BOOK VITHE CARBONIFEROUS VOLCANOES

CHAPTER XXIIITHE CARBONIFEROUS SYSTEM OF BRITAIN AND ITS VOLCANIC RECORDS

Geography and Scenery of the Carboniferous Period—Range of Volcanic Eruptions during that time—I. The Carboniferous Volcanoes of Scotland—Distribution, Arrangement and Local Characters of the Carboniferous System in Scotland—Sketch of the Work of previous Observers in this Subject.

Within the area of the British Isles, the geological record is comparatively full and continuous from the base of the Upper Old Red Sandstone to the top of the Coal-measures. We learn from it that the local basins of deposit in which the later portion of the Old Red Sandstone was accumulated sank steadily in a wide general subsidence, that allowed the clear sea of the Carboniferous Limestone ultimately to spread for some 700 miles from the west coast of Ireland into Westphalia. Over the centre of England this Carboniferous Mediterranean had a breadth of at least 150 miles, gradually shallowing northwards in the direction of land in Scotland and Northern Ireland. The gentle sinking of the floor of the basin continued until more than 6000 feet of sediment, chiefly composed of the remains of crinoids, corals and other marine organisms, had been piled up in the deeper parts. Traces of the southern margin of this sea, or at least of a long insular ridge that rose out of its waters, are to be seen in the protuberances of older rocks which appear at intervals from under the Coal-measures and later formations between the borders of Wales and the heart of Leicestershire, and of which the crags of Charnwood Forest are among the few peaks that still remain visible. To the south of this ridge, open sea extended far southward and westward over the site of the Mendip Hills and the uplands of South Wales.

The Carboniferous period, as chronicled by its sedimentary deposits, was a time of slow submergence and quiet sedimentation, terrestrial and marineconditions alternating along the margins of the sinking land, according as the rate of depression surpassed or fell short of that of the deposition of sediment. There is no trace of any general disturbance among the strata, such as would be marked by an important and widely extended unconformability. But many indications may be observed that the rate of subsidence did not continue uniform, if, indeed, the downward movement was not locally arrested, and even exchanged for a movement in the opposite direction. It is difficult, for instance, to believe the ancient ridge of the Midlands to have been so lofty that even the prolonged subsidence required for the accumulation of the whole Carboniferous system was insufficient to carry its highest crests below the level of the coal-jungles. More probably the depression reached its maximum along certain lines or bands running in a general north-easterly direction, the intervals between these lines sinking less, or possibly even undergoing some measure of uplift. One of the subsiding tracts, that of the wide lowlands of Central Scotland, was flanked on the south by a ridge which, while its north-eastern portion was buried under the Upper Old Red Sandstone and Lower Carboniferous rocks, remained above water towards the south-west, and does not appear to have been wholly submerged there even at the close of the Carboniferous period.

So abundant and varied are the sedimentary formations of Carboniferous time, and so fully have they preserved remains of the contemporary plants and animals, that it is not difficult to realise in some measure the general aspect of the scenery of the time, and the succession of changes which it underwent from the beginning to the end of the period. The land was green with a luxuriant if somewhat monotonous vegetation. Large pine trees flourished on the drier uplands. The lower grounds nourished dense groves of cycads or plants allied to them, which rose as slim trees twenty or thirty feet high, with long hard green leaves and catkins that grew into berries. The swamps and wetter lands bore a rank growth of various gigantic kinds of club-moss, equisetaceous reeds and ferns.

Nor was the hum of insect-life absent from these forests. Ancestral types of cockroaches, mayflies and beetles lived there. Scorpions swarmed along the margins of the shallow waters, for their remains, washed away with the decayed vegetation among which they harboured, are now found in abundance throughout many of the dark shales.

The waters were haunted by numerous kinds of fish quite distinct from those of the Old Red Sandstone. In the lagoons, shoals of small ganoids lived on the cyprids that peopled the bottom, and they were in turn preyed on by larger ganoids with massive armature of bone. Now and then a shark from the opener sea would find its way into these more inland waters. The highest types of animal life yet known to have existed at this time were various amphibians of the extinct order of Labyrinthodonts.

The open sea, too, teemed with life. Wide tracts of its floor supported a thick growth of crinoids whose jointed stems, piled over each other generation after generation, grew into masses of limestone many hundreds of feet in thickness. Corals of various kinds lived singly and in colonies,here and there even growing into reefs. Foraminifera, sponges, sea-urchins, brachiopods, gasteropods, lamellibranchs and cephalopods, in many genera and species, mingled their remains with the dead crinoids and corals to furnish materials for the wide and thick accumulation of Carboniferous Limestone.

Looking broadly at the history of the Carboniferous period, and bearing in mind the evidence of prolonged depression already referred to, we can recognize in it three great eras. During the first, the wide clear sea of the Carboniferous Limestone spread over the centre and south of Britain, interrupted here and there by islands that rose from long ridges whereby the sea-floor was divided into separate basins. Next came a time of lessened depression, when the sea-bottom was overspread with sand, mud and gravel, and was even in part silted up, as has been chronicled in the Millstone Grit. The third stage brings before us the jungles of the Coal-measures, when the former sea-floor became a series of shallow lagoons where, as in the mangrove-swamps of our own time, a terrestrial vegetation sprang up and mingled its remains with those of marine shells and fishes.

Such a state of balance among the geological forces as is indicated by the stratigraphy of the Carboniferous system would not prepare us for the discovery of the relics of any serious display of contemporary volcanic activity. And, indeed, throughout the Carboniferous rocks of Western Europe there is for the most part little trace of contemporaneous volcanic eruptions. Yet striking evidence exists that, along the western borders of the continental area, in France as well as over much of Britain, which had for so many previous geological ages been the theatre of subterranean activity, the older half of Carboniferous time witnessed an abundant, though less stupendous and prolonged, renewal of volcanic energy.

From the very commencement of the Carboniferous period to the epoch when the Coal-measures began to be accumulated, the area of the British Isles continued to be a scene of active volcanism. In the course of that prolonged interval of geological time the vents shifted their positions, and gradually grew less energetic, but there does not appear to have been any protracted section of the interval when the subterranean activity became everywhere entirely quiescent.

The geologist who traces, from older to younger formations, the progress of some persistent operation of nature, observes the evidence gradually to increase in amount and clearness as it is furnished by successively later parts of the record. He finds that the older rocks have generally been so dislocated and folded, and are often so widely covered by younger formations, that the evidence which they no doubt actually contain may be difficult to decipher, or may be altogether concealed from view. In following, for instance, the progress of volcanic action, he is impressed, as he passes from the older to the younger Palæozoic chronicles, by the striking contrast between the fulness and legibility of the Carboniferous records and the comparative meagreness and obscurity of those of the earlier periods. The Carboniferous rocks have undergone far less disturbance than the Cambrianand Silurian formations; while over wide tracts, where their volcanic chapters are fullest and most interesting, they lie at the surface, and can thus be subjected to the closest scrutiny. Hence the remains of the volcanic phenomena of the later Palæozoic periods present a curiously modern aspect, when contrasted with the fragmentary and antique look of those of older date.

The history of volcanic action during the Carboniferous period in Britain is almost wholly comprised in the records of the earlier half of that period, that is, during the long interval represented by the Carboniferous Limestone series and the Millstone Grit. It was chiefly in the northern part of the region that volcanic activity manifested itself. In Scotland there is the chronicle of a long succession of eruptions across the district of the central and southern counties, from the very beginning of Carboniferous time down to the epoch when the Coal-measures began to be accumulated. In England, on the other hand, the traces of Carboniferous volcanoes are confined within a limited range in the Carboniferous Limestone, while in Ireland they appear to be likewise restricted to the same lower division of the system. During the whole of the vast interval represented by the Coal-measures volcanic energy, so far as at present known, was entirely dormant over the region of the British Isles.

These general statements will be more clearly grasped from the accompanying table, which shows the various sections into which the Carboniferous system of Britain has been divided, and also, by black vertical lines, the range of volcanic intercalations in each of the three kingdoms.

Such being the general range in time of the Carboniferous volcanic phenomena, it may be convenient, in this preliminary survey, to take note of the general distribution of the volcanic districts over the British Isles, as in this way we may best realise the extent and grouping of the eruptions, which will then be considered in further detail (seeMap I.).

Not only were the Carboniferous volcanoes most abundant and persistent in Scotland, but they attained there a variety and development which give their remains an altogether exceptional interest in the study of volcanic geology. They were distributed over the wide central valley, from the south of Cantyre to beyond the mouth of the estuary of the Forth. On the southern side of the Silurian Uplands, they were likewise numerous and active. There is thus no considerable tract of Lower Carboniferous rocks in Scotland which does not furnish its evidence of contemporaneous volcanic action.

Although some portions of the Scottish Carboniferous igneous rocks run for a short distance into England, it is remarkable that, when these at last die out southwards, no other relics of contemporaneous volcanic energy take their place. Along the Pennine chain, from the Border into the heart of England, though natural sections are abundant, no trace of included volcanic rocks appears until we reach Derbyshire. The whole of that wide interval of 150 miles, so far as the present evidence goes, remained during Carboniferous time entirely free from any volcanic eruption. But from the picturesque country of the Peak southwards, the sea-floor of the Carboniferous Limestone, in what is now the heart of England, was dotted with vents whence the sheets of "toadstone" were ejected, which have so long been a familiar feature in English geology. Beyond this limited volcanic district the Carboniferous formations of the south-west of England remain, on the whole, devoid of contemporaneous volcanic intercalations, traces of Carboniferous volcanic action having been recognized only in West Somerset and Devonshire. In the Mendip district and in the ridges of limestone near Weston-super-Mare bands of cellular lava and tuff have been observed. To the west of Dartmoor, Brent Tor and some of the surrounding igneous masses may mark the positions of eruptive vents during an early part of the Carboniferous period.

At the south end of the Isle of Man relics remain of a group of vents among the Carboniferous limestones. Passing across to Ireland, where these limestones attain so great a thickness and cover so large a proportion of the surface of the island, we search in vain for any continuation of the abundant and varied volcanic phenomena of Central Scotland. So far as observation has yet gone, only two widely separated areas of Carboniferous volcanic rocks are known to occur in Ireland.[399]One of these shows that a little group of vents probably rose from the floor of the Carboniferous Limestone sea, near Philipstown, in King's County. The other lies far to the west in the Golden Vale of Limerick, where a more important series of vents poured out successive streams of lava with showers of ashes, from an early part of the Carboniferous period up to about the beginning of the time of the Coal-measures.

[399]The supposed Carboniferous volcanic rocks of Bearhaven on the coast of Cork are noticed onp. 49, vol. ii.

The total area within which the volcanic eruptions of Carboniferous time took place was thus less than that over which the volcanoes of theLower Old Red Sandstone were distributed, yet they were scattered across the larger part of the site of the British Isles. From the vents of Fife to those of Limerick is a distance of above 300 miles; from the latter eastward to those of Devonshire is an interval of 250 miles; while the space between the Devonshire volcanoes and those of Fife is about 400 miles. In this triangular space volcanic action manifested itself at each of the apices, to a slight extent along the centre of the eastern side, but with much the greatest vigour throughout the northern part of the area.

Since the volcanic phenomena of Carboniferous time are exhibited on a much more extensive scale in Scotland than in any other region of the world yet studied, it will be desirable to describe that area in considerable detail. The other tracts in Britain where volcanic rocks of the same age occur need not be so fully treated, except where they help to a better comprehension of the general volcanic history.

It is in the southern half of Scotland that the Carboniferous system is developed (Map IV.). A line drawn from Machrihanish Bay, near the Mull of Cantyre, north-eastward across Arran and Bute to the south end of Loch Lomond, and thence eastward by Bridge of Allan, Kinross and Cupar to St. Andrews Bay, forms the northern limit of this system. South of that line Carboniferous volcanic intercalations are to be met with in nearly every county across into the borders of Northumberland.

That we may follow intelligently the remarkably varied volcanic history of this region, it is desirable to begin by taking note of the nature and sequence of the sedimentary formations among which the volcanic rocks are intercalated, for these serve to bring before us the general conditions of the geography of the period. The subjoined table exhibits the subdivisions into which the Carboniferous system in Scotland has been grouped:—

Upper Red Sandstone group, nearly devoid of coal-seams.

Coal-bearing, white, yellow and grey sandstones, dark shales and ironstones (Upper Coal series).

Thick white and reddish sandstones and grits.

Sandstones, shales, fireclays, coal-seams, ironstones and three seams of marine limestone, of which the uppermost is known as the Castlecary seam, the second as the Calmy or Arden, and the lowest as the Index (Lower Coal series).

Bands of marine limestone intercalated among sandstones, shales and some coal-seams. A thick band of limestone lying at or near the bottom of the group, traceable all over Central Scotland, is known as the Hurlet or Main Limestone. Some higher and thinner seams are called Hosie's (seeFig. 155).

In the basin of the Firth of Forth, below the Hurlet Limestone, comes a varied series of white and yellow sandstones, black shales (oil-shales), cyprid shales and limestones (Burdiehouse), and occasional coal-seams (Houston), having a total depth of about 3000 feet. This local group abounds in fossil plants, entomostraca and ganoid fishes. It passes down into the Cement-stone group, which, however, is feebly developed in this district, unless it is partly represented by the sandstones, shales, limestones and coals just mentioned.

Cement-stone group consisting of red, blue and green marls and shales, red and grey sandstones, and thin bands of cement-stone: fossils scarce.

Reddish and grey sandstones and shales, with occasional plant-remains, passing down into the deep red (sometimes yellow) sandstones, red marls and cornstones of the Upper Old Red Sandstone.

[400]The Calciferous Sandstones are the stratigraphical equivalents of the Limestone Shale and lower portion of the Carboniferous Limestone of England.

From this table the gradual geographical evolution of the Carboniferous period in Scotland may be gleaned. We observe that at the beginning, the conditions under which the Old Red Sandstone had been accumulated still in part continued. The great lacustrine basins of the Lower Old Red Sandstone had indeed been effaced, and their sites were occupied by comparatively shallow areas of fresh or brackish water in which the Upper Old Red Sandstone was laid down. Their conglomerates and sandstones had been uplifted and fractured. Their vast ranges of volcanic material, after being deeply buried under sediment, had been once more laid bare, and extended as ridges of land, separating the pools and lagoons which they supplied with sand and silt. This singular topography had not been entirely effaced at the beginning of the Carboniferous period, for we find that many of the ridges which bounded the basins of the Upper Old Red Sandstone remained as land until they sank beneath the waters in which the earliest Carboniferous strata accumulated. Thus, while no trace of an unconformability has yet been detected at the top of the Upper Old Red Sandstone, there is often a strong overlap of the succeeding deposits. At the south end of the Pentland Hills, for example, the Upper Old Red Sandstone attains a thickness of 1000 feet, but only three miles further south it entirely disappears, together with all the overlying mass of Calciferous Sandstones, and the Carboniferous Limestone then rests directly on the Lower Old Red Sandstone. Again, at the north end of the same chain the upper division of the Old Red Sandstone dies out against the lower, which is eventually overlapped by the Calciferous Sandstones.

The change from the physical conditions of the Scottish Old Red Sandstone to those of the Carboniferous system was no doubt gradual and slow. The peculiar red sandy sediment continued to be laid down in basins that were apparently being gradually widened by access of water from the open sea. Yet it would seem that in Scotland these basins still for a long time continued saline or, from some other cause, unfavourable to life; for the red, blue and green shales or marls, and occasional impure limestones or cement-stones and gypseous layers, which were deposited in them, are in general unfossiliferous, though drifted plants from the neighbouring land are hereand there common enough. The sediments of these early Carboniferous waters are met with all over the southern half of Scotland, but in very unequal development, and constitute what is known as the "Cement-stone Group."

It was while these strata were in course of deposition that the earliest Carboniferous volcanoes broke into eruption. In some localities a thickness of several hundred feet of the Cement-stone group underlies the lowest lavas. In other places the lavas occur in and rest on the Upper Old Red Sandstone and have the Cement-stone group wholly above them; while in yet other districts the volcanic rocks seem entirely to take the place of that group. So vigorous was the earliest display of volcanic action in Carboniferous times that from the borders of Northumberland to the uplands of Galloway, and from the slopes of the Lammermuirs to Stirlingshire and thence across the estuary of the Clyde to Cantyre, innumerable vents were opened and large bodies of lava and ashes were ejected.

The Cement-stone group, save where succeeded by volcanic intercalations, passes up conformably into the lowest crinoidal limestones of the Carboniferous Limestone series. In the basin of the Firth of Forth, however, the cement-stones, feebly represented there, are overlain by a remarkable assemblage of white sandstones, black carbonaceous shales, or "oil-shales," cyprid limestones, occasional marine limestones and thin seams of coal, the whole having a thickness of more than 3000 feet. These strata, unlike the typical Cement-stone group, abound in fossils both vegetable and animal. They prove that, over the area of the Forth, the insalubrious basins wherein the red and green sediments of the Cement-stone group were laid down, gave place to opener and clearer water with occasional access of the sea. The peculiar lagoon-conditions which favoured the formation of coal were thus developed in Central Scotland earlier than elsewhere in Britain. We shall see in later pages that these conditions were accompanied by a fresh outbreak of volcanic activity, in a phase less vigorous but more enduring and extensive than that of the first Carboniferous eruptions.

The Carboniferous Limestone sea over the site of the southern half of Scotland appears never to have reached the depth which it attained in England and Ireland. To the north of it lay the land from which large quantities of sand and mud were carried into it, as shown by the deep accumulations of sandstone and shale, which far surpass in thickness the few comparatively thin marine limestones intercalated in them. There is thus a striking contrast between the thick masses of limestone in central and south-western England and their dwindled representatives in the north. Another marked difference between the Scottish and English developments of this formation is to be noticed in the abundant proof that the comparatively shallow waters of the northern basin were plentifully dotted over with active volcanoes. The eruptions were especially vigorous and prolonged in the basin of the Firth of Forth. They continued at intervals, even after the peculiar geographical conditions of the Carboniferous Limestone had ceased. But they had died out by the time of the beginning of the Coal-measures.

Owing to the number and variety of the natural sections, the Carboniferous volcanic rocks of Scotland have been the subject of numerous observations and descriptions, from the early days of geology down to the present time. The mere enumeration of the titles of the various publications regarding them would make a long list. These rocks formed the subject of some of Hutton's early observations, and furnished him with facts from which he established the igneous origin of "whinstone."[401]They supplied Playfair with numerous apt illustrations in support of Hutton's views, and he seems to have made himself thoroughly familiar with them.[402]In the hands of Sir James Hall they became the groundwork of those remarkable experiments on the fusion of whinstone which may be said to have laid the foundation of experimental geology.[403]In the controversies of the Neptunian and Plutonian schools these rocks were frequently appealed to by each side in confirmation of its dogmas. The appointment in 1804 of Jameson to the Chair of Natural History in the Edinburgh University gave increased impetus to the study of the igneous rocks of Scotland. Though he did not himself publish much regarding them, we know that he was constantly in the habit of conducting his class to the hills, ravines and quarries around Edinburgh, and that the views which he taught were imbibed and extended by his pupils.[404]Among the early writers the names of Allan,[405]Townson,[406]Lord Greenock,[407]and Ami Boué,[408]deserve especial mention.

[401]Hutton'sTheory of the Earth, vol. i. p. 155et seq.[402]Playfair'sIllustrations of the Huttonian Theory, § 255et seq.[403]Trans. Roy. Soc. Edin.(1805), vol. v. p. 43.[404]Mem. Wern. Soc.ii. 178, 618; iii. 25;Edin. Phil. Journ.i. 138, 352; xv. 386.[405]Trans. Roy. Soc. Edin.(1811), vi. p. 405.[406]Tracts and Observations in Natural History and Physiology, 8vo, Lond. 1799.[407]Trans. Roy. Soc. Edin.(1833), xiii. pp. 39, 107.[408]Essai géologique sur l'Écosse.Paris; no date, probably 1820.

[401]Hutton'sTheory of the Earth, vol. i. p. 155et seq.

[402]Playfair'sIllustrations of the Huttonian Theory, § 255et seq.

[403]Trans. Roy. Soc. Edin.(1805), vol. v. p. 43.

[404]Mem. Wern. Soc.ii. 178, 618; iii. 25;Edin. Phil. Journ.i. 138, 352; xv. 386.

[405]Trans. Roy. Soc. Edin.(1811), vi. p. 405.

[406]Tracts and Observations in Natural History and Physiology, 8vo, Lond. 1799.

[407]Trans. Roy. Soc. Edin.(1833), xiii. pp. 39, 107.

[408]Essai géologique sur l'Écosse.Paris; no date, probably 1820.

The first broad general sketch of the Carboniferous igneous rocks of a large district of the country was that given by Hay Cunningham in his valuable essay on the geology of the Lothians.[409]He separated them into two series, the Felspathic, including "porphyry" and "clinkstone," and the Augitic or Trap rocks. To these he added "Trap-tufa," which he considered to be identical in origin with modern volcanic tuff. It was the eruptive character of the igneous rocks on which he specially dwelt, showing by numerous sections the effects which the protrusion of the molten masses have had upon the surrounding rocks. He did not attempt to separate the intrusive from the interstratified sheets, nor to form a chronological arrangement of the whole.

[409]Mem. Wern. Soc.vii. p. 1. Published separately, 1838.

Still more important was the sketch given by Maclaren, in his classicGeology of Fife and the Lothians.[410]This author clearly recognized that many of the igneous rocks were thrown out contemporaneously with the strata among which they now lie. He constantly sought for analogies among modern volcanic phenomena, and presented the Carboniferous igneous rocks of the Lothians not as so many petrographical varieties, but as monumentsof different phases of volcanic action previous to the formation of the Coal-measures. His detailed descriptions of Arthur Seat and the rocks immediately around Edinburgh, which alone the work was originally intended to embrace, may be cited as models of exact and luminous research. The portions referring to the rest of the basin of the Forth did not profess to be more than a mere sketch of the subject.

[410]Small 8vo, Edin. 1838, first partly published as articles in theScotsmannewspaper. A second edition, which was little more than a reprint of the first, appeared in 1866.

Various papers of more local interest, to some of which allusion will be made in the sequel, appeared during the next quarter of a century. But no systematic study of the volcanic phenomena of any part of Scotland was resumed until the extension in 1854 of the Geological Survey to the north of the Tweed by A. C. Ramsay. The volcanic rocks of the Lothians and Fife were mapped by Mr. H. H. Howell and myself. The maps of that district began to be published in the year 1859, and the Memoirs two years later. In 1861, in a chronological grouping of the whole of the volcanic phenomena of Scotland, I gave an outline of the Carboniferous eruptions.[411]By degrees the detailed mapping of the Geological Survey was pushed across the whole of the rest of the south of Scotland, and the Carboniferous volcanic rocks of each area were then for the first time carefully traced and assigned to their various stratigraphical horizons. In the following pages reference will be given to the more important features of the Survey maps and Memoirs. In the year 1879, availing myself of the large amount of information which my own traverses and the work of the Survey had enabled me to acquire, I published a Memoir on the geology and petrography of the volcanic rocks of the basin of the Firth of Forth;[412]and lastly, in my Presidential Address to the Geological Society in 1892, I gave a summary of all that had then been ascertained on the subject of the volcanic rocks of Carboniferous time in the British Isles.[413]

[411]Trans. Roy. Soc. Edin.vol. xxii.[412]Ibid.vol. xxix. (1879), p. 437.[413]Quart. Journ. Geol. Soc.xlviii. (1892), p. 104. This summary, with additional details and illustrations, is embodied in the text.

[411]Trans. Roy. Soc. Edin.vol. xxii.

[412]Ibid.vol. xxix. (1879), p. 437.

[413]Quart. Journ. Geol. Soc.xlviii. (1892), p. 104. This summary, with additional details and illustrations, is embodied in the text.

Two well-marked types of volcanic accumulations are recognizable in the British Isles, which may be conveniently termed Plateaux and Puys.

1.Plateaux.—In this type, the volcanic materials were discharged over wide tracts of country, so that they now form broad tablelands or ranges of hills, reaching sometimes an extent of many hundreds of square miles and a thickness of more than 1000 feet. Plateaux of this character occur within the British area only in Scotland, where they are the predominant phase of volcanic intercalations in the Carboniferous system.

It is noteworthy that the Carboniferous plateaux appeared during a well-marked interval of geological time. The earliest examples of them date from the close of the Upper Old Red Sandstone. They were all in vigorous eruption during the time of the Calciferous Sandstones, but in no case did they survive into that of the Hurlet and later limestones. They are thus eminently characteristic of the earliest portion of the Carboniferous period.

2.Puys.—In this type, the ejections were often confined to the discharge of a small amount of fragmentary materials from a single solitaryvent, and even where the vents were more numerous and the outpourings of lava and showers of ash more copious, the ejected material usually covered only a small area round the centres of eruption. Occasionally streams of basic lava and accumulations of tuff were piled up into long ridges. Volcanoes of this character were specially abundant in the basin of the Firth of Forth, and more sparingly in Ayrshire and Roxburghshire. They form the persistent type throughout the rest of the British Isles.

The Puys also occupy a well-defined stratigraphical position. They did not begin until some of the volcanic plateaux had become extinct. From the top of the Cement-stone group up into the Carboniferous Limestone series, their lavas and tuffs are met with on many platforms, but none occur above that series save in Ayrshire, where some of the eruptions appear to have been as late as about the beginning of the Coal-measures.

Arranged in tabular form the stratigraphical and geographical distribution of the two great volcanic types of the Carboniferous system in Scotland will be more easily followed. I have therefore drawn up the accompanying scheme:—

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