CHAPTER XIX.

CARBONIFEROUS.

THE next great division of the Geological Record has received the name of Carboniferous, from the beds of coal (Latin, Carbo) which form one of its most conspicuous features. The rocks of which it consists reach sometimes a thickness of fully 20,000 feet, and contain the chronicle of a remarkable series of geographical changes which succeeded the Devonian period. They include limestones made up in great part of corals, crinoids, polyzoa, brachiopods, and other calcareous organisms which swarmed in the clearer parts of the sea; sandstones often full of coaly streaks and remains of terrestrial plants; dark shales not infrequently charged with vegetation, and containing nodules and seams of clay-ironstone; and seams of coal varying from less than an inch to several feet or yards in thickness, and generally resting on beds of fire-clay.

These various strata are disposed in such a way as to afford us clear evidence of the physical geography of large areas of the earth's surface during the Carboniferous period. The limestones attain a thickness of sometimes several thousand feet, with hardly any intermixture of sedimentary material. They consist partly of aggregated masses of corals and coralloid animals, which grew on the sea-floor somewhat after the manner of modern coral-reefs; partly of aggregated stems and joints of crinoids, which must have flourished in prodigious numbers on the bottom, mixed with fragments of other organisms, and aggregated into sheets of solid stone. This Carboniferous or Mountain Limestone stretches from the west of Ireland eastwards for a distance of 750 miles, across England, Wales, Belgium, and Rhineland into Westphalia. In the basin of the Meuse it is not less than 2500 feet thick, and in Derbyshire, where it attains its maximum development, it exceeds 6000 feet. Such an enormous accumulation of organic remains shows that, during the time of its deposition, a wide and clear sea extended over the centre of Europe. But as the limestone is traced northwards, it is found to diminish in thickness. Beds of sandstone, shale, and coal begin to make their appearance in it, and rapidly increase in importance, as they are followed away from the chief limestone area; while the limestone itself is at last reduced in Scotland to a few beds, each only a yard or two in thickness. From this change in the character of the rocks, the inference may be drawn that, while the sea extended from the west of Ireland eastwards into Westphalia, land lying to the north supplied sand, mud, and drifted plants, which, being scattered over the sea-floor, prevented the thick limestone from extending northwards. These detrital materials now form the masses of sandstone and shale that take the place of the limestone in the north of England and in Scotland. The occasional northward extension of a limestone bed full of marine organisms serves to mark a time when, for a longer or shorter interval, the water cleared, sand and mud ceased to be carried so far southward, and the corals, crinoids, and other limestone-building creatures were able to spread themselves farther over the sea-floor. But the thinness of such intercalated limestones also indicates that the intervals favourable for their formation were comparatively short, the sandy and muddy silt being once again borne southward from the land, killing off or driving away the limestone-builders and spreading new sheets of sand and mud over the site.

There can be no doubt that, while these changes were in progress, the whole wide area of deposition in Western and Central Europe was undergoing a gradual depression. The sea-bottom was sinking, but so slowly that the growth of limestone and the deposit of sediment probably on the whole kept pace with it. The actual depth of the water may not have varied greatly even during a subsidence of several thousand feet. That this must have been the case may be inferred from the structure of the limestone itself. We have seen that this rock sometimes exceeds 6000 feet in thickness. Had there been no subsidence of the seafloor during the accumulation of so thick a mass of organic debris, it is evident that the first beds of limestone must have been begun at a depth of at least 6000 feet below the surface of the sea, and that, by the gradual increase of calcareous matter, the sea was eventually filled up to that amount, if it was not filled up entirely. But we can hardly suppose that the same kinds of organisms could live at a depth of 6000 feet and also at or near the surface. We would expect to find the organic contents of the lower parts of the limestone entirely different from those in the upper parts. But though there are differences sufficient to admit of the limestone being separated into stages, each marked by its own distinctive assemblage of fossils, the general character or facies of the organisms remains so uniform and persistent throughout, as to make it quite certain that the conditions under which the creatures lived on the bottom and built up the limestone continued with but little change during the whole time when the 6000 feet of rock were being deposited. As this could not have been the case had there been a gulf of 6000 feet to fill up, we are led to conclude that the bottom slowly subsided until its original level, on which the limestone began to form, had sunk at least 6000 feet.

This conclusion is borne out by many other considerations. Thus the sedimentary strata that replace the limestone on its northern margin are also several thousand feet thick. But from bottom to top they abound with evidence of shallow-water conditions of deposition. Their repeated alternations of sandstone, grit (even conglomerate), and shale; the presence in them of constant currentbedding; the frequent occurrence of ripple-marked and sun-cracked surfaces; the preservation of abundant remains of terrestrial vegetation-some of it evidently in its position of growth-prove that the mass of sediment was not laid down in a deep hollow of the sea-bottom, but in shallow waters not far from the margin of the land.

But probably the most interesting evidence of longcontinued subsidence during the Carboniferous period is furnished by the history of the coal-seams. Coal is composed of compressed and mineralised vegetation. Each layer of coal is usually underlain by a bed of fire-clay, or at least of shale, through which roots and rootlets, descending from the under surface of the coal-seam, branch out freely. There can be no doubt that each bed of fire-clay is an old soil, and the seam of coal lying upon it represents the matted growth of vegetation which that soil supported. Hence, in each case of the association of a fire-clay and a coal-seam, we have distinct evidence of a terrestrial surface.

a

b

C

b

In many regions the Carboniferous system comprises a series of d sandstones, shales, and other strata, many thousands of feet in thickness, throughout which, on successive platforms, there lie hundreds of seams of d coal. If each of these seams marks

a

a

a former surface of terrestrial vegetation, how is this succession of buried land-surfaces to be accounted for? There is obviously but one solution of a the problem. The area over which

b

C

the coal-seams extend must have been a slowly sinking. During this subsidence, sand, mud, and silt were 6 transported from the neighbouring land, and in such quantity as to fill up the shallow waters. On the muddy flats thus formed, the vegetation of the flat marshy swamps spread

a

d

a

FIG. 140.-Section of part seaward. There may not improbof the Cape Breton coal- ably have been pauses in the downfield, showing a succession of buried trees and landward movement, during which the surfaces; (a) sandstones; maritime jungles and forests con(b) shales; (c) coal-seams; tinued to flourish and to form a thick (d) under-clays or soils. matted mass of vegetable matter. When the subsidence recommenced, this mass of living and dead vegetation was carried down beneath the water and