CHAPTER XXVII.

SILURIAN GROUP.

Silurian strata formerly called transition — Term grauwacké — Subdivisions of Upper and Lower Silurian — Ludlow formation and fossils — Wenlock formation, corals and shells — Caradoc and Llandeilo beds — Graptolites — Lingula — Trilobites — Cystideæ — Vast thickness of Silurian strata in North Wales — Unconformability of Caradoc sandstone — Silurian strata of the United States — Amount of specific agreement of fossils with those of Europe — Great number of brachiopods — Deep-sea origin of Silurian strata — Absence of fluviatile formations — Mineral character of the most ancient fossiliferous rocks.

We come next in the descending order to the most ancient of the primary fossiliferous rocks, that series which comprises the greater part of the strata formerly called "transition" by Werner, for reasons explained in Chap. VIII., pp. [91] and [92.] Geologists have also applied to these older strata the general name of "grauwacké," by which the German miners designate a particular variety of sandstone, usually an aggregate of small fragments of quartz, flinty slate (or Lydian stone), and clay-slate cemented together by argillaceous matter. Far too much importance has been attached to this kind of rock, as if it belonged to a certain epoch in the earth's history, whereas a similar sandstone or grit is found sometimes in the Old Red, and in the Millstone Grit of the Coal, and sometimes in certain Cretaceous and even Eocene formations in the Alps.

The name of Silurian was first proposed by Sir Roderick Murchison, for a series of fossiliferous strata lying below the Old Red Sandstone, and occupying that part of Wales and some contiguous counties of England, which once constituted the kingdom of the Silures, a tribe of ancient Britons. The strata have been divided into Upper and Lower Silurian, and these again in the region alluded to admit of several well-marked subdivisions, all of them explained in the following table.

UPPER SILURIAN ROCKS.
Prevailing Lithological characters. Thickness in Feet. Organic Remains.
1. Ludlow
formation		 Tilestones. Finely laminated reddish and green sandstones and shales. 800? Marine mollusca of almost every order, the Brachiopoda most abundant. Serpula, Corals, Sauroid fish, Fuci.
Upper Ludlow. Micaceous grey sandstone. 2000
Aymestry limestone. Argillaceous limestone.
Lower Ludlow. Shale, with concretions of limestone.
2. Wenlock formation. Wenlock limestone. Concretionary limestone. 1800 Marine mollusca of various orders as before, Crustaceans of the Trilobite family.
Oldest bones of fish yet known.
Wenlock shale. Argillaceous shale.
LOWER SILURIAN ROCKS.
3. Caradoc formation. Caradoc sandstones. Flags of shelly limestone and sandstone, thick bedded white freestone. 2500 Crinoidea, Corals, Mollusca, chiefly Brachiopoda, Trilobites.
4. Llandeilo
formation.		 Llandeilo flags. Dark coloured calcareous flags. 1200 Mollusca, Trilobites.

UPPER SILURIAN ROCKS.

Ludlow formation.—This member of the Upper Silurian group, as will be seen by the above table, is of great thickness, and subdivided into four parts,—the Tilestone, the Upper and Lower Ludlow, and the intervening Aymestry limestone. Each of these may be distinguished near the town of Ludlow, and at other places in Shropshire and Herefordshire, by peculiar organic remains.

1. Tilestones.—This uppermost division was originally classed by Sir R. Murchison with the Old Red Sandstone, because they decompose into a red soil throughout the Silurian region. At the same time he regarded the tilestones as a transition group forming a passage from Silurian to Old Red. It is now ascertained that the fossils agree in great part specifically, and in general character entirely, with those of the succeeding formation.

2. Upper Ludlow.—The next division, called the Upper Ludlow, consists of grey calcareous sandstone, decomposing into soft mud, and contains, among other shells, the Lingula cornea, which is common to it and the lowest, or tilestone beds of the Old Red. But the Orthis orbicularis is peculiar to the Upper Ludlow, and very common; and the lowest or mudstone beds, are loaded for a thickness of 30 feet with Terebratula navicula ([fig. 410.]), in vast numbers. Among the cephalopodous mollusca occur the genera Bellerophon and Orthoceras, and among the crustacea the Homalonotus ([fig. 418.] [p. 354.]). A coral called Favosites polymorpha, Goldf. ([fig. 401.] [p. 346.]) is found both in this subdivision and in the Devonian system.

Fig. 409.

Orthis orbicularis, J. Sow. Delbury. Upper Ludlow.

Fig. 410.

Terebratula navicula, J. Sow. Aymestry limestone; also in Upper and Lower Ludlow.

Among the fossil shells are species of Leptæna, Orthis, Terebratula, Avicula, Trochus, Orthoceras, Bellerophon, and others.[352-A]

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 "mudstones." In these shales many zoophytes are found enveloped 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.

The scales, spines (ichthyodorulites), jaws, and teeth of fish of the genera Onchus, Plectrodus, and others of the same family, have been met with in the Upper Ludlow rocks.

Fig. 411.

Pentamerus Knightii, Sow. Aymestry.

3. Aymestry limestone.—The next group is a subcrystalline and argillaceous limestone, which is in some places 50 feet thick, and distinguished around Aymestry by the abundance of Pentamerus Knightii, Sow. ([fig. 411.]), also found in the Lower Ludlow. This genus of brachiopoda has only been found in the Silurian strata. The name was derived from πεντε, pente, five, and μερος, 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; but neither the structure of this shell, nor the connection of the animal with its several parts, are as yet understood. 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.

Fig. 412.

Lingula Lewisii, J. Sow. Abberley Hills.

Three other abundant shells in the Aymestry limestone are, 1st, Lingula Lewisii ([fig. 412.]); 2d, Terebratula Wilsoni, Sow. ([fig. 413.]), which is also common to the Lower Ludlow and Wenlock limestone; 3d, Atrypa reticularis, Lin. ([fig. 414.]), which has a very wide range, being found in every part of the Silurian system, except the Llandeilo flags.

Fig. 413.

Terebratula Wilsoni, Sow. Aymestry.

Fig. 414.

Atrypa reticularis. Linn. Syn. Terebratula affinis, Min. Con. Aymestry.

4. Lower Ludlow shale.—A dark grey argillaceous deposit, containing, among other fossils, the new genera of chambered shells, the Phragmoceras of Broderip, and the Lituites of Breyn (see [figs. 415], [416.]). The latter is partly straight and partly convoluted, nearly as in Spirula.

Fig. 415.

Phragmoceras ventricosum, J. Sow. (Orthoceras ventricosum, Stein.) Aymestry; 1/4 nat. size.

Fig. 416.

Lituites giganteus, J. Sow. Near Ludlow; also in the Aymestry and Wenlock limestones; 1/4 nat. size.

Fig. 417.

The Orthoceras Ludense ([fig. 417.]), as well as the shell last mentioned, is peculiar to this member of the series. The Homalonotus delphinocephalus ([fig. 418.]) is common to this division and to the Wenlock limestone. This crustacean belongs to a group of trilobites which has been met with in the Silurian rocks only, and in which the tripartite character of the dorsal crust is almost lost.

Fig. 418.

Homalonotus delphinocephalus, König.[354-A] Dudley Castle; 1/2 nat. size.

A species of Graptolite, G. Ludensis, Murch. ([fig. 419.]), a form of zoophyte which has not yet been met with in strata newer than the Silurian, occurs in the Lower Ludlow.

Wenlock formation.—We next come to the Wenlock formation, which has been divided (see Table, [p. 351.]) into

1. Wenlock limestone, formerly well known to collectors by the name of the Dudley limestone, which forms a continuous ridge, ranging for about 20 miles from S.W. to N.E., about a mile distant from the nearly parallel escarpment of the Aymestry limestone. The prominence of this rock in Shropshire, like that of Aymestry, is due to its solidity, and to the softness of the shales above and below. It is divided into large concretional masses of pure limestone, and abounds in trilobites, among which the prevailing species are Phacops caudatus ([fig. 422.]) and Calymene Blumenbachii, commonly called the Dudley trilobite. The latter is often found coiled up like a wood-louse (see [fig. 420.]).

Fig. 419.

Fig. 419. Graptolithus Ludensis, Murchison. Lower Ludlow.

Fig. 420.

Calymene Blumenbachii, Brong. Wenlock, L. Ludlow, and Aym. limest.

Fig. 421.

Leptæna depressa. Wenlock.

Fig. 422.

Phacops caudatus, Brong. Wenlock, Aym. limest., and L. Ludlow.

Leptæna depressa, Sow., is common in this rock, but also ranges through the Lower Ludlow, Wenlock shale, and Caradoc Sandstone.

Fig. 423.

Catenipora escharoides.

Among the corals in which this formation is very rich, the Catenipora escharoides, Lam. ([fig. 423.]), or chain coral, may be pointed out as one very easily recognized, and widely spread in Europe, ranging through all parts of the Silurian group, from the Aymestry limestone to the bottom of the series.

Another coral, the Porites pyriformis, is also met with in profusion; a species common to the Devonian rocks.

Cystiphyllum Siluriense ([fig. 425.]) is a species peculiar to the Wenlock limestone. This new genus, the name of which is derived from κυστις, a bladder, and φυλλον, a leaf, was instituted by Mr. Lonsdale for corals of the Silurian and Devonian groups. It is composed of small bladder-like cells (see [fig. 425. b.]).

2. The Wenlock Shale, which exceeds 700 feet in thickness, contains many species of brachiopoda, such as a small variety of the Lingula Lewisii ([fig. 412.]), and the Atrypa reticularis ([fig. 414.]) before mentioned, and it will be seen that several other fossils before enumerated range into this shale.

Fig. 424.

Porites pyriformis, Ehren. Wenlock limest. and shale. Also in Aymestry limestone, and L. Ludlow.

a. Vertical section, showing transverse lamellæ.

Fig. 425.

LOWER SILURIAN ROCKS.

The Lower Silurian rocks have been subdivided into two portions.

1. The Caradoc sandstone, which abuts against the trappean chain called the Caradoc Hills, in Shropshire. Its thickness is estimated at 2500 feet, and the larger proportion of its fossils are specifically distinct from those of the Upper Silurian rocks. Among them we find many trilobites and shells of the genera Orthoceras, Nautilus, and Bellerophon; and among the Brachiopoda the Pentamerus oblongus and P. lævis ([fig. 426.]), which are very abundant and peculiar to this bed; also Orthis grandis ([fig. 427.]), and a fossil of well-defined form, Tentaculites annulatus, Schlot. ([fig. 428.]), which Mr. Salter has shown to be referable to the Annelids and to the same tribe as Serpula.

Fig. 426.

Pentamerus lævis, Sow. Caradoc Sandstone. Perhaps the young of Pentamerus oblongus.

Fig. 427.

Cast of Orthis grandis, J. Sow. Horderley; two-thirds of nat. size.

Fig. 428.

Tentaculites scalaris, Schlot. Eastnor Park; nat. size, and magnified.

The most ancient bony remains of fish yet discovered in Great Britain are those obtained from the Wenlock limestones; but coprolites referred to fish occur still lower in the Silurian series in Wales.

Fig. 429.

Ogygia Buchii, Burmeister. Syn. Asaphus Buchii, Brong. 1/4 nat. size. Radnorshire.

2. The Llandeilo flags, so named from a town in Caermarthenshire, form the base of the Silurian system, consisting of dark-coloured micaceous grit, frequently calcareous, and distinguished by containing the large trilobites Asaphus Buchii and A. tyrannus, Murch., both of which are peculiar to these rocks. Several species of Graptolites ([fig. 430.]) occur in these beds.

Fig. 430.

a, b. Graptolithus Murchisonii, Beck. Llandeilo flags.

Fig. 431.

G. foliaceus, Murchison. Llandeilo flags.

In the fine shales of this formation Graptolites are very abundant. I collected these same bodies in great numbers in Sweden and Norway in 1835-6, both in the higher and lower shales of the Silurian system; and was informed by Dr. Beck of Copenhagen, that they were fossil zoophytes related to the genera Pennatula and Virgularia, of which the living species now inhabit mud and slimy sediment. The most eminent naturalists still hold to this opinion.

A species of Lingula is met with in the lowest part of the Llandeilo beds; and it is remarkable that this brachiopod is among the earliest, if not the most ancient animal form detected in the lowest Silurian of North America. These inhabitants of the seas, of so remote an epoch, belonged so strictly to the living genus Lingula, as to demonstrate, like the pteriform ferns of the coal, through what incalculable periods of time the same plan and type of organization has sometimes prevailed.

Among the forms of trilobite extremely characteristic of the Lower Silurian throughout Europe and North America, the Trinucleus may be mentioned. This family of crustaceans appears to have swarmed in the Silurian seas, just as crabs, shrimps, and other genera of crustaceans abound in our own. Burmeister, in his work on the organization of trilobites, supposes them to have swum 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. They underwent various transformations analogous to those of living crustaceans. M. Barrande, author of a work on the Silurian rocks of Bohemia, has traced the same species from the young state just after its escape from the egg to the adult form, through various metamorphoses, each having the appearance of a distinct species. Yet, notwithstanding the numerous species of preceding naturalists which he has thus succeeded in uniting into one, he announces a forthcoming work in which descriptions and figures of 250 species of Trilobite will be given.

Fig. 432.

Trinucleus ornatus, Burm.

Cystideæ.—Among the additions which recent research has made to the paleontology of the oldest Silurian rocks, none are more remarkable than the radiated animals called Cystideæ. Their structure and relations were first elucidated in an essay published by Von Buch at Berlin in 1845. They are usually met with as spheroidal bodies covered with polygonal plates, with a mouth on the upper side, and a point of attachment for a stem b (which is almost always broken off) on the lower. (See [fig. 433.]) They are considered by Professor E. Forbes as intermediate between the crinoids and echinoderms. The Sphæronites here represented ([fig. 433.]) occurs in the Llandeilo beds in Wales.[358-A]

Fig. 433.

Sphæronites balticus, Eichwald. (Of the family Cystideæ.)

Lower Silurian, Shole's Hook and Bala.

Thickness and unconformability of Silurian strata.—According to the observation of our government surveyors in North Wales, the Lower Silurian strata of that region attain, in conjunction with the contemporaneous volcanic rocks, the extraordinary thickness of 27,000 feet. One of the groups, called the trappean, consisting of slates and associated volcanic ash and greenstone, is 15,000 feet thick. Another series, called the Bala group, composed of slates and grits with an impure limestone rich in organic remains, is 9,000 feet thick.[359-A]

Throughout North Wales the Wenlock shales rest unconformably upon the Caradoc sandstones; and the Caradoc is in its turn unconformable to the Llandeilo beds, showing a considerable interval of time between the deposition of this group and that of the formations next above and below it. The Caradoc sandstone in the neighbourhood of the Longmynd Hills in Shropshire, appears to Professor E. Forbes to have been a deep-sea deposit formed around the margin of high and steep land. That land consisted partly of upraised Llandeilo flags and partly of rocks of still older date.[359-B]

Such evidence of the successive disturbance of strata during the Silurian period in Great Britain is what we might look for when we have discovered the signs of so grand a series of volcanic eruptions as the contemporaneous greenstones and tuffs of the Welsh mountains afford.

Silurian Strata of the United States.

The position of some of these strata, where they are bent and highly inclined in the Appalachian chain, or where they are nearly horizontal to the west of that chain, is shown in the section, [fig. 379.] [p. 327.] But these formations can be studied still more advantageously north of the same line of section, in the states of New York, Ohio, and other regions north and south of the great Canadian lakes. Here they are found, as in Russia, in horizontal position, and are more rich in well-preserved fossils than in almost any spot in Europe. The American strata may readily be divided into Upper and Lower Silurian, corresponding in age and fossils to the European divisions bearing the same names. The subordinate members of the New York series, founded on lithological and geographical considerations, are most useful in the United States, but even there are only of local importance. Some few of them, however, tally very exactly with English divisions, as for example the limestone, over which the Niagara is precipitated at the great cataract, which, with its underlying shales, agrees paleontologically with the Wenlock limestone and shale of Siluria. There is also a marked general correspondence in the succession of fossil forms, and even species, as we trace the organic remains downwards from the highest to the lowest beds.

Mr. D. Sharpe, in his report on the mollusca collected by me from these strata in North America[359-C], 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 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 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.

Whether the Silurian rocks are of deep-water origin.—The grounds relied upon by Professor E. Forbes, for inferring that the larger part of the Silurian Fauna is indicative of a sea more than 70 fathoms deep, are the following: first, the small size of the greater number of conchifera; secondly, the paucity of pectinibranchiata (or spiral univalves); thirdly, the great number of floaters, such as Bellerophon, Orthoceras, &c.; fourthly, the abundance of orthidiform brachiopoda; fifthly, the absence or great rarity of fossil fish.

It is doubtless true that some living Terebratulæ, on the coast of Australia, inhabit shallow water; but all the known species, allied in form to the extinct Orthis, inhabit the depths of the sea. It should also be remarked that Mr. Forbes, in advocating these views, was well aware of the existence of shores, bounding the Silurian sea in Shropshire, and of the occurrence of littoral species of this early date in the northern hemisphere. Such facts are not inconsistent with his theory; for he has shown, in another work, how, on the coast of Lycia, deep-sea strata are at present forming in the Mediterranean, in the vicinity of high and steep land.

Had we discovered the ancient delta of some large Silurian river, we should doubtless have known more of the shallow, and brackish water, and fluviatile animals, and of the terrestrial flora of the period under consideration. To assume that there were no such deltas in the Silurian world, would be almost as gratuitous an hypothesis, as for the inhabitants of the coral islands of the Pacific to indulge in a similar generalization respecting the actual condition of the globe.[360-A]

Mineral Character of Silurian Strata.

In lithological character, the Silurian strata vary greatly when we trace them through Europe and North America. The shales called mudstones are as little altered from some deposits, found in recent submarine banks, as are those of many tertiary formations. We meet with red sandstone and red marl, with gypsum and salt, of Upper Silurian date, in the Niagara district, which might be mistaken for trias. The whitish granular sandstone at the base of the Silurian series in Sweden resembles the tertiary siliceous grit of Fontainebleau. The Calcareous Grit, oolite, and pisolite of Upper Silurian age in Gothland, are described by Sir R. Murchison as singularly like rocks of the oolitic period near Cheltenham; and, not to cite more examples, the Wenlock or Dudley limestone often resembles a modern coral-reef. If, therefore, uniformity of aspect has been thought characteristic of rocks of this age, the idea must have arisen from the similarity of feature acquired by strata subject to metamorphic action. This influence, seeing that the causes of change are always shifting the theatre of their principal development, must be multiplied throughout a wider geographical area by time, and become more general in any given system of rocks in proportion to their antiquity. We are now acquainted with dense groups of Eocene slates in the Alps, which were once mistaken by experienced geologists for Transition or Silurian formations. The error arose from attaching too great importance to mineral character as a test of age, for the tertiary slates in question having acquired that crystalline texture which is in reality most prevalent in the most ancient sedimentary formations.

CAMBRIAN GROUP.

Below the Silurian strata in North Wales, and in the region of the Cumberland lakes, there are some slaty rocks, devoid of organic remains, or in which a few obscure traces only of fossils have been detected (for which the names of Cambrian and Cumbrian have been proposed). Whether these will ever be entitled by the specific distinctness of their fossils to rank as independent groups, we have not yet sufficient data to determine.

TABULAR VIEW OF FOSSILIFEROUS STRATA,

Showing the Order of Superposition or Chronological Succession of the principal European Groups.

I. POST-TERTIARY.
A. POST-PLIOCENE.
Periods and Groups. Examples. Observations.
1. Recent.
  • Peat mosses and shell-marl, with bones of land animals, human remains, and works of art.
  • Newer parts of modern deltas and coral reefs.
 All the imbedded shells, freshwater and marine, of living species, with occasional human remains and works of art.
2. Post-Pliocene.
  • Clay, marl, and volcanic tuff of Ischia, [p. 113.]
  • Loess of the Rhine, [p. 117.]
  • Newer part of boulder formation, with erratics, [p. 124.]
 All the shells of living species. No human remains or works of art. Bones of quadrupeds, partly of extinct species.
II. TERTIARY.
B. PLIOCENE.
3. Newer Pliocene or Pleistocene.
  • Boulder formation or drift of northern Europe and North America, chaps. 11. & 12.
  • Cavern deposits and osseous breccias, [p. 153.]
  • Fluvio-marine crag of Norwich, [p. 148.]
  • Limestone of Girgenti, in Sicily, [p. 152.]
  • Three-fourths of the fossil shells of existing species.
  • A majority of the mammalia extinct; but the genera corresponding with those now surviving in the same great geographical and zoological province, [p. 157.]
  • During part of this period icebergs frequent in the seas of the northern hemisphere, and glaciers on hills of moderate height.
4. Older Pliocene.
  • A third or more of the species of mollusca extinct.
  • Nearly, if not all, the mammalia extinct.
C. MIOCENE.
5. Miocene.
  • About two-thirds of the species of shells extinct.
  • The recent species of shells often not found in the adjoining seas, but in warmer latitudes.
  • All the mammalia extinct.
D. EOCENE.
6. Upper Eocene.
  • Upper marine of Paris basin, Fontainebleau sandstone, [p. 175.]
  • Upper freshwater and millstone of same.
  • Kleyn Spauwen beds, [p. 176.]
  • Hermsdorf tile-clay, near Berlin.
  • Mayence tertiary strata, [p. 177.]
  • Freshwater beds of Limagne d'Auvergne, [p. 181.]
  • Fossil shells of the Eocene period, with very few exceptions, extinct. Those which are identified with living species rarely belong to neighbouring regions
  • All the mammalia of extinct species, and the greater part of them of extinct genera.
  • Plants of Upper Eocene, indicating a south European or Mediterranean climate; those of Lower Eocene, a tropical climate.
7. Middle Eocene.
  • Paris gypsum with Paleotherium, &c., [p. 191.]
  • Freshwater and fluvio-marine beds of Headon Hill, Isle of Wight, [p. 197.]
  • Barton beds, Hants, [p. 198.]
  • Calcaire Grossier, Paris, [p. 193.]
  • Bagshot and Bracklesham beds, Surrey and Sussex, [p. 198.]
8. Lower Eocene.
  • London clay proper of Highgate Hill and Sheppey,—Bognor beds, Sussex, [p. 200.]
  • Sables inférieurs, and lits coquilliers of Paris basin, [p. 196.]
  • Mottled and plastic clays and sands of the Hampshire and London basins, [p. 203.]
  • Sables inférieurs and argiles plastiques of Paris basin, [p. 196.]
  • Nummulitic formation of the Alps, [p. 205.]
III. SECONDARY.
E. CRETACEOUS.
§ UPPER CRETACEOUS.
9. Maestricht beds.
  • Yellowish white limestone of Maestricht, [p. 209.]
  • Coralline limestone of Faxoe, Denmark, [p. 210.]
 Ammonite, Baculite, and Belemnite, associated with Cypræa, Oliva, Mitra, Trochus, &c. Large marine saurians.
10. Upper White Chalk. White chalk with flints of North and South Downs,— Surrey and Sussex, [p. 211.] Marine limestone formed in part of decomposed corals.
11. Lower White Chalk. Chalk without flints, and chalk marl, ibid.
12. Upper Greensand.
  • Loose sand, with bright green particles, ibid.
  • Firestone of Merstham, Kent, [p. 218.]
  • Marly stone, with layers of chert, south of Isle of Wight.
13. Gault. Dark blue marl at base of chalk escarpment,—Kent and Sussex, [p. 218.] Numerous extinct genera of conchiferous cephalopoda, Hamite, Scaphite, Ammonite, &c.
§§ LOWER CRETACEOUS.
14. Lower Greensand.
  • Sand with green matter,—Weald of Kent and Sussex, [p. 219.]
  • White, yellowish, and ferruginous sand, with concretions of limestone and chert,—Atherfield, Isle of Wight.
  • Limestone called Kentish Rag
 Species of shells, &c., nearly all distinct from those of Upper Cretaceous; most of the genera the same.
F. WEALDEN.
15. Weald Clay. Clay with occasional bands of limestone,—Weald of Kent, Surrey, and Sussex, [p. 227.] Of freshwater origin. Shells of pulmoniferous mollusca, and of Cypris. Land reptiles.
16. Hastings Sand. Sand with calciferous grit and clay,—Hastings, Sussex, Cuckfield, Kent, [p. 229.] Freshwater with intercalated bed of brackish and salt water origin. Shells of fluviatile and lacustrine genera. Reptiles of the genera Pterodactyle, Iguanodon, Megalosaurus, Plesiosaurus, Trionyx, and Emys.
17. Purbeck Beds. Limestones, calcareous slates and marls, [p. 231.] Chiefly freshwater, and divisible into three groups, each containing distinct species of freshwater mollusca and of entomostraca. Alternations of deposits formed in fresh, brackish, and marine water, and of ancient soils formed on land and retaining roots of trees. Plants chiefly cycads and conifers, [p. 231.]
G. OOLITE.
18. Upper Oolite.
  • a. Portland building stone, [p. 259.]
  • b. Portland sand.
  • c. Kimmeridge clay, Dorsetshire, [p. 260.]
  • Ammonites and Belemnites numerous.
  • Large saurians, as Pterodactyles, Plesiosaurs, Ichthyosaurs.
  • No cetaceans yet known, but three species of terrestrial mammalia, [p. 267], [268.] Preponderance of ganoid fish. The plants chiefly cycads, conifers, and ferns, with a few palms.
19. Middle Oolite.
  • a. Coral Rag, [p. 260.] Calcareous freestones, oolitic, } often full of corals. Oxfordshire.
  • b. Oxford clay—Dark blue clay,—Oxfordshire and midland counties, [p. 262.]
20. Lower Oolite.
  • a. Cornbrash and forest marble, Wiltshire, [p. 263.]
  • b. Great oolite and Stonesfield slate,—Bath, Bradford, Stonesfield near Woodstock, Oxfordshire, [p. 266.]
  • c. Fuller's earth,—Clay containing fuller's earth near Bath, [p. 272.]
  • d. Inferior oolite, calcareous freestone, and yellow sands,—Cotteswold Hills, Dundry Hill, near Bristol, [p. 272.]
H. LIAS.
21. Lias. Argillaceous limestone, marl and clay,—Lyme Regis, Dorsetshire, [p. 273.] Mollusca, reptiles, and fish of genera analogous to the oolitic.
I. TRIAS.
22. Upper Trias. Keuper of Germany, or variegated marls—Red, grey, green, blue, and white marls and sandstones with gypsum—Würtemberg, bone-bed of Axmouth, Dorset, [p. 289.] Batrachian reptiles, e.g. Labyrinthodon, Rhyncosaurus, &c. Cephalopoda: Ceratites. No Belemnites. Plants: Ferns, Cycads, Conifers.
23. Middle Trias or Muschelkalk. Compact greyish limestone with beds of dolomite and gypsum,—North of Germany, [p. 287.] Wanting in England. With Equisetites and Calamite.
24. Lower Trias.
  • Variegated or Bunter sandstone of Germans—Red and white spotted sandstone with gypsum and rock-salt, [p. 288.]
  • Part of New Red sandstone of Cheshire with rock-salt, [p. 294.]
 Plants different for the most part from those of the Upper Trias.
IV. PRIMARY.
K. PERMIAN.
25. Upper Permian.
  • Yellow magnesian limestone, Yorkshire and Durham, [p. 301.]
  • Zechstein of Thuringia, Upper part of Permian beds, Russia.
 Organic remains, both animal and vegetable, more allied to primary than to secondary periods.
26. Lower Permian.
  • a. Marl slate of Durham and Thuringia.
  • b. Lower New Red sandstone of north of England and Rothliegendes of Germany.
  • a. and b. Lower part of Permian beds, Russia, [p. 301.]
 Thecodont saurians. Heterocercal fish of genus Palæoniscus, &c.
L. CARBONIFEROUS.
27. Coal measures.
  • a. Strata of sandstone and shale, with beds of coal,—S. Wales and Northumberland, [p. 309.]
  • b. Millstone grit,—S. Wales, Bristol coal-field, Yorkshire, [p. 308.]
  • Great thickness of strata of fluvio-marine origin, with beds of coal of vegetable origin, based on soils retaining the roots of trees.
  • Oldest of known reptiles or Archegosaurus. Sauroid fish.
28. Mountain limestone.
  • Carboniferous or mountain limestone, with marine shells and corals.
  • Mendip Hills, and many parts of Ireland, [p. 340.]
  • Brachiopoda of genus Productus.
  • Cephalopoda of genera Cyrtoceras, Goniatite, Orthoceras.
  • Crustaceans of the genus Phillipsia.
  • Crinoideans abundant.
M. DEVONIAN.
29. Upper Devonian.
  • a. Yellow sandstone of Dura Den, Fife.
  • b. Red sandstone and marl with cornstone of Herefordshire and Forfarshire.
  • Paving and roofing-stone, Forfarshire.
  • Upper part of Devonian beds of South Devon.
  • Tribe of fish with hard coverings like chelonians, Pterichthys, Pamphractus, &c.; also of genera Cephalaspis, Holoptichius, &c.
  • No reptiles yet known.
30. Lower Devonian. Grey sandstone with Ichthyolites,—Caithness, Cromarty, and Orkney, Lower part of Devonian beds of South Devon, and green chloritic slates of Cornwall, limestone of Gerolstein, Eifel. Fish, partly of same genera, but of distinct species from those in Upper Devonian; Glyptolepis, Dipterus, also Osteolepis, Coccosteus, &c.
N. SILURIAN.
31. Upper Silurian.
  • a. Tilestone of Brecon and Caermarthen.
  • b. Limestone and shale, Ludlow, Shropshire.
  • c. Wenlock or Dudley limestone.
  • Oldest of fossil fish yet discovered.
  • Trilobites and Graptolites abundant.
  • Brachiopoda very numerous.
  • Cephalopoda: Bellerophon, Orthoceras.
32. Lower Silurian.
  • a. Caradoc sandstone, Caer Caradoc, Shropshire.
  • b. Llandeilo flags, calcareous flags and schists,—Builth, Radnorshire, Llandeilo, Caermarthenshire.
  • Same genera of invertebrate animals as in Upper Silurian, but species chiefly distinct. Trinucleus caractaci, Cystideæ, [p. 358.]
  • No land plants yet known.
  • Footprints of tortoise, see note, [p. 360.]

CHAPTER XXVIII.

VOLCANIC ROCKS.

Trap rocks — Name, whence derived — Their igneous origin at first doubted — Their general appearance and character — Volcanic cones and craters, how formed — Mineral composition and texture of volcanic rocks — Varieties of felspar — Hornblende and augite — Isomorphism — Rocks, how to be studied — Basalt, greenstone, trachyte, porphyry, scoria, amygdaloid, lava, tuff — Alphabetical list, and explanation of names and synonyms, of volcanic rocks — Table of the analyses of minerals most abundant in the volcanic and hypogene rocks.

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. Suppose a a in the annexed diagram, 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. They also are seen, in some instances, to pass insensibly into the unstratified division of a, or the Plutonic rocks.

Fig. 434.

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 volcanos. They also 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, and amygdaloid. All these, which were recognized 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 on the sides of hills. This configuration appears to be derived from two causes. First, the abrupt original terminations of sheets of melted matter, which have spread, whether on the land or bottom of the sea, over a level surface. For we know, in the case of lava flowing from a volcano, that a stream, when it has ceased to flow, and grown solid, very commonly ends in a steep slope, as at a, [fig. 435.] But, secondly, the step-like appearance arises more frequently from the mode in which horizontal masses of igneous rock, such as b c, intercalated between aqueous strata, have, subsequently to their origin, been exposed, at different heights, by denudation. Such an outline, it is true, is not peculiar to trap rocks; great beds of limestone, and other hard kinds of stone, often presenting similar terraces and precipices: but these are usually on a smaller scale, or less numerous, than the volcanic steps, or form less decided features in the landscape, as being less distinct in structure and composition from the associated rocks.

Fig. 435.

Step-like appearance of trap.

Although the characters of trap rocks are greatly diversified, the beginner will easily learn to distinguish them as a class from the aqueous formations. Sometimes they present themselves, as already stated, in tabular masses, which are not divided into strata: sometimes in shapeless lumps and irregular cones, forming chains of small hills. Often they are seen in dikes and wall-like masses, intersecting fossiliferous beds. The rock is occasionally found divided into columns, often decomposing into balls of various sizes, from a few inches to several feet in diameter. The decomposing surface very commonly assumes a coating of a rusty iron colour, from the oxidation of ferruginous matter, so abundant in the traps in which augite or hornblende occur; or, in the felspathic varieties of trap, it acquires a white opaque coating, from the bleaching of the mineral called felspar. On examining any of these volcanic rocks, where they have not suffered disintegration, we rarely fail to detect a crystalline arrangement in one or more of the component minerals. Sometimes the texture of the mass is cellular or porous, or we perceive that it has once been full of pores and cells, which have afterwards become filled with carbonate of lime, or other infiltrated mineral.

Most of the volcanic rocks produce a fertile soil by their disintegration. It seems that their component ingredients, silica, alumina, lime, potash, iron, and the rest, are in proportions well fitted for vegetation. As they do not effervesce with acids, a deficiency of calcareous matter might at first be suspected; but although the carbonate of lime is rare, except in the nodules of amygdaloids, yet it will be seen that lime sometimes enters largely into the composition of augite and hornblende. (See Table, [p. 377.])

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 and other gases 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 upwards 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 it down on one side, and often it flows out from a fissure at the base of the hill (see [fig. 436.]).[368-A]

Fig. 436.

Part of the chain of extinct volcanos called the Monts Dome, Auvergne. (Scrope.)

Composition and nomenclature.—Before speaking of the connection between the products of modern volcanos and the rocks usually styled trappean, and before describing the external forms of both, and the manner and position in which they occur in the earth's crust, it will be desirable to treat of their mineral composition and names. The varieties most frequently spoken of are basalt, greenstone, syenitic greenstone, clinkstone, claystone, and trachyte; while those founded chiefly on peculiarities of texture, are porphyry, amygdaloid, lava, tuff, scoriæ, and pumice. It may be stated generally, that all these are mainly composed of two minerals, or families of simple minerals, felspar and hornblende; some almost entirely of hornblende, others of felspar.

These two minerals may be regarded as two groups, rather than species. Felspar, for example, may be, first, common felspar, that is to say, potash-felspar, in which the alkali is potash (see table, [p. 377.]); or, secondly, albite, that is to say, soda-felspar, where the alkali is soda instead of potash; or, thirdly, Labrador-felspar (Labradorite), which differs not only in its iridescent hues, but also in its angle of fracture or cleavage, and its composition. We also read much of two other kinds, called glassy felspar and compact felspar, which, however, cannot rank as varieties of equal importance, for both the albitic and common felspar appear sometimes in transparent or glassy crystals; and as to compact felspar, it is a compound of a less definite nature, sometimes containing both soda and potash; and which might be called a felspathic paste, being the residuary matter after portions of the original matrix have crystallized.

The other group, or hornblende, consists principally of two varieties; first, hornblende, and, secondly, augite, which were once regarded as very distinct, although now some eminent mineralogists are in doubt whether they are not one and the same mineral, differing only as one crystalline form of native sulphur differs from another.

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 were different, and 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; and that when this happened, as in some lavas of modern date, the hornblende occurs in the mass of the rock, where crystallization may have taken place more slowly, while the augite merely lines cavities where the crystals may have been produced rapidly. 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 in rocks from Siberia which presented a hornblende cleavage, 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 recognized 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 recrystallizing, 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 recrystallizes, 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 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 find that a mineral determined to be the same by its physical characters, crystalline form, and optical properties, has often been declared by skilful analyzers to be composed of distinct elements. (See the table at [p. 377.]) 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 in a great degree 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 ισος, isos, equal, and μορφη, 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.

Having been led into this digression on the recent progress of mineralogy, I may here observe that the geological student must endeavour as soon as possible to familiarize himself with the characters of five at least of the most abundant simple minerals of which rocks are composed. These are, felspar, quartz, mica, hornblende, and carbonate of lime. This knowledge cannot be acquired from books, but requires personal inspection, and the aid of a teacher. It is well to accustom the eye to know the appearance of rocks under the lens. To learn to distinguish felspar from quartz is the most important step to be first aimed at. In general we may know the felspar because it can be scratched with the point of a knife, whereas the quartz, from its extreme hardness, receives no impression. But when these two minerals occur in a granular and uncrystallized state, the young geologist must not be discouraged if, after considerable practice, he often fails to distinguish them by the eye alone. If the felspar is in crystals, it is easily recognized by its cleavage: but when in grains the blow-pipe must be used, for the edges of the grains can be rounded in the flame, whereas those of quartz are infusible. If the geologist is desirous of distinguishing the three varieties of felspar above enumerated, or hornblende from augite, it will often be necessary to use the reflecting goniometer as a test of the angle of cleavage, and shape of the crystal. The use of this instrument will not be found difficult.

The external characters and composition of the felspars are extremely different from those of augite or hornblende; so that the volcanic rocks in which either of these minerals decidedly predominates, are easily recognized. But there are mixtures of the two elements in every possible proportion, the mass being sometimes exclusively composed of felspar, at other times solely of augite, or, again, of both in equal quantities. Occasionally, the two extremes, and all the intermediate gradations, may be detected in one continuous mass. Nevertheless there are certain varieties or compounds which prevail so largely in nature, and preserve so much uniformity of aspect and composition, that it is useful in geology to regard them as distinct rocks, and to assign names to them, such as basalt, greenstone, trachyte, and others, already mentioned.

Basalt.—As an example of rocks in which augite greatly prevails, basalt may first be mentioned. Although we are more familiar with this term than with that of any other kind of trap, it is difficult to define it, the name having been used so vaguely. It has been very generally applied to any trap rock of a black, bluish, or leaden-grey colour, having a uniform and compact texture. Most strictly, it consists of an intimate mixture of augite, felspar, and iron, to which a mineral of an olive green colour, called olivine, is often superadded, in distinct grains or nodular masses. The iron is usually magnetic, and is often accompanied by another metal, titanium. Augite is the predominant mineral, the felspar being in much smaller proportions. There is no doubt that many of the fine-grained and dark-coloured trap rocks, called basalt, contained hornblende in the place of augite; but this will be deemed of small importance after the remarks above made. Other minerals are occasionally found in basalt; and this rock may pass insensibly into almost every variety of trap, especially into greenstone, clinkstone, and wacké, which will be presently described.

Greenstone, or Dolerite, is usually defined as a granular rock, the constituent parts of which are hornblende and imperfectly crystallized felspar; the felspar being more abundant than in basalt; and the grains or crystals of the two minerals more distinct from each other. This name may also be extended to those rocks in which augite is substituted for hornblende (the dolorite of some authors), or to those in which albite replaces common felspar, forming the rock sometimes called Andesite.

Syenitic greenstone.—The highly crystalline compounds of the same two minerals, felspar and hornblende, having a granitiform texture, and with occasionally some quartz accompanying, may be called Syenitic greenstone, a rock which frequently passes into ordinary trap, and as frequently into granite.

Trachyte.—A porphyritic rock of a whitish or greyish colour, composed principally of glassy felspar, with crystals of the same, generally with some hornblende and some titaniferous iron. In composition it is extremely different from basalt, this being a felspathic, as the other is an augitic, rock. It has a peculiar rough feel, whence the name τραχυς, trachus, rough. Some varieties of trachyte contain crystals of quartz.

Fig. 437.

Porphyry.
White crystals of felspar in a dark base of hornblende and felspar.

Porphyry is merely a certain form of rock, 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. 437.]). Thus trachyte is porphyritic; for in it, as in many modern lavas, there are crystals of felspar; 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.

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, calcedony, calcareous spar, or zeolite, are scattered through a base of wacké, basalt, greenstone, or other kind of trap. It derives its name from the Greek word amygdala, 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. 438.

Scoriaceous lava in part converted into an amygdaloid.

Montagne de la Veille, Department of Puy de Dome, France.

The annexed figure 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.

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-felspar is present.

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, or in consequence of the contact with water in or upon the damp soil.

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 rents 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-felspar.[374-A]

Trap tuff, volcanic 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 volcanos, where showers of these materials, together with small pieces of other rocks ejected from the crater, 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.

Besides the peculiarity of their composition, some tuffs, or volcanic grits, as they have been termed, differ from ordinary sandstones by the angularity of their grains. When the fragments are coarse, the rock is styled a volcanic breccia. Tufaceous conglomerates result from the intermixture of rolled fragments or pebbles of volcanic and other rocks with tuff.

According to Mr. Scrope, the Italian geologists confine the term tuff, or tufa, to felspathose mixtures, and those composed principally of pumice, using the term peperino for the basaltic tuffs.[374-B] The peperinos thus distinguished are usually brown, and the tuffs grey or white.

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 to cause them to resemble many of those kinds of trap which are found in ordinary dikes. The chocolate-coloured mud, which was poured for weeks out of the crater of Graham's Island, in the Mediterranean, in 1831, must, when unmixed with other materials, have constituted a stone heavier than granite. Each cubic inch of the impalpable powder which has fallen for days through the atmosphere, during some modern eruptions, has been found to weigh, without being compressed, as much as ordinary trap rocks, and to be often identical with these in mineral composition.

The fusibility of the igneous rocks generally exceeds that of other rocks, for there is much alkaline matter and lime in their composition, which serves as a flux to the large quantity of silica, which would be otherwise so refractory an ingredient.

It is remarkable that, notwithstanding the abundance of this silica, quartz, that is, crystalline silica, is usually wanting in the volcanic rocks, or is present only as an occasional mineral, like mica. The elements of mica, as of quartz, occur in lava and trap; but the circumstances under which these rocks are formed are evidently unfavourable to the development of mica and quartz, minerals so characteristic of the hypogene formations.

It would be tedious to enumerate all the varieties of trap and lava which have been regarded by different observers as sufficiently abundant to deserve distinct names, especially as each investigator is too apt to exaggerate the importance of local varieties which happen to prevail in districts best known to him. It will be useful, however, to subjoin here, in the form of a glossary, an alphabetical list of the names and synonyms most commonly in use, with brief explanations, to which I have added a table of the analysis of the simple minerals most abundant in the volcanic and hypogene rocks.

Explanation of the names, synonyms, and mineral composition of the more abundant volcanic rocks.

Amphibolite. See [Hornblende rock], amphibole being Haüy's name for hornblende.

Amygdaloid. A particular form of volcanic rock; see [p. 372.]

Augite rock. A kind of basalt or greenstone, composed wholly or principally of granular augite. (Leonhard's Mineralreich, 2d edition, p. 85.)

Augitic-porphyry. Crystals of Labrador-felspar and of augite, in a green or dark grey base. (Rose, Ann. des Mines, tom. 8. p. 22. 1835.)

Basalt. Chiefly augite—an intimate mixture of augite and felspar with magnetic iron, olivine, &c. See [p. 371.] The yellowish green mineral called olivine, can easily be distinguished from yellowish felspar by its infusibility, and having no cleavage. The edges turn brown in the flame of the blow-pipe.

Basanite. Name given by Alex. Brongniart to a rock, having a base of basalt, with more or less distinct crystals of augite disseminated through it.

Claystone and Claystone-porphyry. An earthy and compact stone, usually of a purplish colour, like an indurated clay; passes into hornstone; generally contains scattered crystals of felspar and sometimes of quartz.

[Clinkstone]. Syn. Phonolite, fissile Petrosilex; a greenish or greyish rock, having a tendency to divide into slabs and columns; hard, with clean fracture, ringing under the hammer; principally composed of compact felspar, and, according to Gmelin, of felspar and mesotype. (Leonhard, Mineralreich, p. 102.) A rock much resembling clinkstone, and called by some Petrosilex, contains a considerable percentage of quartz and felspar. As both trachyte and basalt pass into clinkstone, the rock so called must be very various in composition.

Compact Felspar, which has also been called Petrosilex; the rock so called includes the hornstone of some mineralogists, is allied to clinkstone, but is harder, more compact, and translucent. It is a varying rock, of which the chemical composition is not well defined, and is perhaps the same as that of clay. (MacCulloch's Classification of Rocks, p. 481.) Dr. MacCulloch says, that it contains both potash and soda.

Cornean. A variety of claystone allied to hornstone. A fine homogeneous paste, supposed to consist of an aggregate of felspar, quartz, and hornblende, with occasionally epidote, and perhaps chlorite; it passes into compact felspar and hornstone. (De la Beche, Geol. Trans. second series, vol. 2. p. 3.)

[Diallage rock]. Syn. Euphotide, Gabbro, and some Ophiolites. Compounded of felspar and diallage, sometimes with the addition of serpentine, or mica, or quartz. (MacCulloch. ibid. p. 648.)

Diorite. A kind of [greenstone], which see. Components, felspar and hornblende in grains. According to Rose, Ann. des Mines, tom. 8. p. 4., diorite consists of albite and hornblende.

Dioritic-porphyry. A porphyritic greenstone, composed of crystals of albite and hornblende, in a greenish or blackish base. (Rose, ibid. p. 10.)

Dolerite. Formerly defined as a synonym of [greenstone], which see. But, according to Rose (ibid. p. 32.), its composition is black augite and Labrador-felspar; according to Leonhard (Mineralreich, &c. p. 77.), augite, Labrador-felspar, and magnetic iron.

Domite. An earthy trachyte, found in the Puy de Dome, in Auvergne.

Euphotide. A mixture of grains of Labrador-felspar and diallage. (Rose, ibid. p. 19.) According to some, this rock is defined to be a mixture of augite or hornblende, and saussurite, a mineral allied to jade. (Allan's Mineralogy, p. 158.) See [Diallage rock].

Felspar-porphyry. Syn. Hornstone-porphyry; a base of felspar, with crystals of felspar, and crystals and grains of quartz. See also [Hornstone].

Gabbro, see [Diallage rock].

[Greenstone]. Syn. Dolerite and diorite; components, hornblende and felspar, or augite and felspar in grains. See above, [p. 372.]

Greystone. (Graustein of Werner.) Lead grey and greenish rock, composed of felspar and augite, the felspar being more than seventy-five per cent. (Scrope, Journ. of Sci. No. 42. p. 221.) Greystone lavas are intermediate in composition between basaltic and trachytic lavas.

[Hornblende Rock]. A greenstone, composed principally of granular hornblende, or augite. (Leonhard, Mineralreich, &c., p. 85.)

[Hornstone], Hornstone-porphyry. A kind of felspar porphyry (Leonhard, ibid.), with a base of hornstone, a mineral approaching near to flint, differing from compact felspar in being infusible.

Hypersthene Rock, a mixture of grains of Labrador-felspar and hypersthene (Rose, Ann. des Mines, tom. 8. p. 13.), having the structure of syenite or granite; abundant among the traps of Skye. Some geologists consider it a greenstone, in which hypersthene replaces hornblende.

Laterite. A red jaspery rock, composed of silicate of alumina and oxide of iron. Abundant in the Deccan, in India; and referred to the trap formation; from Later, a brick or tile.

Melaphyre. A variety of black porphyry, the base being black augite with crystals of felspar; from μελας, melas, black.

[Obsidian]. Vitreous lava like melted glass, nearly allied to pitchstone.

Ophiolite, sometimes same as Diallage rocks (Leonhard, p. 77.); sometimes a kind of serpentine.

Ophite. A green porphyritic rock composed chiefly of hornblende, with crystals of that mineral in a base of the same, mixed with some felspar. It passes into serpentine by a mixture of talc. (Burat's d'Aubuisson, tom. ii. p. 63.)

Pearlstone. A volcanic rock, having the lustre of mother of pearl; usually having a nodular structure; intimately related to obsidian, but less glassy.

Peperino. A form of volcanic tuff, composed of basaltic scoriæ. See [p. 374.]

Petrosilex. See [Clinkstone] and Compact Felspar.

Phonolite. Syn. of [Clinkstone], which see.

[Pitchstone]. Vitreous lava, less glassy than obsidian; a blackish green rock resembling glass, having a resinous lustre and appearance of pitch; composition various, usually felspar and augite; passes into basalt; occurs in veins, and in Arran forms a dike thirty feet wide, cutting through sandstone; forms the outer walls of some basaltic dikes.

Porphyry. Any rock in which detached crystals of felspar, or of one or more minerals, are diffused through a base. See [p. 372.]

Pozzolana. A kind of tuff. See [p. 36.]

Pumice. A light, spongy, fibrous form of trachyte. See [p. 373.]

Pyroxenic-porphyry, same as augitic-porphyry, pyroxene being Haüy's name for augite.

Scoriæ. Syn. volcanic cinders; reddish brown or black porous form of lava. See [p. 373.]

Serpentine. A greenish rock, in which there is much magnesia; usually contains diallage, which is nearly allied to the simple mineral called serpentine. Occurs sometimes, though rarely, in dikes, altering the contiguous strata; is indifferently a member of the trappean or hypogene series.

Syenitic-greenstone; composition, crystals or grains of felspar and hornblende. See [p. 372.]

Tephrine, synonymous with lava. Name proposed by Alex. Brongniart.

Toadstone. A local name in Derbyshire for a kind of [wacké], which see.

Trachyte. Chiefly composed of glassy felspar, with crystals of glassy felspar. See [p. 372.]

Trap tuff. See [p. 374.]

Trass. A kind of tuff or mud poured out by lake craters during eruptions; common in the Eifel, in Germany.

Tufaceous Conglomerate. See [p. 374.]

Tuff. Syn. Trap-tuff, volcanic tuff. See [p. 374.]

Vitreous lava. See [Pitchstone] and [Obsidian].

Volcanic Tuff. See [p. 374.]

[Wacké]. A soft and earthy variety of trap, having an argillaceous aspect. It resembles indurated clay, and when scratched exhibits a shining streak.

Whinstone. A Scotch provincial term for greenstone and other hard trap rocks.

ANALYSIS OF MINERALS MOST ABUNDANT IN THE VOLCANIC AND HYPOGENE ROCKS.

Silica. Alumina. Magnesia. Lime. Potash. Soda. Iron Oxide. Manganese. Remainder.
Actinolite (Bergman) 64· — 22· — — — 3· — —
Albite (Rose) 68·84 20·53 — a trace — 9·12 — — —
—— (mean of 4 analyses) 69·45 19·44 0·13 0·22 — 9·95 a trace a trace —
Augite (Rose) 53·36 — 4·99 22·19 — — 17·38 0·09 —
—— (mean of 4 analyses) 53·57 1· 11·26 20·9 — — 10·75 0·67 —
Carbonate of Lime (Biot) — — — 56·33 — — — — 43·05 C.
Chiastolite (Landgrabe) 68·49 30·17 4·12 — — — 2·7 — 0·27 W.
Chlorite (Vauquelin) 26· 18·5 8· — — 2· 43· — —
—— (mean of 3 analyses) 27·43 17·9 14·56 0·50 1·56 — 30·63 — 6·92 W.
Diallage (Klaproth) 60· — 27·5 — — — 10·5 — —
—— (mean of 3 analyses) 43·33 2·2 26·41 5·58 — — 11·53 — 8·54 W.
Epidote (Vauquelin) 37· 21· — 15· — — 24· 1·5 —
Felspar, common (Vauq.) 62·83 17·02 — 3· 13· — 1· — —
—— (Rose) 66·75 17·5 — 1·25 12· — 0·75 — —
—— (mean of 7 analyses) 64·04 18·94 — 0·76 13·66 — 0·74 — —
Garnet (Klaproth) 35·75 27·25 — — — — 36· 0·25 —
—— (Phillips) 43· 16· — 20· — — 16· — —
Hornblende (Klap.) 42· 12· 2·25 11· a trace — 30· 0·25 —
—— (Bonsdorff.) 45·69 12·18 18·79 13·85 — — 7·32 0·22 1·5 F.
Hypersthene (Klaproth) 54·25 2·25 14· 1·5 — — 24·5 a trace 1· W.
Labrador-felspar (Klap.) 55·75 26·5 — 11· — 4· 1·25 — 0·5 W.
Leucite (Klap.) 53·75 24·62 — — 21·35 — — — —
Mesotype (Gehlen) 54·64 19·70 — 1·61 — 15·09 — — 9·83 W.
Mica (Klaproth) 42·5 11·5 9· — 10· — 22· 2· —
—— (Vauquelin) 50· 35· — 1·33 — — 7· — —
—— (mean of 3 analyses) 45·83 22·58 — — 11·08 — 14· 1·45 —
Olivine (Klaproth) 50· — 38·5 — — — 12· — —
Schorl or Tourmaline (Gmelin) 35·48 34·75 4·68 — 0·48 1·75 17·44 1·89 4·02 B.
—— (mean of 6 analyses) 36·03 35·82 4·44 0·28 0·71 1·96 13·71 1·62 —
Serpentine (Hisinger) 43·07 0·25 40·37 0·5 — — 1·17 — 12·45 W.
—— (mean of 5 analyses) 37·29 4·97 36·8 2·89 — — 3·14 — 12·77 W.
Steatite (Vauquelin) 64· — 22· — — — 3· — 5· W.
—— (mean of 3 anal. by Klap.) 48·3 6·18 26·65 — — — 2· — 9·5 W.
Talc. (Klaproth) 61·75 — 30·5 — 2·75 — 2·5 — —

In the last column of the above Table, the letters B. C. F. W. represent Boracic acid, Carbonic acid, Fluoric acid, and Water.

CHAPTER XXIX.

VOLCANIC ROCKS—continued.

Trap dikes — sometimes project — sometimes leave fissures vacant by decomposition — Branches and veins of trap — Dikes more crystalline in the centre — Foreign fragments of rock imbedded — Strata altered at or near the contact — Obliteration of organic remains — Conversion of chalk into marble — and of coal into coke — Inequality in the modifying influence of dikes — Trap interposed between strata — Columnar and globular structure — Relation of trappean rocks to the products of active volcanos — Submarine lava and ejected matter corresponds generally to ancient trap — Structure and physical features of Palma and some other extinct volcanos.

Having in the last chapter spoken of the composition and mineral characters of volcanic rocks, I shall next describe the manner and position in which they occur in the earth's crust, and their external forms. Now the leading varieties, such as basalt, greenstone, trachyte, porphyry, 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.

Volcanic dikes.—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 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 the annexed figure.[378-A])

Fig. 439.

Dike in inland valley, near the Brazen Head, Madeira.

In the islands of Arran, Skye, and 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, as represented in the annexed view ([fig. 440.]). In these instances, the greenstone of the dike is usually more tough and hard than the sandstone; but chemical action, and chiefly the oxidation of the iron, has given rise to the more rapid decay.

Fig. 440.

Fissures left vacant by decomposed trap. Strathaird, Skye. (MacCulloch.)

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.

Fig. 441.

Trap veins in Airdnamurchan.

As fissures sometimes send off branches, or divide into two or more fissures of equal size, so also we find trap dikes bifurcating and ramifying, and sometimes they are so tortuous as to be called veins, though this is more common in granite than in trap. The accompanying sketch ([fig. 441.]) by Dr. MacCulloch represents part of a sea-cliff in Argyleshire, where an overlying mass of trap, b, sends out some veins which terminate downwards. Another trap vein, a a, cuts through both the limestone, c, and the trap, b.

In [fig. 442.], 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 5 to 7 feet in width, which will afford a scale of measurement for the whole.

Fig. 442.

Ground plan of greenstone dike traversing sandstone. Arran.

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 downwards in veins or dikes, which probably unite with other masses of igneous rock at a greater depth. The largest of the dikes represented in the annexed diagram, and which are seen in part of the coast of Skye, is no less than 100 feet in width.

Fig. 443.

Trap dividing and covering sandstone near Suishnish in Skye. (MacCulloch.)

Every variety of trap-rock is sometimes found in these dikes, as basalt, greenstone, felspar-porphyry, and more rarely trachyte. The amygdaloidal traps also occur, 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 eruptions 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.

Dikes more crystalline in the centre.—In many cases trap at the edges or sides of a dike is less crystalline or more earthy than in the centre, in consequence of the melted matter having cooled more rapidly by coming in contact with the cold sides of the fissure; whereas, in the centre, the matter of the dike being kept long in a fluid or soft state, the crystals are slowly formed. In the ancient part of Vesuvius, called Somma, a thin band of half-vitreous lava is found at the edge of some dikes. At the junction of greenstone dikes with limestone, a sahlband, or selvage, of serpentine is occasionally observed.

Fig. 444.

Syenitic greenstone dike of Næsodden, Christiania.

b. imbedded fragment of crystalline schist surrounded by a band of greenstone.

On the left shore of the fiord of Christiania, in Norway, I examined, in company with Professor Keilhau, a remarkable dike of syenitic greenstone, which is traced through Silurian strata, until at length, in the promontory of Næsodden, it enters mica-schist. [Fig. 444.] represents a ground plan, where the dike appears 8 paces in width. In the middle it is highly crystalline and granitiform, of a purplish colour, and containing a few crystals of mica, and strongly contrasted with the whitish mica-schist, between which and the syenitic rock there is usually on each side a distinct black band, 18 inches wide, of dark greenstone. When first seen, these bands have the appearance of two accompanying dikes; yet they are, in fact, only the different form which the syenitic materials have assumed where near to or in contact with the mica-schist. At one point, a, one of the sahlbands terminates for a space; but near this there is a large detached block, b, having a gneiss-like structure, consisting of hornblende and felspar, which is included in the midst of the dike. Round this a smaller encircling zone is seen, of dark basalt, or fine-grained greenstone, nearly corresponding to the larger ones which border the dike, but only 1 inch wide.

It seems, therefore, evident that the fragment, b, has acted on the matter of the dike, probably by causing it to cool more rapidly, in the same manner as the walls of the fissure have acted on a larger scale. The facts, also, illustrate the facility with which a granitiform syenite may pass into ordinary rocks of the volcanic family.

Fig. 445.

Greenstone dike, with fragments of gneiss. Sorgenfri, Christiania.

The fact above alluded to, of a foreign fragment, such as b, [fig. 444.], included in the midst of the trap, as if torn off from some subjacent rock or the walls of a fissure, is by no means uncommon. A fine example is seen in another dike of greenstone, 10 feet wide, in the northern suburbs of Christiania, in Norway, of which the annexed figure is a ground plan. The dike passes through shale, known by its fossils to belong to the Silurian series. In the black base of greenstone are angular and roundish pieces of gneiss, some white, others of a light flesh-colour, some without lamination, like granite, others with laminæ, which, by their various and often opposite directions, show that they have been scattered at random through the matrix. These imbedded pieces of gneiss measure from 1 to about 8 inches in diameter.

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 intense heat of melted matter and the entangled gases might be expected to cause.

Plas-Newydd.—A striking example, near Plas-Newydd, in Anglesea, has been described by Professor Henslow.[381-A] The dike is 134 feet wide, and consists of a rock which is a compound of felspar 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, to 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 porcellanous 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.[382-A] 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.[382-B]

Antrim.—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 8 or 10 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 porcellanous 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."[382-C] All traces of organic remains are effaced in that part of the limestone which is most crystalline.

Fig. 446.

Basaltic dikes in chalk in island of Rathlin, Antrim. Ground plan, as seen on the beach. (Conybeare and Buckland. [382-D])

The annexed drawing ([fig. 446.]) 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.[382-E] By another, the slate clay of the coal measures has been indurated, and has assumed the character of flinty slate[383-A]; and in another place the slate clay of the lias has been changed into flinty slate, which still retains numerous impressions of ammonites.[383-B]

It might have been anticipated that beds of coal would, from their combustible nature, be effected 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 9 feet on each side.[383-C]

At Cockfield Fell, in the north of England, a similar change is observed. Specimens taken at the distance of about 30 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.[383-D]

As examples might be multiplied without end, I shall merely select one or two others, and then conclude. The rock of Stirling Castle is a calcareous sandstone, fractured and forcibly displaced by a mass of greenstone which has evidently invaded the strata in a melted state. The sandstone has been indurated, and has assumed a texture approaching to hornstone near the junction. In Arthur's Seat and Salisbury Craig, near Edinburgh, a sandstone which comes in contact with greenstone is converted into a jaspideous rock.[383-E]

The secondary sandstones in Skye are converted into solid quartz in several places, where they come in contact with veins or masses of trap; and a bed of quartz, says Dr. MacCulloch, found near a mass of trap, among the coal strata of Fife, was in all probability a stratum of ordinary sandstone, having been subsequently indurated and turned into quartzite by the action of heat.[383-F]

But although strata in the neighbourhood of dikes are thus altered in a variety of cases, shale being turned into flinty slate or jasper, limestone into crystalline marble, sandstone into quartz, coal into coke, and the fossil remains of all such strata wholly and in part obliterated, 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 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 are 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 caloric supplied by the dike and its gases will act more powerfully.

Fig. 447.

Trap interposed between displaced beds of limestone and shale, at White Force, High Teesdale, Durham. (Sedgwick.[384-A])

Intrusion of trap between strata.—In proof of the mechanical force which the fluid trap has sometimes exerted on the rocks into which it has intruded itself, I may refer to the Whin-Sill, where a mass of basalt, from 60 to 80 feet in height, represented by a, [fig. 447.], is in part wedged in between the rocks of limestone, b, and shale, c, which have been separated from the great mass of limestone and shale, d, with which they were united.

The shale in this place is indurated; and the limestone, which at a distance from the trap is blue, and contains fossil corals, is here converted into granular marble without fossils.

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 was caused by gases propelling the lava upwards.

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 9 feet, and those of Morven an inch or less.[385-A] 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. Among other examples of the last-mentioned phenomenon is the mass of basalt, called the Chimney, in St. Helena (see [fig. 448.]), a pile of hexagonal prisms, 64 feet high, evidently the remainder of a narrow dike, the walls of rock which the dike originally traversed having been removed down to the level of the sea. In [fig. 449.] a small portion of this dike is represented on a less reduced scale.[385-B]

Fig. 448.

Volcanic dike composed of horizontal prisms. St. Helena.

Fig. 449.

Small portion of the dyke in Fig. 448.

Fig. 450.

Lava of La Coupe d'Ayzac, near Antraigue, in the province of Ardèche.

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 descends and occupies 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. The accompanying sketch ([fig. 450.]) represents the remnant of the lava at one of the points where a lateral torrent joins the main valley of the Volant. 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 then horizontal at f, their position having been every where determined, according to the law before mentioned, by the concave form of the original valley.

Fig 451.

Columnar basalt in the Vicentin. (Fortis.)

In the annexed figure ([451.]) 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.[386-A] 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 hornblende or augite predominate; it is also observed in clinkstone, trachyte, and other felspathic rocks of the igneous class, although in these it is rarely exhibited in such regular polygonal forms.

Fig. 452.

Basaltic pillars of the Käsegrotte, Bertrich-Baden, half way between Treves and Coblentz. Height of grotto, from 7 to 8 feet.

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. 452.]). 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. The position of the lava bordering the river in this valley might be represented by a section like that already given at [fig. 450.] [p. 385.], if we merely supposed inclined strata of slate and the argillaceous sandstone called greywacké to be substituted for gneiss.

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.

Fig. 453.

Globiform pitchstone. Chiaja di Luna, Isle of Ponza. (Scrope.)

A striking example of this structure occurs in 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. 453.]). 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."[387-A]

A fissile texture is occasionally assumed by clinkstone and other trap rocks, so that they have been used for roofing houses. Sometimes the prismatic and slaty structure is found in the same mass. The causes which give rise to such arrangements are very obscure, but are supposed to be connected with changes of temperature during the cooling of the mass, as will be pointed out in the sequel. (See Chaps. [XXXV.] and [XXXVI.])

Relation of Trappean Rocks to the products of active Volcanos.

When we reflect on the changes above described in the strata near their contact with trap dikes, and consider how great is the analogy in composition and structure of the rocks called trappean and the lavas of active volcanos, 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. The trappean rocks first studied in the north of Germany, and in Norway, France, Scotland, and other countries, were either such as had been formed entirely under deep 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 they 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 downwards 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.

Fig. 454.

Strata intersected by a trap dike, and covered with alluvium.

We have already stated how frequently dense masses of strata have been removed by denudation from wide areas (see [Chap. VI.]); and this fact prepares us to expect a similar destruction of whatever may once have formed the uppermost part of ancient submarine or subaerial volcanos, 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. 454.]), 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 volcanos.

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 volcanos, 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.

Thus, for example, the recent examination of the igneous rocks of Sicily, especially those of the Val di Noto, has proved that all the more ordinary varieties of European trap have been there produced under the waters of the sea, at a modern period; that is to say, since the Mediterranean has been inhabited by a great proportion of the existing species of testacea.

These igneous rocks of the Val di Noto, and the more ancient trappean rocks of Scotland and other countries, differ from subaerial volcanic formations in being more compact and heavy, and in forming sometimes extensive sheets of matter intercalated between marine strata, and sometimes stratified conglomerates, of which the rounded pebbles are all trap. They differ also in the absence of regular cones and craters, and in the want of conformity of the lava to the lowest levels of existing valleys.

It is highly probable, however, that insular cones did exist in some parts of the Val di Noto: and that they were removed by the waves, in the same manner as the cone of Graham island, in the Mediterranean, was swept away in 1831, and that of Nyöe, off Iceland, in 1783.[389-A] All that would remain in such cases, after the bed of the sea has been upheaved and laid dry, would be dikes and shapeless masses of igneous rock, cutting through sheets of lava which may have spread over the level bottom of the sea, and strata of tuff, formed of materials first scattered far and wide by the winds and waves, and then deposited. Trap conglomerates also, to which the action of the waves must give rise during the denudation of such volcanic islands, will emerge from the deep whenever the bottom of the sea becomes land.

The proportion of volcanic matter which is originally submarine must always be very great, as those volcanic vents which are not entirely beneath the sea, are almost all of them in islands, or, if on continents, near the shore. This may explain why extended sheets of trap so often occur, instead of narrow threads, like lava streams. For, a multitude of causes tend, near the land, to reduce the bottom of the sea to a nearly uniform level,—the sediment of rivers,—materials transported by the waves and currents of the sea from wasting cliffs,—showers of sand and scoriæ ejected by volcanos, and scattered by the wind and waves. When, therefore, lava is poured out on such a surface, it will spread far and wide in every direction in a liquid sheet, which may afterwards, when raised up, form the tabular capping of the land.

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. 373.]); 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.[390-A]

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 volcanos; for they are such in every essential point, although they no longer eject fire and smoke."[390-B] 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.[390-C]

Although the principal component minerals of subaerial lavas are the same as those of intrusive trap, and both the columnar and globular structure are common to both, there are, nevertheless, some volcanic rocks which never occur as lava, such as greenstone, clinkstone, the more crystalline porphyries, and those traps in which quartz and mica appear as constituent parts. In short, the intrusive trap rocks, forming the intermediate step between lava and the plutonic rocks, depart in their characters from lava in proportion as they approximate to granite.

These views respecting the relations of the volcanic and trap rocks will be better understood when the reader has studied, in the 33d chapter, what is said of the plutonic formations.

FORM, STRUCTURE, AND ORIGIN OF VOLCANIC MOUNTAINS.

The origin of volcanic cones with crater-shaped summits has been alluded to in the last chapter ([p. 368.]), and more fully explained in the "Principles of Geology" (chaps. xxiv. to xxvii.), where Vesuvius, Etna, Santorin, and Barren Island were 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 volcanos of still higher antiquity.

The island of Palma, for example, one of the Canaries, offers an excellent illustration of what, in common with many others, I regard as the ruins of a large dome-shaped mass formed by a series of eruptions proceeding from a crater at the summit, this crater having been since replaced by a larger cavity, the origin of which has afforded geologists an ample field for discussion and speculation.

Fig. 455.

View of the Isle of Palma, and of the entrance into the central cavity or Caldera. From Von Buch's "Canary Islands."

Fig. 456.

Map of the Caldera of Palma and the great ravine, called "Barranco de las Angustias." From Survey of Capt. Vidal, R.N., 1837.

Von Buch, in his excellent account of the Canaries, has given us a graphic picture of this island, which consists chiefly of a single mountain ([fig. 455.]). This mountain has the general form of a great truncated cone, with a huge and deep cavity in the middle, about six miles in diameter, called by the inhabitants "the Caldera," or cauldron. The range of precipices surrounding the Caldera are no less than 4000 feet in their average height; at one point, where they are highest, they are 7730 feet above the level of the sea. The external flanks of the cone incline gently in every direction towards the base of the island, and are in part cultivated; but the walls and bottom of the Caldera present on all sides rugged and uncultivated rocks, almost completely devoid of vegetation. So steep are these walls, that there is no part by which they can be descended, and the only entrance is by a great ravine, or Barranco, as it is called (see b b', map, [fig. 456.]), which extends from the sea to the interior of the great cavity, and by its jagged, broken, and precipitous sides, exhibits to the geologist a transverse section of the rocks of which the whole mountain is composed. By this means, we learn that the cone is made up of a great number of sloping beds, which dip outwards in every direction from the centre of the void space, or from the hollow axis of the cone. The beds consist chiefly of sheets of basalt, alternating with conglomerates; the materials of the latter being in part rounded, as if rolled by water in motion. The inclination of all the beds corresponds to that of the external slope of the island, being greatest towards the Caldera, and least steep when they are nearest the sea. There are a great number of tortuous veins, and many dikes of lava or trap, chiefly basaltic, and most of them vertical, which cut through the sloping beds laid open to view in the great gorge or Barranco. These dikes and veins are more and more abundant as we approach the Caldera, being therefore most numerous where the slope of the beds is greatest.

Assuming the cone to be a pile of volcanic materials ejected by a long succession of eruptions (a point on which all geologists are agreed), we have to account for the Caldera and the great Barranco. I conceive that the cone itself may be explained, in accordance with what we know of the ordinary growth of volcanos[392-A], by supposing most of the eruptions to have taken place from one or more central vents, at or near the summit of the cone, before it was truncated. From this culminating point, sheets of lava flowed down one after the other, and showers of ashes or ejected stones. The volcano may, in the earlier stages of its growth, have been in great part submerged, like Stromboli, in the sea; and, therefore, some of the fragments of rock cast out of its crater may not only have been rolled by torrents sweeping down the mountain's side, but have also been rounded by the waves of the sea, as we see happen on the beach near Catania, on which the modern lavas of Etna are broken up. The increased number of dykes, as we approach the axis of the cone, agrees well with the hypothesis of the eruptions having been most frequent towards the centre.

There are three known causes or modes of operation, which may have conduced towards the vast size of the Caldera. First, the summit of a conical mountain may have fallen in, as happened in the case of Capacurcu, one of the Andes, according to tradition, in the year 1462, and of many other volcanic mountains.[393-A] Sections seem wanting, to supply us with all the data required for judging fairly of the tenability of this hypothesis. It appears, however, from Captain Vidal's survey (see [fig. 456.]), that a hill of considerable height rises up from the bottom of the Caldera, the structure of which, if it be any where laid open, might doubtless throw much light on this subject. Secondly, an original crater may have been enlarged by a vast gaseous explosion, never followed by any subsequent eruption. A serious objection to this theory arises from our not finding that the exterior of the cone supports a mass of ruins, such as ought to cover it, had so enormous a volume of matter, partly made up of the solid contents of the dikes, been blown out into the air. In that case, an extensive bed of angular fragments of stone, and of fine dust, might be looked for, enveloping the entire exterior of the mountain up to the very rim of the Caldera, and ought nowhere to be intersected by a dike. The absence of such a formation has induced Von Buch to suppose that the missing portion of the cone was engulphed. It should, however, be remembered, that in existing volcanos, large craters, two or three miles in diameter, are sometimes formed by explosions, or by the discharge of great volumes of steam.

There is yet another cause to which the extraordinary dimensions of the Caldera may, in part at least, be owing; namely, aqueous denudation. Von Buch has observed, that the existence of a single deep ravine, like the Great Barranco, is a phenomenon common to many extinct volcanos, as well as to some active ones. Now, it will be seen by Captain Vidal's map ([fig. 456.] [p. 391.]), that the sea-cliff at Point Juan Graje, 780 feet high, now constituting the coast at the entrance of the great ravine, is continuous with an inland cliff which bounds the same ravine on its north-western side. No one will dispute that the precipice, at the base of which the waves are now beating, owes its origin to the undermining power of the sea. It is natural, therefore, to attribute the extension of the same cliff to the former action of the waves, exerted at a time when the relative level of the island and the ocean were different from what they are now. But if the waves and tides had power to remove the rocks once filling a great gorge which is 7 miles long, and, in its upper part, 2000 feet deep, can we doubt that the same power may have cleared out much of the solid mass now missing in the Great Caldera?

The theory advanced to account for the configuration of Palma, commonly called the "elevation crater theory," is this. All the alternating masses of basalt and conglomerate, intersected in the Barranco, or abruptly cut off in the escarpment or walls of the Caldera, were at first disposed in horizontal masses on the level floor of the ocean, and traversed, when in that position, by all the basaltic dikes which now cut through them. At length they were suddenly uplifted by the explosive force of elastic vapours, which raised the mass bodily, so as to tilt the beds on all sides away from the centre of elevation, causing at the same time an opening at the culminating point. Besides many other objections which may be urged against this hypothesis, it leaves unexplained the unbroken continuity of the rim of the Caldera, which is uninterrupted in all places save one[394-A], namely, that where the great gorge or Barranco occurs.

As a more natural way of explaining the phenomenon, the following series of events may be imagined. The principal vent, from which a large part of the materials of the cone were poured or thrown out, was left empty after the last escape of vapour, when the volcano became extinct. We learn from Mr. Dana's valuable work on the geology of the United States' Exploring Expedition, published in 1849, that two of the principal volcanos of the Sandwich Islands, Mounts Loa and Kea in Owyhee, are huge flattened volcanic cones, 15,000 feet high (see [fig. 457.]), each equalling two and a half Etnas in their dimensions.

Fig. 457.

Mount Loa, in the Sandwich Islands. (Dana)

The dotted lines indicate a supposed column of solid rock caused by the lava consolidating after eruptions.

From the summits of these lofty though featureless hills, and from vents not far below their summits, successive streams of lava, often 2 miles or more in width, and sometimes 26 miles long, have flowed. They have been poured out one after the other, some of them in recent times, in every direction from the apex of the cone, down slopes varying on an average from 4 degrees to 8 degrees; but at some places considerably steeper.[394-B] Sometimes deep rents open on the sides of these cones, which are filled by streams of lava passing over them, the liquid matter in such cases probably uniting in the fissure with other lava melted in subterranean reservoirs below, and thus explaining the origin of one great class of lateral dikes, on Etna, Palma, and other cones.

If the flattened domes, such as those here alluded to in the Sandwich Islands, instead of being inland, and above water, were situated in mid-ocean, like the Island of St. Paul, and for the most part submerged (see [figs. 458], [459], [460.]), and if a gradual upheaval of such a dome should then take place, the denuding power of the sea could scarcely fail to play an important part in modifying the form of the volcanic mountain as it rose. The crater will almost invariably have one side much lower than all the others, namely, that side towards which the prevailing winds never blow, and to which, therefore, showers of dust and scoriæ are rarely carried during eruptions. There will also be one point on this windward or lowest side more depressed than all the rest, by which the sea may enter as often as the tide rises, or as often as the wind blows from that quarter. For the same reason that the sea continues to keep open a single entrance into the lagoon of an atoll or annular coral reef, it will not allow this passage into the crater to be stopped up, but scour it out, at low tide, or as often as the wind changes. The channel, therefore, will always be deepened in proportion as the island rises above the level of the sea, at the rate perhaps of a few feet or yards in a century.

Fig. 458.

Map of the Island of St. Paul, in the Indian Ocean, lat. 38° 44´ S., long. 77° 37´ E., surveyed by Capt. Blackwood, R. N., 1842.

Fig. 459.

View of the Crater of the Island of St. Paul.

The island of St. Paul may perhaps be motionless; but if, like many other parts of the earth's crust, it should begin to undergo a gradual upheaval, or if, as has happened to the shores of the Bay of Baiæ, its level should oscillate, with a tendency upon the whole to increased elevation, the same power which has cut away part of the cone, and caused the cliffs now seen on the north-east side of the island, would have power to undermine the walls of the crater, and enlarge its diameter, keeping open the channel, by which it enters into it. This ravine might be excavated to the depth of 180 feet (the present depth of the crater), and its length might be extended to many miles according to the size of the submerged part of the cone. The crater is only a mile in diameter, and the surrounding cliffs, where loftiest, only 800 feet high, so that the size of this cone and crater is insignificant when compared to those in the Sandwich Islands, and I have merely selected it because it affords an example of a class of insular volcanos, into the craters of which the sea now enters by a single passage. The crater of Vesuvius in 1822 was 2000 feet deep; and if it were a half submerged cone, like St. Paul, the excavating power of the ocean might in conjunction with gaseous explosions and co-operating with a gradual upheaving force, give rise to a caldera on as grand a scale as that exhibited by Palma.

Fig. 460.

Side view of the Island of St. Paul (N.E. side). Nine-pin rocks two miles distant. (Captain Blackwood.)

If, after the geographical changes above supposed, the volcanic fires long dormant should recover their energy, they might, as in the case of Teneriffe, Vesuvius, Santorin, and Barren Island, discharge from the old central vent, long sealed up at the bottom of the caldera, new floods of lava and clouds of elastic vapours. Should this happen, a new cone will be built up in the middle of the cavity or circular bay, formed, partly by explosion, partly perhaps by engulphment, and partly by aqueous denudation. In the island of Palma this last phase of volcanic activity has never occurred; but the subterranean heat is still in full operation beneath the Canary Islands, so that we know not what future changes it may be destined to undergo.

CHAPTER XXX.

ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS.

Tests of relative age of volcanic rocks — Test by superposition and intrusion — Dike of Quarrington Hill, Durham — Test by alteration of rocks in contact — Test by organic remains — Test of age by mineral character — Test by included fragments — Volcanic rocks of the Post-Pliocene period — Basalt of Bay of Trezza in Sicily — Post-Pliocene volcanic rocks near Naples — Dikes of Somma — Igneous formations of the Newer Pliocene period — Val di Noto in Sicily.

Having referred the sedimentary strata to a long succession of geological periods, we have next 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:—1st, superposition and intrusion, with or without alteration of the rocks in contact; 2d, organic remains; 3d, mineral character; 4th, included fragments of older rocks.

Fig. 461.

Tests by superposition, &c.—If a volcanic rock rests upon an aqueous deposit, the former 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 at D in the annexed figure ([fig. 461.]), 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. 461.]), 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 if the stratum a be altered by b at the point of contact, we must then conclude the trap to have been intrusive, 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 a 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 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 strata d or e to have been altered at their junction, as if by heat.

Fig. 462.

When trap dikes were described in the preceding chapter, they were shown to be more modern than all the strata which they traverse. A basaltic dike at Quarrington Hill, near Durham, passes through coal-measures, the strata of which are inclined, and shifted so that those on the north side of the dike are 24 feet above the level of the corresponding beds on the south side (see section, [fig. 463.]). But the horizontal beds of overlying Red Sandstone and Magnesian Limestone are not cut through by the dike. Now here the coal-measures were not only deposited, but had subsequently been disturbed, fissured, and shifted, before the fluid trap now forming the dike was introduced into a rent. It is also clear that some of the upper edges of the coal strata, together with the upper part of the dike, had been subsequently removed by denudation before the lower New Red Sandstone and Magnesian Limestone were superimposed. Even in this case, however, although the date of the volcanic eruption is brought within narrow limits, it cannot be defined with precision; it may have happened either at the close of the Carboniferous period, or early in that of the Lower New Red Sandstone, or between these two periods, when the state of the animate creation and the physical geography of Europe were gradually changing from the type of the Carboniferous era to that of the Permian.

Fig. 463.

Section at Quarrington Hill, east of Durham. (Sedgwick.)

The test of age by superposition is strictly applicable to all stratified volcanic tuffs, according to the rules already explained in the case of other sedimentary deposits. (See [p. 96.])

Test of age by organic remains.—We have seen how, in the vicinity of active volcanos, 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 volcanos now active 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 such evidence we can distinctly establish the coincidence in age of volcanic rocks, and the different primary, secondary, and tertiary fossiliferous strata already considered.

The tuffs now alluded to are not exclusively marine, but include, in some places, freshwater 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 10 feet, and for a distance of 8 leagues from the crater in a southerly direction. Birds, cattle, and wild animals were scorched to death in great numbers, and buried in these ashes. Some volcanic dust fell at Chiapa, upwards of 1200 miles to windward of the volcano, 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.[399-A]

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; as in Iceland in 1783, when 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.[399-B] 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 so much in mineral composition and texture as to render these characters of minor importance when compared to their value in the chronology of the fossiliferous rocks.

It will, however, be seen in the description which follows, of the European trap rocks of different ages, that they had often a peculiar lithological character, resembling the differences before remarked as existing between the modern lavas of Vesuvius, Etna, and Chili. (See [p. 378.])

It has been remarked that 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; but the great mass of trachyte occupies in general an inferior position, and is cut through and overflowed by basalt. It can by no means be inferred that trachyte predominated greatly 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 more felspathic lavas 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 felspathic lavas; thus, for example, hornblende, augite, and olivine are each more than three times the weight of water; whereas common felspar, albite, and Labrador felspar, have each scarcely more than 21/2 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 felspathic lavas and traps. 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 upwards to the surface by the expansive power of gases. Those materials, therefore, which occupied 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 conglomerates almost exclusively composed of rolled pebbles of trap, associated with stratified rocks in the neighbourhood of masses of intrusive trap. If the pebbles agree generally in mineral character with the latter, we are then enabled to determine the age of the intrusive rock 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 volcanic islands, or at the base of Etna.

Post-Pliocene Period (including the Recent).—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, and that they have been ever shifting the places where they have broken out at the earth's surface.

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 species of testacea. The southern and eastern flanks of Etna are skirted by a fringe of alternating sedimentary and volcanic deposits, of submarine origin, as at Adernò, Trezza, and other places. Of sixty-five species of fossil shells which I procured in 1828 from this formation, near Trezza, it was impossible to distinguish any from species now living in the neighbouring sea.

Fig. 464.

View of the Isle of Cyclops in the Bay of Trezza.[401-A]

The Cyclopian 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 Cyclopian 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. 464.]) 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 the drawing ([fig. 464.]), 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.

Fig. 465.

Contortions of strata in the largest of the Cyclopian Islands.

In the annexed woodcut ([fig. 465.]) I have represented a portion of the altered rock, a few feet square, where the alternating thin laminæ of sand and clay have put on the appearance which we often observe in some of the most contorted of the 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. 466.])

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 honeycombed 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.

Fig. 466.

Post-Pliocene strata invaded by lava, Isle of Cyclops (horizontal section).

Post-Pliocene formations near Naples.—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 20 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 was also stated in this work ([p. 113.]), 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. These post-pliocene strata, containing recent marine shells, alternate with distinct currents and sheets of lava which were of contemporaneous origin; and we find that in Vesuvius itself, the ancient cone called Somma is of far greater volume than the modern cone, and is intersected by a far greater number of dikes. In contrasting this ancient part of the mountain with that of modern date, one principal point of difference is observed; namely, the greater frequency in the older cone of fragments of altered sedimentary rocks ejected during eruptions. We may easily conceive that the first explosions would act with the greatest violence, rending and shattering whatever solid masses obstructed the escape of lava and the accompanying gases, so that great heaps of ejected pieces of rock would naturally occur in the tufaceous breccias formed by the earliest eruptions. But when a passage had once been opened, and an habitual vent established, the materials thrown out would consist of liquid lava, which would take the form of sand and scoriæ, or of angular fragments of such solid lavas as may have choked up the vent.

Among the fragments which abound in the tufaceous breccias of Somma, none are more common than a saccharoid dolomite, supposed to have been derived from an ordinary limestone altered by heat and volcanic vapours.

Carbonate of lime enters into the composition of so many of the simple minerals found in Somma, that M. Mitscherlich, with much probability, ascribes their great variety to the action of the volcanic heat on subjacent masses of limestone.

Dikes of Somma.—The dikes seen in the great escarpment which Somma presents towards the modern cone of Vesuvius are very numerous. They are for the most part vertical, and traverse at right angles the beds of lava, scoriæ, volcanic breccia, and sand, of which the ancient cone is composed. 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 1 to 12 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 half way, and a few terminate at both ends, either in a point or abruptly. 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.[404-A] Examples are not rare of one dike cutting through another, and in one instance a shift or fault is seen at the point of intersection.

In some cases, however, the rents seem to have been filled laterally, when the walls of the crater had been broken by star-shaped cracks, as seen in the accompanying woodcut ([fig. 467.]). But the shape of these rents is an exception to the general rule; for 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 facts:—"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.

Fig. 467.

Dikes or veins at the Punta del Nasone on Somma. (Necker.[405-A])

"These channels, upon examination after an eruption, I have found 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."[405-B]

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 upwards, 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 texture of the Vesuvian dikes is different at the edges and in the middle. Towards the centre, observes M. Necker, the rock is larger grained, the component elements being in a far more crystalline state; while at the edge the lava is sometimes vitreous, and always finer grained. A thin parting band, approaching in its character to pitchstone, occasionally intervenes, on the contact of the vertical dike and intersected beds. M. Necker mentions one of these at the place called Primo Monte, in the Atrio del Cavallo; and when on Somma, in 1828, I saw three or four others in different parts of the great escarpment. These phenomena are in perfect harmony with the results of the experiments of Sir James Hall and Mr. Gregory Watt, which have shown that a glassy texture is the effect of sudden cooling, and that, on the contrary, a crystalline grain is produced where fused minerals are allowed to consolidate slowly and tranquilly under high pressure.

It is evident that the central portion of the lava in a fissure would, during consolidation, part with its heat more slowly than the sides, although the contrast of circumstances would not be so great as when we compare the lava at the bottom and at the surface of a current flowing in the open air. In this case the uppermost part, where it has been in contact with the atmosphere, and where refrigeration has been most rapid, is always found to consist of scoriform, vitreous, and porous lava; while at a greater depth the mass assumes a more lithoidal structure, and then becomes more and more stony as we descend, until at length we are able to recognize with a magnifying glass the simple minerals of which the rock is composed. On penetrating still deeper, we can detect the constituent parts by the naked eye, and in the Vesuvian currents distinct crystals of augite and leucite become apparent.

The same phenomenon, observes M. Necker, may readily be exhibited on a smaller scale, if we detach a piece of liquid lava from a moving current. The fragment cools instantly, and we find the surface covered with a vitreous coat; while the interior, although extremely fine-grained, has a more stony appearance.

It must, however, be observed, that although the lateral portions of the dikes are finer grained than the central, yet the vitreous parting layer before alluded to is rare in Vesuvius. This may, perhaps, be accounted for, as the above-mentioned author suggests, by the great heat which the walls of a fissure may acquire before the fluid mass begins to consolidate, in which case the lava, even at the sides, would cool very slowly. Some fissures, also, may be filled from above, as frequently happens in the volcanos of the Sandwich Islands, according to the observations of Mr. Dana; and in this case the refrigeration at the sides would be more rapid than when the melted matter flowed upwards from the volcanic foci, in an intensely heated state. Mr. Darwin informs me that in St. Helena almost every dike has a vitreous selvage.

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.

Newer Pliocene Period—Val di Noto.—I have already alluded (see [p. 150.]) to the igneous rocks which are associated with a great marine formation of limestone, sand, and marl, in the southern part of Sicily, as at Vizzini and other places. In this formation, which was shown to belong to the Newer Pliocene period, large beds of oysters and corals repose upon lava, and are unaltered at the point of contact. In other places we find dikes of igneous rock intersecting the fossiliferous beds, and converting the clays into siliceous schist, the laminæ being contorted and shivered into innumerable fragments at the junction, as near the town of Vizzini.

The volcanic formations of the Val di Noto usually consist of the most ordinary variety of basalt, with or without olivine. The rock is sometimes compact, often very vesicular. The vesicles are occasionally empty, both in dikes and currents, and are in some localities filled with calcareous spar, arragonite, and zeolites. The structure is, in some places, spheroidal; in others, though rarely, columnar. I found dikes of amygdaloid, wacké, and prismatic basalt, intersecting the limestone at the bottom of the hollow called Gozzo degli Martiri, below Melilli.

Fig. 468. Fig. 469. Ground-plan of dikes near Palagonia.

Dikes.—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 peperino 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. After the melted matter that filled the rent in [fig. 468.] had cooled down, it must have been fractured and shifted horizontally by a lateral movement.

In the second figure ([fig. 469.]), 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.