at, [412]
Fusilina cylindrica, [438]
Fusion of quartz, [500]
Fusus contrarius (Trophon antiquum), [196]
—— quadricostatus, Maryland, [228]
GABBRO, [505]
Gaillonella ferruginea, and G. distans, [52]
Galapagos Islands, living marine saurian in, [362]
Galeocerdo latidens, Bracklesham, [262]
Galerites albogalerus, White Chalk, [294]
Galestes in Middle Purbeck, [328]
Ganoids, the type of Old Red Sandstone fish, [443]
—— of the Wealden, [316]
—— of the Trias, [383]
Gaps in the sequence of fossil remains, [138]
Garnet, [500]
Gases, corrosion of rocks by, [586]
Gaudin on Lower Miocene of Switzerland, [236]
—— on Pliocene flora of Italy, [209]
—— on Proteaceæ in Bournemouth Eocene, [263]
Gault, thickness and fossils of, [300]
Geikie, Mr. A., on Ayrshire Permian trap-rocks, [545]
——, on subaërial denudation, [115]
——, on ice erosion of lake-basins, [187]
——, on Isle of Mull volcanic rocks, [539]
——, on Pentland Old Red volcanic rocks, [548]
——, on Silurian metamorphic rocks, [602]
——, on syenite of Skye, [568]
Geinitz, M., on Permian flora, [393]
Gemunder Maar, volcanic rocks of, [534]
Geneva, Lower Miocene of, [236]
Geology defined, [25]
Gergovia, tuffs and associated lacustrine strata of, [542]
Germany, Lower Miocene of, [242]
——, Triassic fauna of, [375]
Gers, Upper Miocene of, [215]
Gervillia anceps, Neocomian, [310]
—— socialis, Muschelkalk, [379]
Giant’s Causeway basalt, age of, [248]
——, laterite of the, [509]
——, columnar basalt of, [510]
Girgenti, Newer Pliocene of, [207]
Glacial drift, distribution and nature of, [166]
—— epoch in the Post-pliocene, [166]
—— formations of Pliocene age, [189-92]
Glaciation of Russia and Scandinavia, [174]
—— of Scotland, [175]
—— of Wales and England, [180]
—— of North America, [182]
Glaciers, transporting and abrading power of, [168]
Glasgow, marine strata near, [146]
Glauconie grossiere, [275]
Glen Tilt, junction of granite and schist at, [559]
Globiform pitchstone, [512]
Globigerina bulloides, [288]
Globular structure of volcanic rocks, [510]
Glyptostrobus, Europæus, Œningen, [223]
Gneiss, granite veins traversing, [560]
—— defined and figured, [577]
——, fundamental, of Scotland, [493]
Gold mines of Australia and Chili, [616]
—— veins of Russia, [616]
—— of California, of age of alluvium, [617]
Goldenberg, Professor, on Saarbrück coal insects, [406]
Goldfuss, Professor, on reptiles in coal, [406]
Goniatites crenistria, [436]
—— Listeri, coal-measures, [405]
Göppert, on American forms in Swiss Miocene flora, [223]
—— on petrification, [68]
—— on plants of coal-measures, [398]
Gorgonia infundibuliformis, Permian, [388]
Graham’s Island, forming ashy conglomerate, [549]
Grampians, Old Red conglomerates of, [73]
——, trap-rocks of the, [547]
——, former glaciers in the, [175]
Grand Canary, Upper Miocene, shelly tuffs of, [558]
Granite, composition of, [552]
——, graphic and columnar, [553], [554]
——, how far connected with trap-rocks, [558]
——, hydrothermal action in formation of, [555]
—— metamorphosing fossiliferous strata, [581]
——, porphyritic, [556]
——, oldest, [574]
——, protrusion of solid, [574]
——, passage of, into trap, [558]
——, schorly, [557]
—— veins, [559]
—— veins in talcose gneiss, [560]
Granton, angiosperm found in coal at, [429]
Graptolites of Llandeilo flags, [474]
Graptolites Murchisonii. Llandeilo flags, [473]
Graptolithus priodon, Silurian, [467]
Gray’s, Essex, pachyderms found at, [161]
Great (or Bath) Oolite, [342]
Greece, Upper Miocene formations of, [226]
Greenland, continental ice of, [170]
——, gradual sinking of, [72]
Greenstone, [505]
Gres de Beauchamp, Paris basin, [273]
Gres de Fontainebleau, age of the, [230]
Griffiths, Sir R., on yellow sandstone of Ireland, [441]
Grit defined, [36]
Groups, older, rise highest above the sea, [139]
—— why the newest to be studied first, [140]
Gryllacris lithanthraca, coal, [405]
Gryphæa coated with serpulæ, [48]
—— columba, Chloritic Sand, [300]
—— convexa, Chalk, [295]
—— incurva (G. arcuata), [54], [354]
—— virgula, Kimmeridge clay, [336]
Gryphite Limestone, [354]
Guadaloupe, glass cavities in quartz of, [555]
Gulf-Stream, probable abrading power of, [105]
Gümbel, M., on Rhætic beds, [366]
Gunn, Mrs., on pot-stones in the chalk, [291]
Gutbier, Colonel, on Permian flora, [393]
Gymnogens, term explained, [303]
Gypseous marls of Auvergne, [233]
Gypsum and gypseous marl defined, [38], [39]
Gyrolepis tenuistriatus, Rhætic beds, [367]
HAIME, Mr., on palæozoic corals, [431]
Hakea silicina and Hakea saligna, Œningen, [222]
Hall, Captain Basil, on Cyclopean Isles, [530]
Hall, Sir James, on curved strata, [75]
Hall, Mr. J., on Appalachian palæozoic rocks, [110]
Hallstadt and St. Cassian beds, [376]
Halysites catenularis, Silurian, [465]
Hamilton, Sir W., on eruption of Vesuvius, 1779, [526]
Hamites spiniger, Gault, [301]
Hancock, Mr., on Protosaurus in Permian, [390]
Harkness, Professor, on Silurian metamorphic rocks, [602]
Harlech grits, fossils of the, [486]
Harris, Major, on the Salt Lakes, [374]
Harpactor maculipes, Œningen, [224]
Harpe, M. de la, on Bournemouth Eocene flora, [263]
Hartung, Mr., cited, [496]
Hartz mountains, mineral veins of, [608]
——, Bunter Sandstein of, [380]
Hastings Sands, subdivisions of the, [316]
Hautes Alpes, granite of the, [571]
Hauy on isomorphism, [502]
Headon series, fossils of the, [255]
Heat, powerful in consolidating rocks, [65]
——, rocks upraised and folded by, [92]
Hébert, M., on age of Sables de Bracheux, [330]
——, comparison of Sables Moyens and Barton shells, [258]
——, on pisolitic limestone, [285]
Hebrides, dikes in the, [514]
Heer, Professor, on American genera in Swiss Miocene, [239]
——, on age of Madeira leaf-bed, [532]
——, on Arctic Miocene flora, [239]
——, on Bear Island flora, [441]
——, on Bovey Tracey Miocene flora, [247]
——, on fossil plants of Switzerland, [215], [219], [221], [224], [236]
——, on Lower Miocene plants of Mull, [248]
——, on Monte Bolca Eocene plants, [263], [543]
——, on Proteas of Lower Miocene, [237]
——, on plants of Hempstead beds, [246]
——, on plants of coal-field, Virginia, [383]
——, on Swiss Miocene insects, [223]
——, on supposed Proteaceæ of Œningen beds, [221]
——, on Superga fossil plants, [244]
Heidelberg, varieties of granite near, [560]
Heliolites porosa, Devonian, [451]
Helix hispida (plebeia), [155]
—— labyrinthica, Headon, [255]
—— occlusa, Bembridge, [253]
—— Turonensis, faluns, [56]
Hemicidaris Purbeckensis, Purbeck, [324]
Hemipneustes radiatus, Chalk, [284]
Hemitelites Brownii, Inferior Oolite, [350]
Hempstead beds, subdivisions of the, [244]
Henry, on absorption of carbonic acid gas in water, [585]
Henslow, Professor, on dike in Anglesea, [515]
——, on Red Crag coprolite bed, [197]
Herschel, Sir J., on slaty cleavage, [590]
Hertfordshire pudding-stone, [62]
Heterocercal tail of fish, [389]
Hicks, Dr., on fossils of Arenig beds, [476]
——, on fossils of Harlech grits, [486]
——, on Menevian beds, [485]
Himalaya, shells 18,000 feet high in, [29]
——, Upper Miocene of, [226]
Hippopodium ponderosum, Lias, [355]
Hippopotamus, tooth of, [164]
Hippurite Limestone, [304]
Hippurites organisans, Chalk, [306]
Histioderma hibernica, [486]
Hitchcock, Professor, on Trias footprints, [381]
Holoptychius nobilissimus, scale of, and restoration, [442]
Homalonotus Delphinocephalus, [467]
—— armatus, Devonian, [454]
Homfray, Mr., on fossils of Tremadoc beds, [483]
Homocercal tail of fish, [389]
Hooghly River, analysis of water, [69]
Hooker, Dr., on coniferæ, [429], [430]
——, on structure of sigillaria, [426]
——, on sporangia of Silurian plant, [460]
Horizontality of strata, [40]
Horizontal strata, upheaval of, [71]
Hornblende, [499], [502]
Hornblende-schist, [578]
Hörnes, Dr., on fossil mollusca of Vienna basin, [225]
Horstead, pot-stones at, [291]
Hour-glass illustrating the destruction and renovation of land, [119]
Howse, Mr., on Protosaurus in Permian, [390]
Hubbard, Professor, on granite of White Mountains, [565]
Hudson River Group, fossils of the, [479]
Hughes, Mr. T. McKenny, cited, [450]
——, on slaty cleavage, [589]
——, on protrusion of solid granite, [575]
Hull, Mr. E., on breccias in Permian, [391]
——, on carboniferous of Lancashire, [395]
——, on carboniferous rocks of north of England, [111]
——, on faults in Lancashire coal-field, [91]
——, on anticlinals and synclinals, Lancashire, [85]
——, on thickness of the Upper Trias, [369]
——, on thickness of Permian, [386]
——, on three lines of flexure since the coal in Lancashire, [94]
Human remains of Recent Period, [157]
—— in cavern deposits, [156]
Humboldt, on mineral character of rocks, [602]
Humphrey and Abbot on Mississippi denudation, [114]
Hungary, trachyte of, [558]
Hunt, Sterry, on action of water in metamorphism, [585]
Huronian series, thickness of the, [490]
Huxley, Professor, on Atlantic chalk-mud, [287]
——, on affinity between reptiles and birds, [338]
——, on batrachians of the coal, [407]
——, on fish of Old Red Sandstone, [443-5]
——, on Pteraspis, [463]
Hyæna den of Kirkdale cave, [157]
Hyæna spelæa, tooth of, [165]
Hybodus plicatilis, Rhætic beds, [367]
—— reticulatus, Lias, [359]
Hydrothermal action producing metamorphism, [584]
—— in formation of granite, [555]
—— forming granite veins, [573]
Hymenocaris vermicauda, [484]
Hyperodapedon Gordoni, Trias, [370]
Hypersthene, [499], [502]
—— rock, [505]
—— rocks of Skye, [491]
Hypogene rocks, uniformity of mineral character in, [602]
—— rocks, term defined, [26]
Hypsiprymnus Gaimardi, molar of recent, [327]
Hythe, Neocomian beds of, [308]
ICE, erosion of lake-basins considered, [184], [188]
——, abrading power of, [168]
——, continental, of Greenland, [170]
Icebergs, drift carried by, [172]
—— stranded in Baffin’s Bay, [173]
Ice-borne erratics at Chichester, [181]
Iceland, glass cavities in quartz of, [555]
——, flow of lava in, [523]
Ichthyosaurus communis, Lias, [361]
Idocrase, [500]
Ichthyodorulite of the Lias, [359]
Iguanodon Mantelli, Weald Clay, [315]
Ilfracombe Group of Devon, [449]
Inclined strata, [73]
India, Miocene formations of, [226]
India, Upper Miocene of, [226]
Inferior Oolite, thickness and fossils of, [349]
Infusoria in tripoli, [51]
Inland sea-cliffs, [103]
Inoceramus Lamarckii, White Chalk, [295]
Insect in American coal, [416]
—— beds of the Lias, [363]
Insect wing of neuropterous, [363]
Insects, Devonian, of Canada, [457]
—— in European coal, [405]
——, Miocene, of Croatia, [243]
——, Upper Miocene, at Œningen, [223]
Intrusion, a test of age of Plutonic rocks, [565]
——, a test of age of volcanic rocks, [521]
Inundation mud of rivers, [153]
Ireland, glacial drift of, [190]
——, yellow sandstone of, [441]
Iron pyrites, [500]
—— weapons of Swiss lake-dwellings, [148]
Isastræa oblonga, Portland Sand, [335]
Isle of Bourbon, lava current of the, [566]
—— Wight, Hempstead beds, [244]
—— Wight, Eocene beds, [255]
—— Mull, Miocene leaf-bed of, [247]
—— Mull, volcanic rocks, [248]
Isomorphism, theory of, [502]
Italy, Lower Miocene of, [244]
——, Older Pliocene volcanoes of, [523]
——, Pliocene of, [207]
——, Older Pliocene flora of, [208]
——, Upper Miocene strata of, [226]
JAMIESON, Mr. T. F., on Scotch glacial drift, [175]
Jaws of mammalia in Purbeck, [327]
Jeffreys, Mr. Gwyn, on Atlantic mud, [288]
Jointed structure of metamorphic rocks, [589]
Jones, Dr. Rupert, on Eozoon Canadense, [491]
Jorullo, lava stream of, [566]
Judd, Mr., on Speeton clay, [311]
Jukes, Mr., on Tarannon shales, [468]
Jura, erratic blocks on the, [169]
——, structure of the, [82]
KANGAROO, jaws of, [159]
Käsegrotte, Bertrich Baden, Basaltic pillars of, [512]
Kaup, Professor, on footprints of the Trias, [373]
Keilhau, Professor, on granite veins, [562]
——, on planes of foliation, [595]
——, on Silurian granite of Norway, [573]
——, on protrusion of granite, [581]
Keller, Dr. F., on lake-dwellings, [148]
Kelloway Rock, percentage of Oxford clay fossils in, [341]
Kentish Rag, [308]
Keuper, of Germany, [375]
—— or Upper Trias of England, [369]
Kilkenny, fossil plants of, [441]
Killas, altered by granite in Cornwall, [582]
Kiltorkan, yellow sandstone of, with Anodonta, [441]
Kimmeridge Clay, [335]
King, Dr., on reptile footprints in coal, [407]
King, Mr., on Permian fossils, [388]
Kirkdale cave, hyæna’s den of, [157]
Kitchen-middens of Denmark, [146]
Kleyn Spawen beds, [242]
Könen, Baron von, on Brockenhurst shells, [257]
Koninck, M. de, on Mountain Limestone fish, [436]
——, on shells of Mayence basin, [242]
Koninckia Leonhardi, Hallstadt, [377]
LABRADOR rock, [505]
—— series, [490]
Labradorite, [499], [501]
Labyrinthodon Jægeri, section of tooth, [371]
——, tooth of, [370]
Labyrinthodonts of Coal, [407]
Lake-craters of the Eifel, [534]
Lake districts, southern limits of the, [184]
Lake-dwellings, scarcity of human remains in, [149]
—— of Switzerland, [148]
Lakes, deposits in, [27]
——, connection of, with glacial action, [184-8]
Lamarck on bivalve mollusca, [54]
Lamination of clay slate, [594]
Lamna elegans, Bracklesham, [262]
Lancashire, vast thickness of rocks without corresponding altitude in, [111]
Land, balance of dry, how preserved, [116], [118]
—— has been raised, not the sea lowered, [70]
——, mean height of, above the sea, [115]
——, rise of, in Sweden, [72]
——, rise and fall of, affecting denudation, [101]
Land-ice, action of, in Greenland, [171]
Land’s End, columnar granite at, [553]
——, porphyritic granite at, [556]
La Roche, recent deposits in estuary of, [40]
Lartet, M., on mammalia of Faluns, [214]
——, on Gastornis Parisiensis, [276]
——, on reindeer period, [150]
Lastræa stiriaca, Monod, [239]
Lateral compression causing curved strata, [75]
Laterite of Giant’s Causeway, [509]
Laurentian gneiss of Scotland, [493]
—— Group, Upper and Lower, [491]
—— metamorphic rocks, [601]
—— volcanic rocks, [549]
Lava, [507]
—— consolidating on slopes, [496]
—— currents of Auvergne, [541]
—— streams, effect of, [30]
—— of La Coupe d’Ayzac, [511]
—— of Jorullo, [566]
Lead veins, age of, [616]
Leaf-bed of Madeira in basalt and scoriæ, [532]
——, Isle of Mull Miocene, [248]
Leda amygdaloides, London Clay, [266]
—— Deshayesiana (Nucula Deshayesiana), [241]
—— lanceolata (L. oblonga), Scotch drift, [176]
—— truncata, Scotch drift, [177]
Lee, Mr. J.E., on Pteraspis of Lower Ludlow, [463]
Leidy, Dr., on fossil quadrupeds of Nebraska, [249]
Leperditia inflata, coal-measures, [405]
Lepidodendron, Griffithsii, [441]
—— corrugatum, carboniferous., [417]
—— Sternbergii, coal-measures, [423]
Lepidolite, [499], [501]
Lepidostrobus ornatus, Coal, [424]
Lepidotus gigas, Lias, [358]
—— Mantelli, Wealden, [317]
Leptæna depressa, Wenlock, [466]
—— Moorei, Lias, [355]
Level of surface altered by change of subterranean heat, [119]
Lewis, hornblendic gneiss of, [601]
Lias, fishes of the, [358]
——, fossils of the, [354]
—— and Oolite, origin of the, [364]
——, reptiles of the, [360]
——, insects of the, [363]
——, plants of the, [364]
——, plutonic rocks of the, [571]
——, subdivisions of the, [353]
——, volcanic rocks of the, [544]
Liebig, on conversion of coal into anthracite, [403]
——, on origin of stalactite, [156]
Liége, limestone caverns at, [156]
Lightbody, Mr., on Lower Ludlow shales, [461]
Lignite, conversion of into coal, [403]
Lima giganteum, [354]
—— Hoperi, Chalk, [300]
—— spinosa, White Chalk, [294]
Limagne d’Auvergne, Lower Miocene mammalia of the, [234]
Limburg beds, [242]
Lime, scarcity of, in metamorphic rocks, [604]
—— in solution, source of, [69]
Limestone, block of striated, [168]
——, brecciated, [387]
—— of chemical and organic origin, [61]
——, compact, [501]
——, Hippurite, [304]
——, magnesian, [387]
——, metamorphic or crystalline, [579]
——, Mountain, and its fossils, [430-8]
——, striated, [168]
Limnæa longiscata, [55]
Lingula beds, volcanic tuffs of the, [549]
Lingula Credneri, Permian, [388]
Lingula Flags, fossils of the, [484]
Lingula Dumortieri, Crag, [200]
—— Lewisii, Ludlow, [462]
Lingulella Davisii, [484]
Lipari Isles, tufas in, [586]
Liquidambar europæum, [209]
Lithrostrotion basaltiforme, Carboniferous, [432]
Lits coquilliers, [275]
Littoral denudation defined, [102]
Lituites giganteus, Ludlow, [463]
Llanberis slates, [486]
Llandeilo Flags, fossils of the, [473-5]
Llandeilo formation, thickness of the, [475]
——, Lower, [475]
Llandovery Group, classification of the, [468]
—— Rocks, thickness of the Lower, [469]
Loam defined, [38], [153]
Lodes, shells and pebbles in, [608]
—— See Mineral Veins.
Loess of fluviatile loam described, [153]
——, fossil shells of the, [154]
Logan, Sir W., on Eozoon Canadense, [490]
——, on Gaspe sandstones, [455]
——, on Huronian and Laurentian, [490]
——, on stigmaria in under-clays, [398]
——, on thickness of Nova Scotia coal, [409]
——, on thickness of Laurentian in Canada, [113]
Loire, faluns of the, [211]
London Clay, fossils of the, [264], [266]
Longevity, relative, of mammalia and testacea, [162]
Longmynd Group, fauna of the, [486]
Lonsdale, Mr., on corals of America, [229]
——, on Devonian fossils, [449]
——, on Stonesfield slate, [345]
——, on United States Miocene corals, [229]
Lonsdaleia floriformis, Carboniferous, [432]
Lowe, Reverend R. T., on Mogador shells, [537]
Lubbock, Sir J., on the two stone-periods, [147]
Lucina serrata, Bracklesham, [262]
Ludlow formation, Upper, [459]; Lower, [461]
——, bone-bed of the Upper, [459]
Lulworth Cove, dirt-bed of, [333]
Lycett, Mr., on fossils of the Great Oolite, [344]
Lycopodiaceæ of Coal, [422]
Lycopodium densum, living species, [423]
Lym-fiord, mingled fresh-water and marine strata of, [59]
Lymnea caudata, Headon, [256]
—— longiscata, Bembridge, [253]
Lynton Group of Devon, [454]
MACLAREN, Mr., on Pentland Hills, volcanic rocks, [548]
Macclesfield, marine shells 1,200 feet high at, [181]
MacClintock, Sir L., on Atlantic mud, [287]
MacCulloch, Dr., on Aberdeenshire granite, [558]
——, on basaltic columns in Skye, [510]
——, on formation of hornblende-schist, [582]
——, on trap, [519]
MacMullen, Mr. J., on Eozoon Canadense, [491]
Macropus atlas, lower jaw of, [158]
—— major (living), lower jaw of, [159]
Madeira, beds of laterite in, [509]
——, dike in valley in, [513]
——, Pliocene leaf-bed and shells in lavas of, [532]
——, Miocene volcanic rocks of, [536]
——, wind, removing scoriæ in, [97]
Maestricht beds and their fossils, [283]
Maffiotte, Don Pedro, cited, [538]
Magas pumila, White Chalk, [294]
Magnesian Limestone defined, [38]
—— and marl-slate, [387]
Magnetite, [500]
Maidstone, Upper Cretaceous fossils of, [297]
Malacolite, [502]
Malaise, Professor, on Engihoul cave, [157]
Mammalia, anterior to Paris gypsum, table of, [329]
——, extinct, coeval with man, [152], [157]
——, fossil, of Middle Purbeck, [325]
——, fossil, in Pliocene in Val d’Arno, [208]
——, fossil, in the Crag, [193], [197]
——, fossil, of Vienna basin, [225]
—— of the Limagne d’Auvergne, [234]
—— of Siwalik Hills, [227]
—— of the Stonesfield slate, [345]
——, teeth of Post-pliocene, [165]
Mammalia and testacea, comparative longevity of, [162]
Mammoth, rude carving of in Perigord cave, [150]
—— in Scotch till, [175]
—— See Elephas primigenius.
Man, antiquity of, [152]
Manfredi on amount of subaërial denudation, [114]
Mantell, Dr., on iguanodon of Wealden, [313]
——, on Oxford Clay belemnites, [340]
——, on Wealden fossils, [316]
Mantellia nidiformis, Purbeck, [331]
Map of Chalk formation in France, [305]
—— of Eocene tertiary basins, [250]
—— of Hallstadt and St. Cassian beds, [376]
Marble defined, [37]
—— of Carrara, metamorphic, [599]
Marcou, M., on age of Wealden beds, [319]
Margaric acid, [591]
Marine fauna of the Carboniferous, [432]
—— beds underlying the London Clay, [269]
—— and brackish-water strata in coal, [404]
—— strata, how distinguished from fresh-water, [53-59]
Marl from Lake Superior, [63]
—— and marl-slate defined, [38]
——, red, green, and white, of Auvergne, [233]
—— slate of Middle Permian, [387]
Marsupials, extinct, of Australia, [159]
Marsupites Milleri, White Chalk, [294]
Massachusetts, plumbago of, [583]
Mastodon arvernensis, molar of, Norwich crag, [193]
—— giganteus, in United States after the drift, [183]
Mayence basin tertiaries, [242]
May-Hill Sandstone, [468]
Mechanical and chemical deposits, [60]
—— theory of cleavage, [592]
Mediterranean, one zoological province, [127]
Megalodon cucullatus, Devonian, [452]
Melania inquinata (Cerithium melanoides), [55], [268]
Melania turritissima, Bembridge, [253]
Melanopsis buccinoidea, [55]
Melaphyre, a variety of basalt, [504]
Menevian beds and their fossils, [484]
Mesozoic, term explained, [123]
—— and Cainozoic periods, gap between the, [282]
—— and Palæozoic rocks, limits of the, [385]
Metals, relative age of different, [614]
Metamorphic limestone, [579]
—— strata, origin of, [579]
—— theory, objections to, considered, [587]
—— rocks defined, [32]
Metamorphic rocks, [576]
——, cleavage of, [588]
——, scarcity of lime in, [604]
——, ages of, [597]
——, order of succession of, [602]
——, uniformity of mineral character in, [602]
Metamorphism, hydrothermal action producing, [584]
Metamorphosis of trilobites, [471], [487]
Meteorites, minerals in, [501]
Mexico, Gulf of, terrestrial remains washed into, [128]
Meyer, Mr. Karl, on fossil shells of Madeira, [537]
——, M. H. von, on reptiles in coal, [407]
——, on Wealden of Germany, [319]
Miascite, [558]
Mica and its varieties, [499], [501]
——, how deposited, [40]
—— schist or micaceous schist, [578]
Micaceous sandstone, origin of, [36]
Micraster cor-anguinum, [294]
Microconchus carbonarius, coal-measures, [405]
Microlestes antiquus, Upper Trias, [368]
Migrations of quadrupeds, [161]
Miliolite limestone, [274]
Miller, Hugh, on Old Red Sandstone fish, [443]
——, on salt lakes, [375]
Milne Edwards, Mr., on Palæozoic corals, [432]
Minchinhampton, Great Oolite of, [344]
Mineral composition a test of age of volcanic rocks, [523]
—— a test of age of plutonic rocks, [565]
—— a test of age of strata, [124]
—— character of hypogene rocks, [602]
—— springs of Auvergne, [604]
Mineral veins, [605]
—— formed in fissures, [606]
——, successive formation of, [609]
——, swelling and contraction of, [611]
——, relative age of, [614]
——, pebbles in, [608]
Mineralisation of organic remains, [65]
Minerals in meteorites, [501]
——, table of the most abundant in hypogene rocks, [499]
Miocene of Bordeaux and south of France, [214]
—— and Eocene, line between the, [230], [251]
——, Lower, of England, [244]
——, Lower, of Germany and Croatia, [242]
——, Lower, of Central France, [231]
——, Lower, of Italy, [244]
——, Lower, of Nebraska, United States, [248]
——, term defined, [143]
——, Upper, of the Bolderberg, [224]
——, Upper, of France, [211]
——, Upper, of Italy, [226]
——, Upper, of Greece, [226]
——, Upper, of India, [226]
——, Upper, of Vienna basin, [224]
Mississippi, sediment of, used as a test of denudation by rivers, [114]
—— valley, deposition and denudation in the, [102]
Mitchell, Mr., on Aralia fruit in Alum Bay, Eocene, [263]
Mitchell, Sir T., on Wellington caves, [158]
Mitchell, Rev. Hugh, on Pteraspis, [446]
Mitra Scabra, Barton clay, [259]
Mitscherlich, on Isomorphism, [502]
Modiola acuminata, Permian, [387]
Moel Tryfaen, shells found at, [181]
Mohs on isomorphism, [502]
Molasse, Lower, of Switzerland, [235]
——, Middle, or Marine, of Switzerland, [223]
——, Upper, fresh-water, of Switzerland, [217]
——, term explained, [217]
Mollusca. See Shells.
——, longevity of species of, [162]
—— of Hallstadt beds, [377]
——, value of, in classification, [142]
—— of the Carboniferous, [435]
Monitor of Thuringia, [463]
Monoclinic feldspars, [501]
Monod, flora of the Lower Molasse at, [236]
Mons, unconformable strata near, [95]
Montblanc, talcose granite of, [568]
—— Dor, Auvergne, extinct volcanoes of, [232]
——, age of volcano of, [541]
Monte Bolca, fossil fish of, [543]
—— Calvo, section of cross stratification, [44]
—— Mario, age of volcanic deposits of, [533]
—— Nuovo, formed 1538, [525]
Montmartre, gypseous series of, [270]
Monts Dome, Auvergne, extinct volcanoes, [495]
Moore, Mr. C., on Rhætic beds, [366]
——, on Upper Trias quadrupeds, [369]
Moraines described, [169]
Morea, cretaceous volcanic rocks of, [544]
Mortillet, M. de, on ice-erosion of lake-basins, [184]
Morton, Dr., on age of American cretaceous rocks, [307]
Mosasaurus Camperi, Chalk, [284]
Mountain Limestone, fossils of the, [433-8]
Mull, Isle of, leaf-bed, [247]
Münster, Count, on fossils of Solenhofen, [337]
Murchison, Sir R., on brackish-water strata in coal, [404]
——, on Devonian series, [439], [449], [454]
——, on Devonian ichthyolites, [453]
——, on Eocene igneous rocks, [278]
——, on Llandovery beds, [468]
——, on Laurentian gneiss of Scotland, [492]
——, on metamorphic rocks of North Highlands, [601]
——, on Monte Bolca fish-beds, [543]
——, on name Permian, [385]
——, on Old Red Sandstone, [449]
——, on Palæozoic strata, Queenaig, [112], [113]
——, on protrusion of solid granite, [574]
——, on Silurian, [458], [459], [461], [467], [470], [473], [475]
——, on Tertiary volcanic rocks of Italy, [533]
——, on thickness of chalk in Russia, [287]
——, on thickness of the Trias, [369]
——, on the Upper “Old Red”, [468]
Murchisonia gracilis, [479]
Murex vaginatus, [204]
Muschelkalk, fossils of the, [378]
Muscovite, or common mica, [499], [501]
Musk-ox, fossil, in Thames valley, [161]
Myliobates Edwardsi, Bracklesham, [261]
Mytilus septifer, Permian, [387]
NAPLES, Post-pliocene volcanic rocks of, [525]
——, escape of carbonic acid near, [604]
Natica clausa, Scotch drift, [176]
—— helicoides, Chillesford beds, [192]
Natrolite, [500]
Nautilus centralis, London Clay, [266]
—— Danicus, Faxoe Chalk, [286]
—— plicatus, Hythe beds, [309]
—— truncatus, Lias, [356]
—— ziczac (Aturia ziczac), [266]
Nebraska, Miocene strata of, [248]
Necker, M., on “underlying” igneous rocks, [562]
——, on dikes in Vesuvius, [526]
Neocomian, Upper, [308]
——, Middle, [312]
——, Lower, [312]
——, use of the term, [282]
Neolithic era, [147]
Neozoic type of corals, [431]
Nerinæa Goodhallii, Coral Rag, [339]
Nerinæan limestone, [340]
Nerita conoidea (N. Schmidelliana), [275]
—— costulata, Great Oolite, [345]
—— granulosa, [55]
Neritina concava, Headon, [255]
—— globulus, [55]
Neufchâtel, coins and iron tools in lake of, [149]
Newberry, Dr., on flora of American cretaceous rocks, [307]
Newcastle coal-field, faults in, [90]
Newfoundland bank described, [106]
New Jersey, mastodon in, [183]
New Madrid, “Sunk Country” in, [402]
New Red sandstone of Connecticut Valley, [381]
——, trappean rocks of the, [545]
New York, Devonian strata of, [456]
——, Cambrian strata of, [490]
——, Silurian strata of, [478]
——, Laurentian strata of, [491]
Niagara Limestone, fossils of the, [479]
Nidau, iron tools in lake of, [148]
Nile, homogeneous mud of the, [154]
Ninety-fathom dike in coal, [90]
Nipadites ellipticus, Sheppey, [264]
Nodules in strata, how formed, [63]
Noeggerathia cuneifolia, Permian, [393]
Nomenclature of rocks, [140]
—— of volcanic minerals, [499]
Norfolk cliffs, drift of, [190]
North America. See America.
Norway, Cambrian of, [489]
——, foliation of crystalline schists in, [595]
——, granite veins in gneiss of, [573]
——, granite altering fossiliferous strata in, [581]
Norwich, or Fluvio-marine crag, [193]
Nova Scotia coal-measures, [409]
—— coal, reptiles and shells in, [414]
——, folding and denudation of beds in, [417]
Nucula Cobboldiæ, Crag, [194]
Nummulites lævigata, Bracklesham, [260]
—— Puschi, Pyrenees, [278]
—— variolaria, Bracklesham, [259]
Nummulitic formations, [277]
OBOLUS APOLLINIS, in Russian grit, [478]
Obsidian, [505]
Oceanic areas, permanence of, [117]
Œningen, Upper Miocene beds of, [215]
Oeynhausen, M. von, on Cornish granite veins, [560]
Ogygia Buchii, [474]
Oldhamia radiata: O. antiqua, [487]
Old Red Sandstone, Upper, [440]
——, Middle, with fish, [443]
——, Lower, [446]
——, trap of the, [547]
——, classification of, [439]
Olenus micrurus, [484]
Oligocene, term for Lower Miocene, [230], [244]
Oligoclase, [499], [500]
Oliva Dufresnii, Bolderberg, Belgium, [224]
Olivine, [499]
Omphyma turbinatum, Silurian, [466]
Onchus tenuistriatus, Silurian, [460]
Oolite, classification and physical geography of the, [321]
——, defined, [37]
——, Inferior, fossils of the, [349], [350]
—— and Lias, origin of the, [364]
—— and Chalk, Palæontological break between, [338]
Oolitic strata, palæontological relations of, [351]
—— volcanic rocks, [545]
Ophioderma tenuibrachiata, Lias, [357]
Oppel on zones of Lias, [353]
Orbigny, Alcide de, on foraminifera of Vienna basin, [225]
——, on orbitoidal limestone, [279]
——, on Pisolitic limestone, [285]
——, on Sénonian, [302]
Oreodaphne Heerii, Italian Pliocene, [209]
Organic remains, mineralisation of, [65]
——, tests of age of strata, [125]
——, tests of age of volcanic rocks, [522]
——, geological provinces of, [127]
Oriskany Sandstone, [478]
Orthis elegantula, Ludlow, [46]
—— grandis, Caradoc beds, [470]
—— tricenaria, Bala beds, [470]
—— vespertilio, Bala beds, [470]
Orthoceras duplex, [474]
—— Ludense, Silurian, [463]
—— laterale, [436]
—— ventricosum, Silurian, [462]
Orthoclase, [499], [500]
Orthoclastic feldspars, [501]
Osborne or St. Helen’s series, Eocene, [255]
Osteolepis, Old Red Sandstone, [444]
Ostraceon, spine of, Bracklesham, [261]
Ostrea acuminata, Fuller’s earth, [349]
—— carinata, Chalk marl, [300]
—— columba, Chloritic sand, [300]
—— gregarea, Coral Rag, [339]
—— deltoidea, Kimmeridge clay, [336]
—— distorta, Middle Purbeck, [324]
—— expansa, Portland sand, [336]
—— Marshii, Oolite, [351]
—— vesicularis, Chalk, [295]
Otodus obliquus, Bracklesham, [262]
Outcrop of strata, [83]
Overlapping strata, [95]
Owen, Professor on Archæopteryx, [337]
——, on Eocene Zeuglodon, [279]
——, on footprints in Trias, [382]
——, on fauna of Sheppey, [265], [267]
——, on Gastornis Parisiensis, [276]
——, on Labyrinthodon, [370]
——, on mammalia of Stonesfield, [347]
——, on Purbeck mammalia, [326], [328]
——, on reptiles of coal, [407], [414]
——, on zoological provinces of extinct animals, [160]
Ox, tooth of (recent), [165]
Oxford Clay, thickness and fossils of, [340]
PAGHAM, erratic block at, [182]
Palæaster asperimus, [472]
Palæchinus gigas, Mountain Limestone, [43]
Palæocoma tenuibrachiata, Lias, [357]
Palæoniscus, Permian fish, [389]
—— comptus, P. elegans, P. glaphyrus, [390]
Palæotherium magnum, [254]
Palæophis typhoeus, Bracklesham, [261]
Palæozoic or Paleozoic, term defined, [123]
—— Plutonic rocks, [572]
—— rocks, [458]
—— type of corals, [431]
Palagonia, dikes of lava in, [531]
Paleolithic era, [147], [149]
——, alluvial deposits of, [150]
Palm in Swiss Miocene, [237]
Palma, volcanic crater of, [497]
Paludina lenta, Hempstead beds, [55]
—— orbicularis, Bembridge, [253]
Paradoxides Bohemicus, [488]
—— Davidis, Lower Cambrian, [485]
Parallelism of folded strata for long distances, [93]
Paris basin, Tertiary group first studied in, [141]
——, Tertiaries of the, [270]
Parka decipiens, “Old Red,” [448]
Parkfield Colliery, ground-plan of, [400]
Patagonia, strata of, rich in soda, [587]
Patella rugosa, Great Oolite, [345]
Paterson, Dr., on angiosperm of the Coal, [429]
Peach, Mr. C, cited, [601]
——, Pteraspis, found by, [443]
Pearlstone, [505]
Pebbles in mineral veins, [608]
—— in chalk, [292]
Pecopteris elliptica, Coal, [421]
Pecten Beaveri, White Chalk, [294]
—— cinctus, Neocomian, [312]
—— islandicus, Scotch Drift, [176]
—— jacobæus, in tertiary of Sicily, [206]
—— quinque-costatus, [300]
—— Valoniensis, Rhætic beds, [366]
Pegmatite, [553]
Penarth beds, [366]
Pengelly, Mr., on Bovey Tracey lignite, [246]
——, on flint-knives of Brixham Cave, [157]
Pentacrinus Briareus, Lias, [357]
Pentamerus Knightii, Aymestry, [461]
—— oblongus, and P. lirata, [469]
Pentland Hills, volcanic rocks of the, [548]
Perigord cave, carving of mammoth in, [150]
Permanence of continents and oceans, [117]
Permian Flora, [392]
—— of Germany, [393]
—— strata, thickness of, in north of England, [386]
——, Upper and Middle, [386], [387]
——, Lower, [390]
Perna Mulleti, Neocomian, [310]
Petherwyn, Devonian fossils of, [450]
Petrifaction, process of, [67]
Petrophiloides Richardsoni, Sheppey, [25]
Pahcops caudatus, Silurian, [467]
—— latifrons, Devonian, [450]
Phascolotherium Bucklandi, [348]
Phasianella Heddingtonensis, and cast, [66]
Phillippi, on tertiary shells of Sicily, [205]
Phillips, Professor, on fossils distorted by cleavage, [592]
——, on ninety fathom dike, [90]
——, on Wenlock limestone and shale, [465], [467]
——, on Yoredale series, [395]
Phillips, Mr. J. Arthur, on origin of gold of California, [617]
Phlebopteris contigua, Inferior Oolite, [350]
Phlogopite, [499], [501]
Pholadomya fidicula, Inferior Oolite, [350]
Phonolite, [506]
Phorus extensus, London Clay, [266]
Phragmoceras ventricosum, Silurian, [463]
Physa Bristovii, Middle Purbeck, [325]
—— columnaris, [55]
—— hypnorum, [55]
Piedmont, absence of lakes in, [186]
Pile dwellings of Switzerland, [148]
Pilton, group of, Devon, [449]
Pinnularia in Atlantic mud, [288]
Pinus sylvestris in peat, [147]
Pisolitic limestone of France, [285]
Pitchstone, [505]
Placodus gigas, Muschelkalk, [380]
Placoids, rare in Old Red Sandstone, [443]
Plagiaulax Becklesii, jaw and molar of, [327]
Plagioclastic feldspars, [501]
Plagiostoma giganteum, Lias, [354]
—— Hoperi, Chalk, [300]
Planorbis discus, Bembridge, [253]
—— euomphalus, [55], [255]
Plants of Bovey Tracey, Miocene, [247]
——, fossil fresh-water, [57]
—— of the Coal, [420]
—— of the Lias, [364]
—— of the Swiss Upper Miocene, [219]
Plas Newydd, rock altered by dike near, [515]
Plastic Clay, Eocene, [267]
Platanus aceroides, Miocene, [221]
Platystoma Suessii, Hallstadt, [377]
Playfair, on amount of subaërial denudation, [114]
—— on faults, [87]
Plectrodus mirabilis, Ludlow, [460]
Plesiosaurus dolichodeirus, Lias, [361]
Pleurotoma attenuata, Bracklesham, [262]
—— exorta, Eocene, [57]
Pleurotomaria anglica, and cast, [66]
—— carinata (flammigera), [434]
—— granulata, Inferior Oolite, [351]
—— ornata, Inferior Oolite, [351]
Plieninger, Professor, on Triassic mammifer, [368]
Pliocene glacial formations, [189-92]
—— Period, [189]
—— plutonic rocks, [565]
—— strata of Sicily, [204]
——, term defined, [143]
—— volcanic rocks, [529]
Plombières, alkaline waters of, [585]
Plumbago of Massachusetts, [583]
Plutonic and sedimentary formations, diagram of, [567]
——, origin of the term, [551]
—— rocks, Mesozoic, [570]
——, Recent and Pliocene, [565]
——, Miocene and Eocene, [568]
——, uncertain tests of age of, [564]
—— defined, [31]
Podocarya Bucklandi, Oolite, [348]
Polypterus of the Nile, [444]
Polyzoa and Bryozoa, terms explained, [197]
Pomel, M., on fossil mammalia of the Limagne, [235]
Ponza Islands, globiform pitchstone of, [512]
Porites pyriformis, Devonian, [451]
Porphyritic granite, [556]
Porphyry, [506]
Portland, Cycads in dirt-bed of, [331]
—— oolite and sand, [334]
“Portland screw,” a cast of a shell, [335]
Porto Santo, marine shells in volcanic tuff of, [536]
Post-pliocene period, climate of the, [161]
—— mammalia, teeth of, [163]
——, term defined, [145]
—— lakes of Switzerland, [185]
—— volcanic rocks, [524]
Potamides cinctus, [56]
Pothocites Grantonii, coal-measures, [429]
Potsdam Sandstone, [480], [489]
Pot-stones in the Chalk, [290]
Pottsville, coal seams of, [400]
Powrie, Mr., on Cephalaspis beds, [446]
——, on Parka decipiens, [448]
Pratt, Mr., on Eocene Isle of Wight mammalia, [254]
Predazzo, altered rocks at, [571]
Pressure, solidifying rocks, [65]
Prestwich, Mr., on age of Sables inferieurs, [276]
——, on Chillesford beds, [192]
——, on Coalbrook Dale insects, [405]
——, on Eocene strata, [267], [269]
——, on faults in coal-measure of Coalbrook Dale, [88]
——, on shells of London clay, [264]
——, on thickness of Coralline Crag, [198]
Prévost, M. Constant, on Paris basin, [270]
Primary Limestone, [579]
—— rocks, [458]
——, term defined, [123]
“Primordial Zone” of Bohemia, [481], [482]
Productus horridus, Permian, [388]
—— semireticulatus (antiquatus), [434]
Progressive development indicated by low grade of early mammals, [384]
Proteaceæ of Aix-la-Chapelle flora, [304]
—— of Lower Molasse, Switzerland, [237]
—— of Œningen beds, [221]
Protogine, [578]
Protosaurus of Thuringia, [390], [464]
Protrusion of solid granite, [574]
Provinces of animals and plants, [126]
Psammodus porosus, [437]
Pseudocrinites bifasciatus, Silurian, [466]
Psilophyton princeps, Devonian, [455]
Pteraspis in Lower Ludlow shale, [463]
Pterichthys, Old Red Sandstone, [445]
Pterodactyl of Kentish chalk, [297]
Pterodactylus anglicus, Old Red, [447]
—— crassirostris, Solenhofen, [337]
Ptychodus decurrens, White Chalk, [297]
Pudding-stone or conglomerate, [36]
——, formation of, [62]
Pumice, [508]
Punfield beds, brackish and marine, [318]
Pupa muscorum, [155]
—— tridens, Loess, [56]
—— vetusta, Coal, [415]
Purbeck beds, Upper, Middle, and Lower, [323], [324], [336]
——, fossil mammalia of the Middle, [325]
—— marble, [324]
——, subdivisions of the, [333]
Purity of coal, cause of, [402]
Purpura tetragona, Red Crag, [196]
Purpuroidea nodulata, Great Oolite, [345]
Puy de Côme, cone and lava-current of, [528]
—— de Tartaret, lava-current and cone of, [527], [542]
—— de Pariou, crater of the, [529]
Puzzuoli, elevation of land at, [525]
Pygopterus mandibularis, Permian, [390]
Pyrenees, chalk altered by granite in the, [570]
——, curved strata in, [86]
——, lamination of clay-slate in, [596]
Pyroxene group of minerals, [499], [502]
Pyrula reticulata, Crag, [200]
QUADER-SANDSTEIN, Cretaceous age of the, [293]
Quadrumana of Gers, [215]
Quadrupeds, extinct, in Paleolithic gravels, [152]
Quartz, specific gravity of, [499], [500], [555]
Quartzite or Quartz Rock, [579]
Queenaig, unconformable Palæozoic strata at, [112]
Quenstedt on zones of Lias, [353]
RADABOJ Miocene, brown coal of, [242]
Radiolites foliaceus, White Chalk, [306]
—— Mortoni, White Chalk, [295]
—— radiosa, White Chalk, [306]
Radnorshire, stratified trap in, [549]
Rain-prints with worm tracks in Coal, [416]
——, carboniferous, [416]
Ramsay, Professor, on break between Upper and Lower Cretaceous, [301]
——, on breccias in Permian, [391]
——, on escarpments, [104]
——, on denudation, [98]
——, on ice-erosion of lake-basins, [184]
——, on Lingula Flags, [484]
——, on position of Tremadoc beds, [483]
——, on Silurian metamorphic rocks, [602]
——, on submergence in glacial period, [181]
——, on thickness of the Lower Trias, [372]
——, on thickness of Llandeilo beds, [475]
——, on thickness of the Bala beds, [473]
——, on volcanic tuffs of Snowdon, [549]
——, on zones of the Lias, [353]
Rastrites peregrinus, Llandeilo Flags, [473]
Rath, Von, on Tridymite, [500]
Recent Period defined, [145]
—— volcanic rocks, [524]
Record, imperfection of, in the earth’s crust, [138]
Red Crag, older Pliocene, [194]
—— Sandstone, Origin of, [374]
—— Sea and Mediterranean, distinct species in, [127]
Redruth, Cornwall, section of veins in mine, [607]
Reindeer Period in South of France, [149]
Relistran mine, pebbles in tin of, [609]
Reptiles of the Coal, [406], [413]
Reptiles of the Lias, [360]
Retepora flustracea, Permian, [388]
Rhætic beds between Lias and Trias, [366]
Rhine, fresh-water strata of the, [53]
——, loess of the, [154]
Rhinoceros in drift of Abbeville, [153]
—— leptorhinus (megarhinus), molar of, [164]
—— tichorhinus, molar of, [164]
Rhode Island, metamorphic rocks of, [583]
Rhynchonella navicula, Ludlow, [460]
—— octoplicata, White Chalk, [294]
—— spinosa, Inferior Oolite, [350]
—— Wilsoni, Aymestry, [462]
Richmond, Virginia, Triassic coal-field of, [382]
Rigi and Speer, Lower Miocene of the, [235]
Rimula clathrata, Great Oolite, [345]
Rink, Mr., on Greenland land-ice, [171]
Ripple-marked sandstone, how formed, [46]
Rise and fall of land, [146]
Rissoa Chastelii, Hempstead beds, [245]
Rivers, denuding powers of, [101], [114]
Roches moutonnees described, [169]
Rock, term defined, [26]
Rocks altered by volcanic dikes, [514]
—— altered by subterranean gases, [586]
——, analysis of minerals in, [499]
——, aqueous or stratified, [27]
——, classification of, [121]
——, great thickness of palæozoic, [110]
——, glacial scorings on, [169]
——, metamorphic, age of, [597]
——, plutonic age of, [564]
——, volcanic, age of, [520]
——, trappean, [497]
——, metamorphic, defined, [32]
——, four classes of contemporaneous, [33]
——, plutonic, defined, [31]
——, tests of age of, [123], [125], [520], [564], [597]
——, four contemporaneous classes of, [122]
——, underlying, not always the oldest, [122]
——, volcanic, defined, [29]
Rock-salt of Trias, [371]
——, origin of, [374]
Rogers, Mr. H. D., on blending of coal-seams, [400]
——, on Virginian fault, [92]
Rose, Gustavus, on isomorphism, [502]
——, on Fifeshire dike, [546]
——, on quartz in granite, [555]
Rosso antico, red porphyry of Egypt, [506]
Rostellaria (Hippocrenes) ampla, London Clay, [266]
Roth, M., on Miocene of Greece, [226]
Runn of Cutch, salt of, [375]
Rupelian beds of Dumont, [241], [242]
Russia, glaciation of, [174]
——, Devonian of, [454]
——, Silurian strata of, [478]
SAARBRUCK, reptiles in coal-field of, [406]
Sabal major, Lower Miocene, [237]
Sables de Bracheux, [276]
—— moyens, Paris basin, [273]
Sahlite, [502]
St. Abb’s Head, curved strata of, [76]
——, unconformable stratification at, [94]
St. Andrews, carboniferous trap-rocks of, [545]
St. Cassian, fossil mollusca of, [377]
—— and Hallstadt beds, [376]
St. David’s, Menevian beds of, [485]
St. Mary’s, shells of, [539]
Salt, rock, origin of, [372]
Salter, Mr., on fossils of Arenig group, [476]
——, on Menevian beds, [485]
——, on Tremadoc fossils, [483]
Sandberger, Dr. F., on Mayence basin, [242]
Sandstone, New Red, [369]
——, Old Red, [439]
—— slab with cracks, [317]
——, slab of ripple-marked, [45]
—— slab with footprints, [408]
Sao hirsuta, [488]
Saurians of the Lias, [361]
——, sudden destruction of, [362]
Saurichthys apicalis, Rhætic Beds, [367]
Saussure, on vertical conglomerates, [73]
Saxicava rugosa, Scotch drift, [176]
Saxony, beds of minerals in, [609]
Scandinavia, glaciation of, [174]
Scaphites æqualis, Chloritic marl, [299]
Scapolite, [506]
Scheerer on action of water in metamorphism, [585]
Schist, mica, [578]
——, argillaceous, [579]
——, hornblende, [578]
Schizodus Schlotheimi, Permian, [387]
—— truncatus, Permian, [387]
Schmerling, Dr., on Liége caverns, [157]
Schorl-rock, and schorly granite, [557]
Schwab, M., on Celtic coins in lake-dwellings, [149]
Scoliostoma, St. Cassian, [377]
Scoresby, on Arctic icebergs, [172]
Scoriaceous lava, [507]
Scoriæ, [508]
Scotland, “Fundamental gneiss” of, [493]
——, Old Red Sandstone of, [440]
——, glaciation of, [175]
Screws, fossil, internal casts of shells, [66]
Scrope, Mr., on Isle of Ponza, globiform pitchstone, [512]
——, on minerals in lava, [524]
——, on water in lava, [555]
Scudder, Mr., on Devonian insects of Canada, [457]
Sea, apparent fall of, caused by rise of land, [70]
——, denuding power of the, [105]
——, deep soundings in, [287]
——, mean depth of the, [118]
—— cliffs, inland, [103]
Secondary and Tertiary, gap between the, [281]
——, term defined, [123]
Section of Auvergne alluvium, [100]
—— of carboniferous rocks, Lancashire, [85]
—— of chalk and greensand, [287]
—— of crags near Woodbridge, Suffolk, [198]
—— of cross-stratification, [42-44]
—— of curved strata of the Jura, [82]
—— of dirt-bed in Isle of Portland, [332]
—— of Forfarshire, showing curved strata, [74]
—— of fossil tree, showing texture, [67]
—— of folded and denuded carboniferous beds, Nova Scotia, [418]
—— of the Oolitic strata, [322]
—— of Recent and Post-pliocene alluvial deposits, [151]
—— showing creeps in coal-mines, [79]
—— of slaty cleavage, [589]
—— showing valleys of denudation, [98]
—— showing the Weald formation, [313]
—— of strata thinning out, [41]
—— of superimposed groups at Dundry Hill, [130]
—— of unconformable strata near Mons, [95]
Sections illustrating faults, [88], [90], [91]
Sedgwick, Professor, on the Cambrian Group, [481], [482], [486]
——, on classification of Arenig group, [476]
——, on Devonian series, [439], [449]
——, on position of the May-Hill beds, [568]
——, on protrusion of solid granite, [574]
——, on slaty cleavage, [588], [591]
——, on garnet in altered rock, [515]
——, on concretionary structure, [63]
Sediment, accumulation of, causing a shifting of the subterranean, [117]
isothermals. Sedimentary beds of the Carboniferous, [396]
Selsea Bill, erratics at, [182]
Senarmont on action of water in metamorphism, [585]
Sequoia Langsdorfii, [238]
“Seraphim,” head of Pterygotus anglicus, [446]
Serapis, marine littoral deposits of, [146]
Serpentine, [578]
Serpulæ attached to Gryphæa, [48]
—— attached to Spatangus, [49]
—— attached to Apiocrinus, [343]
Shale defined, [36]
—— of the Lower Ludlow, [461]
Sharpe, Mr. D., on American Silurian fossils, [479]
——, on fossils distorted by cleavage, [592]
Shell-mounds of Denmark, [146]
Shells, Arctic, in Scotch drift, [177]
——, derivative, in the Crag, [195-203]
——, marine, found at great heights above the sea, [29]
——, proportion of living, in the Crags, [194], [195], [199]
——, value of, in classification, [142]
——, fossil, of Virginia, [228]
—— of the London clay, [266]
—— of the mountain limestone, [433]
—— of the Barton clay, [258]
—— of the Oolite, [335], [345], [350]
——, marine, of Moel Tryfaen, [180]
Sheppey, fauna and flora of, [264]
——, Eocene fish of, [267]
Sherringham, erratic block at, [191]
Shetland, granite of, [558]
——, hornblende-schist of, [583]
Sicily, fauna and flora of, older than the country itself, [207]
——, newer Pliocene strata of, [204]
——, subterranean igneous action in, [569]
——, undulating gypseous marls of, [86]
——, volcanic dikes of, [531]
Sidlaw Hills, trap of, [548]
Sigillaria in coal-measures, [380], [411], [425]
Sigillaria lævigata, coal-measures, [426]
Siliceous limestone defined, [37]
Silurian, derivation of the name, [458]
——, granite of Norway, [573]
——, metamorphic, of North Highlands, [601]
—— rocks, classification of, [458]
—— strata of the continent of Europe, [477]
—— strata of United States, [478]
—— volcanic rocks, [548]
Siphonotreta unguiculata, obolus grits, [478]
Siwâlik Hills, fresh-water deposits of, [226]
Skaptar Jokul, flow of lava from, [523]
Skye, hypersthene rocks of, [491]
——, Isle of, Miocene syenite of the, [568]
——, trap dike in, [514]
Slaty cleavage, [588]
Slicken-sides, in opposite walls of veins, [608]
——, term defined, [87]
Smilax sagittifera, Œningen, [222]
Smith, Mr. W., on White Lias bed, [366]
Snowdon, volcanic tuffs of, [549]
Soissonnais sands, [275]
Solenastræa cellulosa, Brockenhurst, [257]
Solenhofen stone, fossils in the, [337]
Solfatara, decomposition of rocks in the, [586]
Somma, cone and dikes of, [526]
Sopwith, Mr. T., models of outcrop of strata, [85]
Sorby, Mr., on action of water in metamorphism, [585]
——, on glass cavities in quartz, [555]
——, on mechanical theory of cleavage, [592]
——, on ripple-marks in mica schist, [596]
South Joggins, section of cliffs at, [410]
Spalacotherium, Purbeck, [346]
Spatangus radiatus, Chalk, [284]
—— with serpula attached, [49]
Species, gradual change of, [139]
—— older than the land they inhabit, [207]
——, similarity of conditions causing reappearance of, [311]
Specific gravity of basalt and trachyte, [504]
Speer and Rigi, Lower Miocene of the, [235]
Speeton Clay, [311]
Sphærexochus mirus, Silurian, [467]
Sphærulites agariciformis, White Chalk, [306]
—— of volcanic minerals, [499]
Sphenophyllum erosum, Coal, [425]
Sphenopteris gracilis, Hastings sands, [318]
Spheroidal concretions in limestone, [64]
Spicula of sponge, Atlantic mud, [288]
Spirifera disjuncta, Devonian, [450]
—— alata, Permian, [388]
—— mucronata, [454]
—— trigonalis, and S. glabra, [434]
Spiriferina Walcotti, Lias, [355]
Spirolina stenostoma, Eocene, [275]
Spirorbis carbonarius, coal-measures, [405]
Spondylus spinosus, White Chalk, [294]
Sponge in flint from White Chalk, [296]
Sponges, vitreous, in the chalk, [291]
Springs, mineral of Auvergne, [604]
Staffa, age of columnar basalt of, [539]
Stalactite, origin of, explained, [156]
Starfish in Silurian strata, [472]
Stations of species affecting distribution of fossils, [354]
Stauria astræiformis, [431]
Stereognathus of Stonesfield, [348]
Sternberg, Count, on insects in coal, [495]
Stigmaria attached to trunk of Sigillaria, [427]
—— in coal-measures, [398], [411], [426]
—— ficoides and surface showing tubercles, Coal, [427]
Stilbite, [500]
Stiper-Stones or Arenig Group, [475]
Stockwerk, assemblage of veins, [605]
Stonesfield slate, mammalia of the, [345]
Strata, term defined, alternations of fresh-water, and shallow and deep, [27]
sea. ——, alternations of marine and fresh-water, [108]
——, curved, inclined, and vertical, [73]
——, apparent horizontality of inclined, [81]
——, contorted in drift, [178]
——, contortion of, in Cyclopean Isles, [530]
——, general table of fossiliferous, [131]
——, horizontality of, [40]
——, origin of metamorphic, [83]
——, overlapping, [95]
—— repeated by being doubled back, [87]
——, slow growth of, attested by fossils, [47-50]
—— of organic origin, [51]
——, tests of age of, [123]
——, unconformability of, [94], [138]
——, vast thickness of, not forming high mountains, [109-13]
Stratification, diagonal or cross, [42], [44]
——, different forms described, [39]
—— of metamorphic rocks considered, [580]
Stratified rocks, composition of, [35]
Striæ, production of, [168]
Strickland, Mr., on thickness of the Trias, [369]
Stricklandinia lirata, [469]
Strike, term explained, [80]
Stringocephalus Burtini, [452]
Stromboli, lava of, [566]
Strophomena depressa, Wenlock, [466]
—— grandis, [471]
Studer, Mr., on gneiss of the Jungfrau, [599]
subaërial denudation, average annual amount of, [113]
Subapennine beds, proportion of recent species in, [143]
—— strata, older Pliocene, [208]
Submarine denudation, [105]
Subsidence of land must preponderate over upheaval, [116]
Succinea amphibia, [55]
—— elongata, [155]
Suess, M., on fossils of St. Cassian beds, [376], [377]
——, on Vienna basin, [225]
Suffolk, Crag of, [195]
“Sunk country,” New Madrid, [402]
Superga, Lower Miocene of the, [244]
Superior, Lake, marl in, [63]
Superposition of deposits, a test of age, [124]
—— a test of age of volcanic rocks, [521]
Sutherlandshire, unconformable Palæozoic strata in, [112]
Swanage, fossil mammalia found at, [326]
Sweden, Cambrian of, [489]
——, slow rise of land in, [72]
——, small thickness of Silurian strata in, [477]
Switzerland, lake-dwellings of, [148]
——, Lower Molasse of, [235]
——, Middle or Marine Molasse of, [223]
——, Upper Miocene of, at Œningen, [215]
Sydney coal-field, rain-prints in, [416]
Syenite, composition of, [552], [557]
——, how far connected with trap-rocks, [558]
Syenitic granite, [557]
Symonds, Rev. W. S., on Moel Tryfaen shells, [180]
Synclinal and anticlinal curves, [74], [85]
TABLE of Botanical Nomenclature, [303]
—— of St. Cassian fossil mollusca, [377]
—— of Cretaceous formations, [283]
—— of Devonian series in Devon, [449]
—— of divisions of Hastings Sand, [316]
—— of English and French Eocene strata, [252]
—— of ages of fossil vertebrata, [464]
—— of Neocomian strata, [308]
—— of mammalia older than Paris gypsum, [329]
—— of marine testacea in the Crag, [202]
—— of Oolitic strata, [321]
—— of volcanic minerals, [499]
—— of Silurian strata of United States, [478]
—— of Silurian rocks, [458]
—— of Triassic strata, [375]
—— of Cambrian strata, [482]
—— of Permian of north of England, [386]
—— of Welsh coal-measures, [394]
—— of thicknesses of Carboniferous limestone, [395]
——, general, of fossiliferous strata, [131]
Table Mountain, granite veins in clay-slate of, [560]
Tails of homocercal and heterocercal fish, [389]
Talcose granite, [557]
—— gneiss, [578]
Tarannon shales, [468]
Tartaret cone, and lava of, [527], [542]
Tate, Mr., on St Cassian fossils, [377]
Tealby series, Middle Neocomian, [312]
Teeth of extinct mammalia, [163], [164]
Tellina balthica (T. solidula), [190]
—— calcarea (T. proxima), [177]
—— obliqua, Crag, [194]
Temnechinus excavatus, [200]
Temnopleurus excavatus, [200]
Tentaculites annulatus, Silurian, [489]
Terebellum fusiforme, Barton, [259]
—— sopita, Barton, [259]
Terebratula affinis, Aymestry, [462]
—— biplicata, White Chalk, [294]
—— carnea, White Chalk, [294]
—— digona, Bradford clay, [345]
—— fimbria, Inferior Oolite, [350]
—— hastata, Mountain Limestone, [434]
—— sella, Neocomian, [310]
—— Wilsoni, Aymestry, [462]
Terebratulina striata, White Chalk, [294]
Terebrirostra lyra, Chloritic Sand, [300]
Teredo navalis, boring wood, [50]
Tertiary formations, classification of, [137], [143]
—— strata, subdivisions of, [143]
——, term defined, [123]
Testacea. See Shells.
Thallogens, [303]
Thamnastræa, Coral Rag, [339]
Thanet sands, [269]
Theca operculata, Tremadoc beds, [483]
Thecodontosaurus, tooth of, [374]
Thecodus parvidens, Ludlow, [460]
Thecosmilia annularis, Coral Rag, [339]
Thirria, M., on Nerinæan limestone, [340]
Thompson, Dr., on Nummulites of Thibet, [277]
Thomson, Wyville, on Atlantic mud, [288]
——, on sponges in chalk mud, [292]
Thuringia, monitor of, [390], [463]
Thurmann, M., on Bernese Jura Oolite, [344]
——, on structure of the Jura, [83]
Thylacotherium Prevostii, Stonesfield, [347]
Tile-stones of the Upper Ludlow, [459]
Tilgate forest, fossil Iguanodon in, [315]
Till described, [166]
——, mammoth in Scotch, [175]
—— of North America, [182]
Tin veins, age of, in Cornwall, [615]
Titanoferrite, [500]
Torell, Dr., on ice-action in Greenland, [172]
——, on Swedish Cambrian fossils, [489]
Touraine, faluns of, [211]
Tourmaline, [500]
Trachytic rocks, [505]
—— tuff, [506]
—— porphyry, [506]
—— lava, age of, [523]
Trap, term defined, [498]
—— dike, intercepting strata, [518]
—— dikes, [513-17]
——, intrusion of, between strata, [517]
—— rocks, ages of, [524-50]
—— rocks passing into granite, [559]
—— tuff described, [508]
Trappean rocks, nomenclature of, [497]
—— rocks, their relation to active volcanoes, [517]
Trass of Lower Eifel, [535]
Travertin, how deposited, [60]
——, inférieur of Paris basin, [273]
Tree ferns, living, [422]
Trees erect in coal, Nova Scotia, [411]
Tremadoc slates and their fossils, [482]
Tremolite, [499], [502]
Trenton limestone, fossils of the, [479]
Trezza, volcanic rocks of, [529]
Trias, beds of passage between Lias and, [366]
—— of England, [369-74]
—— of Germany, [375]
——, Saurians of the, [370]
—— of the United States, [381]
Triassic mammifer, North Carolina, [383]
Triclinic feldspars, [501]
Tridymite, crystallised silica, [500]
Trigonellites latus, Kimmeridge clay, [336]
Trigonia caudata, Neocomian, [310]
—— gibbosa, Portland stone, [335]
Trigonocarpum ovatum, and T. olivæforme, Coal, [429]
Trigonotreta undulata, Permian, [388]
Trilobites of Bala and Caradoc beds, [471]
——, metamorphosis of, [471], [488]
—— of primordial zone, [487]
Triloculina inflata, Eocene, [275]
Trimmer, Mr., on contorted strata, [179]
——, on shells of Moel Tryfaen, [186]
Trinucleus concentricus, T. Caractaci, [472]
Trionyx, carapace of, Bembridge, [253]
Tripoli composed of diatomaceæ, [51]
Trochoceras giganteus, Ludlow, [463]
Trophon antiquum (Fusus contrarius), [196]
—— clathratum, Scotch drift, [176]
Tuff defined, [30]
——, shelly, of the Grand Canary, [538]
——, trappean, of Llandeilo rocks, [473]
——, shelly, of Gergovia, [542]
Tupaia Tana, recent, [347]
Turner, Dr., on chemical decomposition, [68]
Turrilites costatus, Chalk, [299]
Turritella multisulcata, Bracklesham, [262]
Tuscany, mineral springs of, [604]
Tylor, Mr., on amount of subaërial denudation, [114]
Tyndall, Dr., on slaty cleavage, [594]
Tynedale fault, [90]
Tynemouth cliff, brecciated limestone of, [387]
Typhis pungens, Barton clay, [259]
UNCITES Gryphus, Devonian, [452]
Unconformability of strata, [94], [138]
Underlying, term applied to plutonic rocks, [34]
Unger on American forms in Swiss Miocene flora, [223], [239]
—— on Miocene plants of Croatia, [243]
Ungulite, or Obolus grit of Russia, [477]
Unio littoralis, [54]
—— Valdensis, Hastings Sands, [317]
United States, Cambrian of the, [489]
——, Cretaceous rocks of, [307]
——, Devonian of, [455]
——, Eocene strata in the, [278]
——, footprints in Carboniferous of, [407]
——, Lower Miocene of, [248]
——, older Pliocene and Miocene formations of, [227]
——, Silurian strata of, [478]
——, Trias of the, [381]
Upheaval of land more than counteracted by subsidence, [116]
——, power of denudation to counteract, [105], [115]
Upper Greensand, or Chloritic series, [298]
Upsala, erratics on modern marine drift near, [174]
Ural Mountains, auriferous alluvium of, [616]
Uralite, [499]
Ursus spelæus, tooth of, [165]
Urville, Captain de, on size of icebergs, [172]
VAL D’ARNO, Newer Pliocene of, [207]
Valleys, origin of, [102]
Valorsine, granite veins in talcose gneiss in, [599]
Valvata piscinalis, [55]
Vanessa Pluto, Lower Miocene, Croatia, [243]
Vegetation of the Coal, [420]
—— of the Devonian of America, [455]
——. See Plants.
Veins, chemical deposits in, [612]
——, granite rocks altered by, [559]
——, different kinds of minerals, [605]
——. See Mineral veins.
Vein-stones, [610]
Venericardia planicosta, [260]
Venetz, M., on Alpine glaciers, [170]
Ventriculites radiatus, Chalk, [292]
Verneuil, M. de, on Russian Silurian, [462]
——, on Permian flora, [392]
Vertebrata, progress of discovery of fossil, [464]
Vertical strata, [73]
Vesuvius, Recent and Post-pliocene volcanic rocks of, [525]
——, basaltic lavas of, [508]
——, tufaceous strata of, [522]
——, dikes of, [527]
Vicarya Lujani, Punfield, [319]
Vicentin, columnar basalt of the, [511]
Vienna Basin, Upper Miocene beds of, [224]
Vine in Upper Miocene beds at Œningen, [221]
Virginia, eighty miles of fault in, [92]
——, coal-field of, [382]
Virlet, M, on corrosion of rocks near Corinth, [586]
——, on Cretaceous traps of Greece, [544]
——, on fossils in veins, [608]
——, on volcanic rocks of the Morea, [544]
Volcanic ash or tuff, [508]
—— breccia, [509]
—— dikes, [513-16]
—— force and denudation opposing powers, [117]
—— mountains, structure and origin of, [494]
Volcanic rocks defined, [29]
——, mineral composition of, [498]
——, Recent and Post-pliocene, [524]
——, Pliocene, [529]
——, Miocene, [536-43]
——, Eocene, [543]
——, Cretaceous and Liassic, [544], [545]
——, New Red, Permian and Carboniferous, [545]
——, Old Red Sandstone, [547]
——, Silurian, Cambrian and Laurentian, [548], [549]
—— of Auvergne, [540]
——, columnar and globular, structure of, [510]
—— of Grand Canary, [528]
—— of Silurian age, [477]
——, special forms of structure of, [506]
——, tests of age of, [520-4]
Volcanoes, extinct, [30]
—— of Auvergne, [495]
Voltzia heterophylla, Bunter, [380]
Voluta ambigua, Barton clay, [259]
—— athleta, Barton, [259]
—— Lamberti, coralline and Red Crag, [196]
—— Lamberti, faluns, [214]
—— nodosa, London clay, [266]
—— Selseïensis, Bracklesham, [262]
Von Buch, Leopold, on “elevation craters,” [496]
——, on Silurian plutonic rocks, [572]
WACKE described, [508]
Wagner, M., on Miocene of Greece, [226]
Walchia piniformis, Permian, [392]
Wales and England, glaciation of, [180]
Wallich, Dr., on Atlantic mud, [287]
Water, denuding power of running, [98], [115]
——, action of, in metamorphism, [584]
Watt, Gregory, on fusion of rock, [584]
Weald clay and its fossils, [317]
Wealden area, thickness of the, [319]
—— formation, [313]
—— flora, [320]
Webster, Mr. T., on Tertiary strata, [141]
Wellington Valley caves, [158]
Wenlock formation, fossils of the, [465-8]
—— limestone, [465]
—— shale, [467]
Werner on mineral veins in Saxony, [609]
—— on isomorphism, [502]
Westwood, Mr., on Lias beetles, [363]
Wexford, veins of copper at, [615]
Whitaker, Mr., on subaërial origin of escarpments, [104]
White or coralline crag, [197]
—— sand of Alum Bay, [38]
Whymper, Mr., on Arctic Miocene plants, [240]
Williams, Mr., on Cornish lodes, [607]
Williamson, Professor, on Conifers of the Coal, [428]
——, on structure of calamite, [425]
Wind, denuding action of the, [97]
Wood, Mr. Searles, on Bridlington shells, [190]
——, on Chillesford and Aldeby beds, [192]
——, on shells of the Crags, [194], [195], [199]
——, on shells of Crag and faluns compared, [213]
——, on fish of Headon series, [255]
——, table of marine testacea of the Crag, [202]
——, on thickness of coralline crag, [198]
Woodward, Dr., on St. Cassian fossils, [377]
Woodward, Mr. H., on Pterygotus, [447]
Woolhope beds, [467]
Woolwich and Reading series, [267]
Wright, Dr., on Barton shells, [258]
——, on zones of the Lias, [353]
Wunsch, Mr. E. A., on trees in volcanic ash, [546]
Wyville Thomson. See Thomson.
XIPHODON gracile, Paris basin, [271]
Xylobius Sigillariæ, Nova Scotia coal, [415]
YOREDALE beds, thickness of the, [395]
Yorkshire, Oolite of, [349]
Young, Mr., on seeds washed out of mammoth tusks, [176]
ZECHSTEIN of Germany, [392]
Zeolites, secondary volcanic minerals, [500]
Zeuglodon cetoides, Eocene, United States, [280]
Zircon-syenite, [558]
Zoantharia rugosa and Z. aporosa, [431]
Zones of the Lias, [353]
Zonites priscus, Coal, [415]
Zoological provinces, great extent of, [127]
Zoophytes, fossil, [48]
——. See Corals, Bryozoa, etc.
Zurich, lake-dwellings in Lake of, [148]