II. The Upper Silurian Series
The latest volcanic eruptions of Silurian time yet definitely known took place during the accumulation of the Wenlock and Ludlow rocks in the far west of Ireland. No satisfactory record of any contemporaneous phenomena of a like kind has yet been met with in any other Upper Silurian district in the British Isles, unless at Tortworth in Gloucestershire, as above described. So far as at present known, only one centre of activity has been preserved. It lies among the headlands of Kerry, where the land projects furthest west into the stormy Atlantic. The occurrence of volcanic rocks in this remote area and their geological horizon have been clearly indicated on the maps of the Geological Survey. More than thirty years, however, have elapsed since some of the mapping was done, and we must therefore be prepared to find it, more especially in its petrography, capable of modification and improvement now.
In the country known as the Dingle promontory, these traces of contemporaneous volcanic rocks are to be observed at various localities and on several horizons. To the east, near Anascaul, on the northern shore of Dingle Bay, some tuffs occur in what are believed to be Llandovery strata. But it is on the western coast, among the headlands and coves that lie to the north and south of Clogher Head, that the best sections are to be seen. The succession of the rocks in this locality was well worked out by Du Noyer, and the Memoir prepared by him, with the general introduction by Jukes, is an invaluable guide to the geologist who would explore this somewhat inaccessible region.[293] The most important correction that will require to be made in the work arises from a mistake as to the true nature of certain rocks which were described as pisolitic tuffs, but which are nodular felsites.
[293] Sheets 160 and 171 of the one-inch map, and Memoir on Sheets 160, 161, 171 and 172.
By far the most striking geological feature of this singularly interesting and impressive coast-line is to be found in the interstratification of lavas with bands of tuff among abundantly fossiliferous strata which, from their organic contents, are unmistakably of the age of the Wenlock group. These lavas occur in a number of sheets, separated from each other by tuffs and other fragmental deposits. They thus point to a series of eruptions over a sea-bottom that teemed with Upper Silurian life. They consist for the most part of remarkably fine typical nodular felsites. The nodules vary in dimensions from less than a pea to the size of a hen's egg. They are sometimes hollow and lined with quartz-crystals. They vary greatly in number, some parts being almost free from them and others entirely made up of them. The matrix, where a fresh fracture can be obtained, is horny in texture, and often exhibits an exceedingly beautiful and fine flow-structure. On weathered faces there may be seen thick parallel strips and lenticles of flow-structure like those of the Snowdon lavas. The upper portions of some of the sheets enclose fragments of foreign rocks. The microscopic examination of a few slices cut from these lavas shows them to be true felsites (rhyolites) composed of a microcrystalline aggregate of quartz and felspar, with layers and patches of cryptocrystalline matter, and only occasional porphyritic crystals of orthoclase and plagioclase.
The pyroclastic rocks associated with these lavas vary from exceedingly fine tuff to coarse agglomerate. Some of the finer tuffs contain pumiceous fragments and pieces of grey and red shale; they pass into fine ashy sandstones and shales, crowded with fossils, and into gravelly breccias made up of fragments of different volcanic rocks.
But the most extraordinary of these intercalated fragmental strata is a breccia or agglomerate, about 15 feet thick, which lies in a thick group of fossiliferous dull-yellow, ashy and ochreous sandstones. The stones of this bed consist chiefly of blocks of different felsites, varying up to three feet in length. Some of them show most perfect flow-structure; others are spongy and cellular, like lumps of pumice. The calcareous sandstone on the top of the breccia is crowded with fossils chiefly in the form of empty casts, and the same material, still full of brachiopods, crinoids, corals, etc., fills up the interstices among the blocks down to the bottom of the breccia, where similar fossiliferous strata underlie it.
Nowhere has the volcanic history of a portion of Palæozoic time been more clearly and eloquently recorded than in this remote line of cliffs swept by the gales of the Atlantic. We see that the ordinary sedimentation of Upper Silurian time was quietly proceeding, fine mud and sand being deposited, and enclosing the remains of the marine organisms that swarmed over the sea-bottom when volcanic eruptions began. First came discharges of fine dust and small stones, which sometimes fell so lightly as not seriously to disturb the fauna on the sea-floor, but at other times followed so rapidly and continuously as to mask the usual sediment and form sheets of tuff and volcanic gravel. Occasionally there would come more paroxysmal explosions, whereby large blocks of lava were hurled forth until they gathered in a thick layer over the bottom. But the life that teemed in the sea, though temporarily destroyed or driven out, soon returned. Corals, crinoids and shells found their way back again, and fine sediment carried their remains with it and filled up the crevices. The ejected volcanic blocks are thus enclosed in a highly fossiliferous matrix.
A succession of lava-streams, of which the strongly-nodular sheet of Clogher Head is the thickest and most conspicuous, mark the culmination of the volcanic energy, and show how at this late part of the Silurian period felsites that reproduce some of the most striking peculiarities of earlier time were once more poured out at the surface. A few more discharges of tuff and the outflow of a greenish flinty felsite brought this series of eruptions to an end, and closed in Britain the long and varied record of older Palæozoic volcanic activity.[294]
[294] As this sheet is passing through the press, the interesting paper by Messrs. S. H. Reynolds and C. J. Gardiner, "On the Kildare Inlier" has appeared (Quart. Journ. Geol. Soc. vol. lii. p. 587). These authors give petrographical details regarding the lavas, which they show to be andesites and basalts of Bala age, associated with highly fossiliferous tuffs.
MAP OF THE SILURIAN VOLCANIC DISTRICTS OF NORTH WALES
Reduced from the Maps of the Geological Survey.
TO ACCOMPANY SIR ARCHIBALD GEIKIE'S "ANCIENT VOLCANOES OF BRITAIN" Map II
Click on map to view larger sized.
BOOK V
THE VOLCANOES OF DEVONIAN AND OLD RED SANDSTONE TIME
CHAPTER XV
THE DEVONIAN VOLCANOES
Throughout the whole region of the British Isles, wherever the uppermost strata of the Silurian system can be seen to graduate into any later series of sedimentary deposits, they are found to pass up conformably into an enormous accumulation of red sandstones, marls, cornstones, and conglomerates, which have long been grouped together under the name of "Old Red Sandstone." In England and Wales, in Scotland and in Ireland, this upward succession is so well shown that at first British geologists were naturally disposed to believe it to represent the normal order of the geological record. When, however, Sedgwick and Murchison demonstrated that in the counties of Devon and Cornwall a very different group of strata contained an abundant assemblage of organic remains, including types which Lonsdale showed to be intermediate between those of the Silurian and the Carboniferous systems; when, moreover, this palæontological facies of the south-west of England, termed by its discoverers "Devonian," was found to be abundantly developed on the Continent, and to be there indeed the prevalent stratigraphical type of the formations intervening between Silurian and Carboniferous, geologists began to perceive that the Old Red Sandstone must be regarded as the record of peculiar local conditions of sedimentation, while the Devonian type was evidently the more usual development of the same geological period.
From the remote Shetland Isles, across the whole of Scotland and England, down to the northern shores of the Bristol Channel, the Old Red Sandstone maintains its general characters. Nowhere, indeed, are these characters more typically developed than in South Wales, where many thousands of feet of red sediments, almost entirely devoid of organic remains, emerge from under the escarpments of Carboniferous Limestone, and stretch into broad uplands until they are lost at the top of the Silurian system.
But when the geologist crosses the Bristol Channel to the opposite shores of North Devon, he encounters a remarkably different assemblage of rocks. It is true that he has not yet been able to detect there any equivalents of the uppermost Silurian strata of Glamorganshire, nor does he find any conspicuous band of Carboniferous Limestone, such as that which encircles the Welsh Coal-field. He is thus unable to start from a known definite horizon in the attempt to work out the order of succession, either in an upward or downward direction. Lithological characters likewise afford him no means of establishing any satisfactory parallelism. As he follows the Devonian strata, however, he finds them to disappear conformably under the Culm-measures, which, though strangely unlike the Carboniferous strata on the opposite coast, are yet proved by their fossils to belong to the Carboniferous system. Hence the Devonian type, like the Old Red Sandstone, is proved to be immediately anterior to, and to graduate into, the Carboniferous rocks.
There is no stratigraphical change in Britain so rapid and complete as that from the Old Red Sandstone on the one side of the Bristol Channel to the Devonian series on the other. No satisfactory explanation has yet been found for this sudden transformation, which still remains one of the unsolved problems in British geology.
As the observer follows the Devonian assemblage across the land to the southern coast-line, he is conscious that its general characters, both lithological and palæontological, depart more and more from the type of the Old Red Sandstone, and approach more closely to the common Devonian facies of the Continent. He is forced to admit that the Old Red Sandstone, notwithstanding its extensive development in Britain, must be regarded as an exceptional type of sedimentation, while the Devonian facies represents that which is most widely prevalent, not only in Europe, but generally over the world.
The broad estuary of the Bristol Channel unfortunately conceals from view the tract which lies between the typical Old Red Sandstone of Glamorganshire and the typical Devonian formations of Devonshire. Whether this intervening space of some fifteen miles was occupied by a physical barrier, which separated the respective areas of deposit of these two types, or the circumstances of sedimentation in the one region merged insensibly into those of the other, must remain matter for speculation.
The geographical conditions betokened by the Old Red Sandstone will be considered in the next chapter. There can be no doubt that those indicated by the Devonian system were marine. The organic remains so plentifully distributed through the argillaceous and arenaceous sediments of that system, and so crowded together in its limestones, were obviously denizens of the open sea. Yet the only tract of Britain over which this sea can be shown to have spread was the south of England. To the north of that belt, the site of Britain during Devonian time appears to have been partly land and partly wide water-basins in which the Old Red Sandstone was deposited.
In that half terrestrial half lacustrine territory that stretched northwards to beyond the Shetland Isles, many volcanoes were active, of which the chronicles will be described in later pages. The most southerly of these centres of eruption yet known was the district of the Cheviot Hills. Throughout the rest of England and Wales no trace of any contemporary volcanic action has been detected in the Old Red Sandstone. It is true that over most of that region rocks of this age have been concealed under younger formations. Yet throughout Wales, where the Old Red Sandstone attains so vast a thickness, and covers so wide an area, it has not yet yielded a vestige of any contemporaneous volcanic eruptions.
But over the sea-floor that covered the south of England, and stretched thence into the heart of Europe, abundant volcanoes have left behind them proofs of their activity. The first geologist who recognized these proofs and traced their extent on the ground appears to have been De la Beche, who, by his detailed maps and careful description of the igneous rocks of Devonshire, did so much to advance the study of ancient volcanic action. This great pioneer not only determined the former existence of Devonian volcanoes, but he was likewise the first to detect and map the volcanic rocks associated with the Carboniferous and "New Red Sandstone" formations of the same region. The broad outlines traced by him among the volcanic products of these three geological periods in the south-west of England still remain but little changed. Nor are they likely to be much improved until the ground is resurveyed on a larger and more accurate map, and with better petrographical equipment than were available in his day.
Not long after the observations of De la Beche came those of A. C. Godwin-Austen, who devoted much time to a sedulous exploration of the rocks of South Devon, and satisfied himself that contemporaneous volcanic sheets were intercalated among the limestones of that district. "The coral limestones," he says, "are in many places superincumbent on great sheets of volcanic materials, with which, in some instances, as at North Whilborough, they alternate." He pointed out that the interstratified volcanic rocks are of two periods, one Devonian and the other Carboniferous.[295]
[295] Trans. Geol. Soc. 2nd ser. vol. vi. (1842), pp. 465, 470, 473.
In his Geological Maps of Devon and Cornwall, which are to the present time those issued by the Geological Survey, De la Beche made no attempt to discriminate between the varieties of igneous rocks, save that the basic "greenstones" were distinguished from the acid bosses of granite and the elvans. But in his classic "Report" much more detail was inserted, showing that he clearly recognized the existence both of volcanic ashes and of lavas, as well as of intrusive sheets. At the outset of his account of the "Grauwacke," he remarks that the sedimentary deposits are accompanied with igneous products, "a portion of which may also be termed sedimentary, inasmuch as it would seem to have been deposited in beds among contemporaneous rocks of the former description by the agency of water, after having been ejected from fissures or craters in the shape of ashes and cinders, precisely as we may now expect would happen with the ashes and cinders ejected from volcanoes, particularly insular and littoral volcanoes, into the sea."[296] Again he speaks of "two kinds of trappean rocks having probably been erupted, one in the state of igneous fusion, and the other in that of ash, during the time that the mud, now forming slates, was deposited, the mixtures of volcanic and sedimentary materials being irregular from the irregular action of the respective causes which produced them; so that though the one may have been derived from igneous action, and the other from the ordinary abrasion of pre-existing solid rocks, they were geologically contemporaneous."[297] He recognized the origin of the amygdaloidal varieties of rock, and by dissolving out the calcite from their cells showed how close was their resemblance to modern pumice.[298]
[296] "Report on the Geology of Cornwall, Devon and West Somerset," Mem. Geol. Survey, 1839, p. 37.
[297] Op. cit. p. 57.
[298] Op. cit. pp. 57, 61.
Since these early researches many geologists have studied the igneous rocks of Devonshire. I would especially refer to the labours of Mr. Allport,[299] the late J. A. Phillips,[300] Mr. Rutley,[301] the late Mr. Champernowne,[302] Mr. W. A. E. Ussher,[303] Mr. Hobson,[304] and General M'Mahon.[305] Mr. Champernowne in particular has shown the abundance of volcanic material among the rocks of Devonshire, and the resemblance which in this respect they offer to the Devonian system of North Germany.
[299] Quart. Journ. Geol. Soc. xxxii. (1876), p. 418.
[300] Op. cit. xxxi. (1875) p. 325, xxxii. (1876) p. 155, xxxiv. (1878) p. 471.
[301] "Brent Tor," Mem. Geol. Surv. p. 18; Quart. Journ. Geol. Soc. lii. (1896), p. 66.
[302] See in particular his last paper "On the Ashprington Volcanic Series of South Devon," Quart. Journ. Geol. Soc. vol. xlv. (1889), p. 369.
[303] This geologist has spent many laborious years in the investigation of the geology of Devonshire, and has published numerous papers on the subject, in the Transactions of the Devonshire Association and of the Royal Cornwall Geological Society, in the Proceedings of the Somersetshire Archæological and Natural History Society, and of the Geologists' Association, in the Geological Magazine, and the Quarterly Journal of the Geological Society. Reference may especially be made to his Memoir in the last named journal, vol. xlvi. (1890), p. 487.
[304] Quart. Journ. Geol. Soc. xlviii. (1892), p. 496.
[305] Op. cit. xlix. (1893), p. 385.
Unfortunately the geological structure of the Palæozoic rocks of the South-west of England has been complicated to an amazing extent by plication and fracture, with concomitant cleavage and metamorphism. Hence it is a task of extreme difficulty to trace out with any certainty definite stratigraphical horizons, and to determine the range of contemporaneous volcanic action. Mr. Ussher has shown with what success this task may be accomplished when it is pursued on a basis of minute mapping, combined with a sedulous collection and determination of fossils.[306] But years must necessarily elapse before such detailed work is carried over the whole Devonian region, and probably not till then will the story of the volcanic history of the rocks be adequately made out.
[306] See Memoir cited in a previous note.
In the meantime, it has been established that while there is a singular absence of igneous rocks in North Devon, a strip of country extending from Newton Abbot and Torquay westwards by Plymouth across Cornwall to Penzance contains abundant records of volcanic action. It has not yet been possible to map out, among what were formerly all grouped together as "greenstones," the respective limits of the bedded lavas and the tuffs, to distinguish the true sills, and to fix on the position of the chief vents of eruption. So intense have been the compression and shearing of the rocks that solid sheets of diabase have been crushed into fissile schists, which can hardly be distinguished from tuffs. Moreover, owing perhaps in large measure to the mantle of red Permian (or Triassic) strata, which has been stripped off by denudation from large tracts of this region once overspread by it, the Devonian rocks have been deeply "raddled," or stained red. But probably one of the main sources of difficulty in studying the petrography of the area is to be found in the results of atmospheric weathering. Devonshire lies in that southern non-glaciated strip of England, where the rocks have been undergoing continuous decay since long before the Ice Age. No ploughshare of ice has there swept off the deep weathered crust, so as to leave hard surfaces of rock, fresh and bare, under a protecting sheet of boulder-clay. It is seldom that a really fresh piece of any igneous rock can be procured among the lanes and shallow pits of Devon, where alone, for the most part, the materials are exposed.
Much, therefore, remains to be done, both in the stratigraphy and petrography of the Devonian volcanic rocks of this country. To the late J. A. Phillips geology is indebted for the first detailed chemical and microscopical investigation of these rocks. He clearly showed the truly volcanic origin of many of the so-called "greenstones." He believed that certain "slaty blue elvans," which he found to have a composition identical with that of altered dolerites, might be highly metamorphosed tuffs, and that others might have been originally sheets of volcanic mud. After studying the chemical composition and minute structure of a large collection of "greenstones," he demonstrated that in all essential particulars they were dolerites, though somewhat altered from their original character.[307] Subsequently they were studied by Dr. Hatch, who found the fresher specimens generally to possess an ophitic structure, while some are granular, others porphyritic.[308]
[307] See especially Quart. Journ. Geol. Soc. vols. xxxii. and xxxiv.
[308] A few of the eruptive rocks of Devonshire have recently been studied by K. Busz. He finds most of his specimens (chiefly from the Torquay district) to be varieties of diabase, but describes a palæopicrite from Highweek near Newton Bushel, and a kersantite from South Brent on the south-east edge of Dartmoor (Neues Jahrb. 1896, p. 57).
Although the rocks have undergone so much crushing, solid cores of them, showing the original structure, may be obtained, also examples of the amygdaloidal, vesicular or slaggy character. They occur in sheets either singly or in groups, and appear generally to be regularly interstratified in the slates and grits. While some of these intercalations, especially the amygdaloidal sheets, may be true superficial lavas, it can hardly be doubted that others are sills, especially those which assume the crystalline structure and composition of gabbros, and show an entire absence of the vesicular structure. But no one has yet attempted to separate the two types from each other.
With these rocks are associated abundant diabase-tuffs (schalstein), frequently mingled with ordinary non-volcanic detrital matter, and shading off into the surrounding grits and slates. There is thus clear evidence of the outpouring of basic lavas and showers of ashes during the Devonian period in the south-west of England, under conditions analogous to those which characterized the deposition of the Devonian system in Nassau and the Harz.
The exact range of these eruptions in geological time has still to be ascertained. So far as at present determined, volcanic activity was not awakened during the accumulation of the Lower Devonian formations. It was not until the sporadic coral-reefs and shell-banks had grown up, which form the basement limestones of the Middle Devonian group, that the first eruptions took place. As Godwin-Austen, Champernowne and Mr. Ussher have shown, some of these reefs were overwhelmed with streams of lava or buried under showers of ashes. The volcanic discharges, however, were peculiarly local, probably from many scattered vents, and never on any great scale. Some districts remained little or not at all affected by them, so that the growth of limestone went on without interruption, or at least with no serious break. In other areas again the place of the limestone is taken by volcanic materials.
The chief epoch of this volcanic action, marked by the "Ashprington Volcanic Series," appears to have occurred about midway in the Middle Devonian period. But in certain districts it extended into Upper Devonian time. Intrusive sills of diabase may mark the later phases of the volcanic history. But the occurrence of such sills even in the Upper Devonian rocks, and the alteration of the strata in contact with them (spilosite), point to the continuance or renewal of subterranean disturbance even in the later Devonian ages, if not in subsequent geological time. That volcanic activity accompanied the deposition of the Carboniferous rocks of Devonshire has long been well known (see [Chapter xxix.]).
CHAPTER XVI
THE VOLCANOES OF THE OLD RED SANDSTONE
Geological Revolutions at the close of the Silurian Period—Physical Geography of the Old Red Sandstone—Old Lake-basins, their Flora and Fauna—Abundance of Volcanoes—History of Investigation in the Subject.
We now enter upon the consideration of the records of a notable era in the geological evolution of north-western Europe. Up to the close of the Silurian period the long history embodied in the rocks presents a constant succession of slowly sinking sea-floors. Wide tracts of ocean stretched over most of Europe, and across the shifting bottom, sand and mud, washed from lands that have long vanished, spread in an ever-accumulating pile. Now and then, some terrestrial movement of more than usual potency upraised this monotonous sea-bed, but the old conditions of ceaseless waste continued, and fresh sheets of detritus were thrown down upon the broken-up heaps of older sediment. All through the vast cycles of time denoted by these accumulations of strata, generations of sea-creatures came and went in long procession, leaving their relics amidst the ooze of the bottom. Genera and families, once abundant, gradually died out, and gave place to others, the onward march of life being slow but uninterrupted. Of the land of the time or of the plants and animals that lived on its surface, hardly anything is known. The chronicles that have come down to us are almost wholly records of the vicissitudes of the ocean-bed.
Over the centre and south of Europe, the marine conditions of Silurian time were prolonged, as we have seen, into the next period, when the Devonian formations were deposited. In that wide region, no marked break has been traced between either the sedimentation or the animal life of the Silurian and Devonian periods. But in the north-west of Europe a striking departure took place from the protracted monotony of marine conditions. By a series of terrestrial movements that affected the area lying to the north of the line of the Bristol Channel, and extended not only to the furthest limit of the British Isles, but probably as far as Norway, and perhaps even into northern Russia, the previous widespread conditions of marine sedimentation were entirely altered. Instead of the fine oceanic silts and sands with their abundant organic remains, and the thick limestones with their masses of coral and crowds of crinoids, there were now laid down, over these northern regions, vast piles of deep red sediment, from which traces of animal life are almost wholly absent. The shelving land against which these ferruginous sands and gravels gathered can still in part be recognized. As the observer follows its margin, notes the varying local peculiarities of its sediment, and detects, sometimes in great abundance, remains of the vegetation which clothed it, the conviction grows in his mind that the remarkable contrast between these deposits, known as the Old Red Sandstone, and those of the Silurian and Devonian systems is not to be accounted for by any mere rearrangement of the sea-bottom, or redistribution of the land that supplied that sea-bottom with sediment. It has long been the general belief among geologists that the subterranean movements which, over the greater part of Britain, brought the deposition of the Upper Silurian formations to a close, led to a total alteration of the geography of the region affected, that the sea-floor was elevated, and that, over the upraised tract, large lakes or inland seas were eventually formed, in which the peculiar sediments of the Old Red Sandstone were accumulated.
The records of this series of geographical changes are too fragmentary to enable us to follow, except in a very general way, the sequence of events in the transformation of the Silurian sea into the peculiar topographical conditions in which the Old Red Sandstone was laid down. While there was a widespread elevation of the sea-floor, and of such ridges of insular land as may have risen above sea-level, the upheaval appears to have been of a somewhat complicated kind, and to have been combined with many local subsidences. The area of disturbance was probably thrown into a series of parallel ridges and troughs, the former continuing to be pushed upward, while the latter tended to subside. The ridges thus became land surfaces, and their prolonged elevation may have more or less compensated for the denudation to which, on their emergence, they were necessarily exposed. The troughs, on the other hand, which sank down, may in many cases have subsided below the sea. But where the general upheaval of the crust was most pronounced, some of the depressions would be isolated above sea-level and become lake-basins in the terrestrial areas.
Of some of these water-basins the outlines can still in some measure be defined. The rocks that rose into hills around them, and from which their enormous accumulations of detritus were derived, still partially survive. We can explore these piles of sediment, and from them can form some idea of the condition of the water in the lakes, and the nature of the vegetation on the surrounding land. The frequent occurrence and exceeding coarseness of the conglomerates, which appear on many successive horizons throughout the deposits of these basins, probably indicate contemporaneous terrestrial disturbances. The same causes that led to the wrinkling of the crust into parallel ridges and troughs no doubt still continued in operation. From time to time the ridges, much worn down by prolonged denudation, were pushed upward, either by gradual uprise or by more rapid jerks. The troughs may in like manner have been still affected by their old tendency to subsidence. Hence, in spite of the effects of degradation and deposition, it is possible that the ridges may not, on the whole, have varied much in height, nor the basins in depth, during the time when thousands of feet were stripped off the land and strewn in detritus over the bottoms of the lakes.
Let us try to realize a little more definitely the general aspect of the region in which the Old Red Sandstone water-basins lay. As the axes of the folds into which the crust of the earth was thrown ran in a north-east and south-west direction, they gave this trend to the lakes and to the tracts of land that separated them. These intervening ridges must in some instances have been hilly or even mountainous. Thus, the Scottish Highlands rose between two of the lakes, and poured into them an abundant tribute of gravel, sand and silt. The terrestrial vegetation of the time has been partially preserved. The hills seem to have been clothed with conifers, while the lower slopes and swamps were green with sigillariæ, lepidodendra and calamites. One of the most characteristic plants was Psilophyton, of which large matted sheets were drifted across the lakes and entombed in the silt of the bottom. A grass-like vegetation, with long linear leaves, seems to have grown thickly in some of the shallows of the lakes.
Of the land animals we have still less knowledge than of the vegetation. Doubtless various forms of insect life flitted through the woodlands, though no relics of their forms have yet been recovered. But the remains of myriapods have been found in Forfarshire.[309] These early relics of the animal life of the land inhabited the woodlands, like our modern gally-worms, and were swept down into the lakes, together with large quantities of vegetation.
[309] Mr. B. N. Peach, Proceedings of Royal Physical Society of Edinburgh, vol. vii. (1882).
Some of the lakes, especially in the earlier part of their history, abounded in eurypterid crustacea. These animals inhabited the seas in Upper Silurian time, and appear to have been isolated in the water-basins of the Old Red Sandstone. Certain species of Pterygotus, a Silurian genus found also in the Lower Old Red Sandstone, reached a length of six feet. But the most abundant forms of animal life were fishes. These furnish additional evidence in favour of the lacustrine nature of the waters in which they lived. Such characteristically marine forms as the sharks and rays of the Silurian seas were replaced by genera of Acanthodians, Ostracoderms, Dipnoids, Teleostomes, Placoderms, and Palæoniscids, which abounded in the more northerly waters. The distinctive outward characters of many of these early vertebrates were their bony scales and plates. Some of them had their heads encased in an armature of bone, of large size and massive thickness. In several genera the bone was coated with a layer of glittering enamel. Even now, after the vast lapse of time since their day, the cuirasses and scale-armour of these fishes keep their bright sheen in the hardened sand and mud from which they are disinterred.
A difference is observable between the faunas of the different water-basins. Even where the same genus occurs in two adjacent areas, the species are often distinct. Two large lakes, separated by the tract of the Scottish Highlands, had each its own assemblage of fishes, not a single genus being common to the two basins. Such contrasts, whether the two lakes were geologically contemporaneous, or the northern arose later than the southern, undoubtedly indicate long-continued isolation and the gradual evolution of new forms under different conditions of environment.[310]
[310] In my memoir "On the Old Red Sandstone of Western Europe" (Trans. Roy. Soc. Edin. vol. xxviii. 1878), I argued for the probable geological contemporaneity of the conglomerates, sandstones and flagstones on either side of the Grampian chain, even although their organic contents were so unlike. The stratigraphical evidence favours this view. In each case a thick series of strata is covered unconformably by Upper Old Red Sandstone, containing Holoptychius nobilissimus and other fishes. The question cannot perhaps be definitely settled by the data available in Scotland. It is quite possible that the basin on the northern side of the Grampians, which I have termed "Lake Orcadie," came into existence after that on the southern side. But I do not think the differences in their respective faunas are to be accounted for simply by lapse of time and the gradual organic evolution in progress over one continuous region. The more the Old Red Sandstone is studied, the more local do its various fish-faunas appear to have been. These strongly-marked diversities appear to me rather to point to prolonged isolation of the basins from each other, as stated above. Dr. Traquair has drawn attention to the remarkable fact that, even in what appears to be one continuous series of strata of no great thickness forming the Upper Old Red Sandstone of the Moray Firth basin, the fishes found about Nairn are entirely different from those met with in the rest of the region.
Such, in brief, were the aspects of the physical geography of the time on the further consideration of which we are now to enter. The subterranean disturbances, so characteristic of the period, were accompanied by a display of volcanic activity more widespread, perhaps, than any which had yet taken place in the geological history of Britain. Nevertheless, it is worthy of remark that this manifestation of underground energy did not begin with the commencement of these displacements of the crust. The earliest eruptions only took place after the geography of the region had been completely changed; at least no trace of them is to be found in the earliest portions of the Old Red Sandstone. After the last lingering Silurian volcanoes in the west of Ireland had died out, a protracted quiescence of the subterranean fires ensued. In the latest ages of Silurian time there was not in Britain, so far as at present known, a single volcanic eruption. Not until after the inauguration of the Old Red Sandstone topography, when the lakes had taken shape and had begun to be filled with sediment from the surrounding hills, did a series of new volcanoes burst into activity over the northern half of Britain. Rising in the midst of the lakes in groups of separate cones, these vents poured out floods of lava, together with clouds of ashes and stones. Their sites, the history of their eruptions, and the piles of material ejected by them, can still be ascertained, and I shall now proceed to give some account of them.
The thick mass of sedimentary material known as the Old Red Sandstone, lying between the top of the Silurian and the base of the Carboniferous system, has been divided into two sections, which, however, are of unequal dimensions, and doubtless represent very unequal periods of time. The older series, or Lower Old Red Sandstone, is by far the more important and interesting in its extent, thickness, palæontological riches, and, what specially concerns us in the present inquiry, in its volcanic records. Wherever its true base can be seen, this series passes down conformably into Upper Silurian strata. It sometimes reaches a thickness of 15,000 and even 20,000 feet. There is generally a marked break between its highest visible strata and all younger formations. Even the upper division of the Old Red Sandstone rests unconformably upon the lower.[311] Such a hiatus undoubtedly points to a considerable lapse of geological time, and to the advent of important geographical changes that considerably modified the remarkable topography of the older part of the period.
[311] Quart. Journ. Geol. Soc. vol. xvi. (1860), p. 312. In Wales no break has actually been discovered between the two divisions of the Old Red Sandstone, though it is suspected to exist there also.
The younger division or Upper Old Red Sandstone passes upward conformably into the base of the Carboniferous system. Its red and yellow sandstones, conglomerates and breccias, covering much more restricted areas, and attaining a much less thickness than those of the lower division, indicate the diminution and gradual effacement of the lakes of the older time, and the eventual return of the sea to the tracts from which it had been so long excluded. So vast an interval elapsed between the time recorded in the deposits respectively of the two sections of the Old Red Sandstone that the characteristic forms of animal life in the earlier ages had entirely passed away, and their places had been taken by other types when the diminished lake-basins of the second period began to be filled up. Volcanic action also dwindled to such a degree that in contrast to the abundant vents of the older period, only one or two widely scattered groups of vents are known to have existed in the area of the British Isles during the later period, and these, after a feeble activity, gave way to a prolonged volcanic quiescence, which lasted until the earlier ages of the succeeding or Carboniferous period.
Although geologists are in the habit of grouping the Old Red Sandstone and the Devonian rocks as equivalent or homotaxial formations, deposited in distinct areas under considerably different conditions of sedimentation, the attempt to follow out the sequence of strata in Devonshire, and to trace some analogy between the Devonian succession and that of the Old Red Sandstone, presents many difficulties for which no obvious solution suggests itself. Into these problems it is not needful to enter further than was done in the last chapter. We may assume that not improbably some of the eruptions now to be described were coeval with those of Devonian time in the south-west of England, though we may hesitate to decide which of them should be brought into parallelism.
As we trace the shore-lines of the ancient basins of the Lower Old Red Sandstone, and walk over the shingle of their beaches, or as we examine the silt of their deeper gulfs, and exhume the remains of the plants that shaded their borders, and of the fishes that swarmed in their waters, we gradually learn that although the sediments which accumulated in some of these basins amount to many thousand feet in thickness; yet from bottom to top they abound in evidence of shallow-water conditions of deposit. The terrestrial disturbances above referred to continued for a vast interval, and while, as already suggested, the floors of the basins sank, and the intervening tracts were ridged up, as the results of one great movement of the earth's crust, the denudation of the surface of the land contributed to the basins such a constant influx of sediment as, on the whole, compensated for the gradual depression of their bottoms.
We need not suppose that these movements of subsidence and upheaval were uninterrupted and uniform. Indeed, the abundant coarse conglomerates, which play so prominent a part in the materials thrown into the basins, suggest that at various intervals during the prolonged sedimentation subterranean disturbances were specially vigorous. But the occurrence of strong unconformabilities among the deposits of the basins sets this question at rest, by proving that the terrestrial movements were so great as sometimes to break up the floor of a lake, and to place its older sediments on end, in which position they were covered up and deeply buried by the succeeding deposits.[312]
[312] An unconformability of this kind occurs between the south end of the Pentland Hills and Tinto in Lanarkshire, and another in Ayrshire.
It is not surprising to discover, among these evidences of great terrestrial disturbance, that eventually groups of volcanoes rose in long lines from the waters of most of the lakes, and threw out enormous quantities of lava and ashes over tracts hundreds of square miles in extent. So vast, indeed, were these discharges, across what is now the Midland Valley of Scotland, that the portions of sheets of lava and tuff visible at the surface form some of the most conspicuous ranges of hills in that district, stretching continuously for 40 or 50 miles and reaching heights of more than 2000 feet above the sea. Exposed in hundreds of ravines and escarpments, and dissected by the waves along both the eastern and western coasts of the country, these volcanic records may be studied with a fulness of detail which cannot be found among earlier Palæozoic formations.
It might have been supposed that a series of rocks so well displayed and so full of interest, would long ere this have been fully examined and described. But they can hardly be said to have yet received, as a whole, the attention they deserve. Without enumerating all the writers who, each in his own measure, have added to the sum of our knowledge of the subject, I may refer to the labours of Jameson,[313] Macknight[314] and Fleming,[315] among the observers who began the investigation. But of the early pioneers, by far the most important in regard to the igneous rocks of the Old Red Sandstone was Ami Boué. While attending the University of Edinburgh, where he took the degree of M.D. in the year 1816, he imbibed from Jameson a love of mineralogy and geognosy, and for several years spent his leisure time in personally visiting many parts of Scotland, in order to study the geological structure of the country. Probably in 1820 he published in French his now classic Essai.[316] The value of this work as an original contribution to the geology of the British Isles has probably never been adequately acknowledged. For this want of due recognition the author himself was no doubt in some measure to blame. He refers distinctly enough to various previous writers, notably to Jameson and Macculloch, but he mingles the results of his own personal examinations with theirs in such a way that it is hardly possible to ascertain what portions are the outcome of his own original observations. Less credit has accordingly been given to him than he could fairly have claimed for solid additions to the subjects of which he treated. In the later years of his life I had opportunities of learning personally from him how extensive had been his early peregrinations in Scotland, and how vivid were the recollections which, after the lapse of half a century, he still retained of them. Judged simply as a well-ordered summary of all the known facts regarding the geology of Scotland, his Essai must be regarded as a work of very great value. Especially important is his arrangement of the volcanic phenomena of the country, which stands far in advance of anything of the kind previously attempted. Under the head of the "Terrain Volcanique," he treats of the basaltic formations, distinguishing them as sheets (nappes, coulées) and dykes; and of the felspathic or trachytic formations, which he subdivides into phonolites, trachytes, porphyries (forming mountains and also sheets) and felspathic or trachytic dykes. In the details supplied under each of these sections he gives facts and deductions which were obviously the result of his own independent examination of the ground, and he likewise marshals the data accumulated by Jameson, Macculloch and others, in such a way as to present a more comprehensive and definite picture of the volcanic phenomena of Scotland than any previous writer had ventured to give.
[313] Memoirs of the Wernerian Society, vol. ii. (1811), pp. 178, 217, 618; vol. iii. (1820), p. 220, 225.
[314] Op. cit. vol. ii. pp. 123, 461.
[315] Op. cit. vol. i. (1808), p. 162; vol. ii. (1811), pp. 138, 339.
[316] Essai géologique sur l'Écosse (Paris; no date, but probably about 1820). He acknowledges his indebtedness to Jameson, whose demonstrations of the geology of the Edinburgh district he partly reproduced in his book. Jameson's early writings in the Wernerian Memoirs and in separate works were mere mineralogical or "geognostical" descriptions. His later lectures became more valuable but were never published, save indirectly in so far as they influenced the opinions of his pupils who published writings on the same subjects. See, for instance, Hay Cunningham's Geology of the Lothians, p. 59, footnote. Compare an article on Boué, Edinburgh Review for May 1823 (vol. xxxviii. p. 413).
The account which Boué wrote of the Old Red Sandstone and its associated igneous rocks marked the first great forward step in the investigation of this section of the geological record. He was the earliest observer to divide what he calls the "roches feldspathiques et trappéennes" into groups according to their geological position and mineralogical character, and to regard them as of igneous origin and of the age, or nearly of the age, of the red sandstone of Central Scotland.
Of later writers who have treated of the volcanic rocks of the Old Red Sandstone, my old friend Charles Maclaren deserves special recognition. His survey and description of the Pentland Hills embodied the first detailed and accurate investigation of any portion of these rocks, and his Geology of Fife and the Lothians may still be read with pleasure and instruction.[317] Boué had indicated roughly on the little sketch-map accompanying his Essai the chief bands of his felspathic and trappean rocks of the Old Red Sandstone, but their position and limits were more precisely defined in Macculloch's "Geological Map of Scotland," which was published in 1840, five years after the sudden and tragic death of its author. The observers who have more recently studied these rocks have been chiefly members of the Geological Survey, and to some of the more important results obtained by them I shall refer in the sequel.
[317] Geology of Fife and the Lothians, 1839. More detailed reference will be made in later pages to this classic.
For many years I have devoted much time to the investigation of the Old Red Sandstone and its volcanic rocks. In the year 1859 I ascertained the existence of the great hiatus between the Lower and Upper divisions of the system.[318] A first sketch of the volcanic history of the Old Red Sandstone was given by me in 1861,[319] which was subsequently enlarged and filled in with more detail in 1879.[320] But it was not until 1892 that I published a somewhat detailed outline of the whole subject, tracing the history of volcanic action during the period of the Old Red Sandstone, the distribution of the volcanoes, and the character of the materials erupted by them.[321] This outline I now proceed to amplify, filling in details that were necessarily omitted before, though there are still several districts regarding which information is scanty.
[318] "On the Old Red Sandstone of the South of Scotland," Quart. Journ. Geol. Soc. xvi. (1860), p. 312.
[319] "On the Chronology of the Trap-Rocks of Scotland," Trans. Roy. Soc. Edin. vol. xxii. (1861), p. 63.
[320] Article "Geology," in Ninth Edition of the Encyclopædia Britannica, vol. x. (1879), p. 343. Reprinted in my Text-Book of Geology, of which the first edition appeared in 1882.
[321] "Presidential Address to the Geological Society," Quart. Journ. Geol. Soc. vol. xlviii. (1892).
In arranging the treatment of the subject I shall divide the record into two main sections, the first and much the more important being devoted to the Lower and the second to the Upper Old Red Sandstone. In the first of these divisions it will be convenient to begin by taking note of the distribution of the various districts over which the geological evidence is spread. We may then proceed to consider the general character of the volcanic rocks and the manner in which they are arranged in the stratigraphy of the country, taking in consecutive order (1) the superficial lavas and tuffs; (2) the vents; (3) the dykes and sills. From these general considerations we may pass to the detailed history of events in each of the separate volcanic areas, and thus obtain, as far as the evidence at present permits, a broad view of the progress of volcanic action during the time of the Lower Old Red Sandstone in Britain.