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:—
| Coal-measures. | ||
 | Upper Red Sandstone group, nearly devoid of coal-seams. | |
Coal-bearing, white, yellow and grey sandstones, dark shales and ironstones (Upper Coal series). | ||
| Millstone Grit. | ||
 | Thick white and reddish sandstones and grits. | |
| Carboniferous Limestone series. | ||
 | 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 (see [Fig. 155]). | ||
| Calciferous Sandstones.[400] | ||
 | 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 here and 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's Theory of the Earth, vol. i. p. 155 et seq.