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in their aggregation; into the character of its fossils, which thus throw light on the geographical conditions of the period; and, finally, into its industrial value, as bearing on the wants and progress of civilisation. Adopting this method, we find the Carboniferous system composed in the main of sandstones, shales, fire-clays, ironstones, limestones, and coals, all many times alternating with each other, and in some districts attaining to a thickness of 12,000 or 14,000 feet. Of course, during the deposition of such a vast thickness of strata, and which necessarily implies the lapse of long ages, there must have been frequent changes in the relative levels of sea and land; hence some of these sediments were laid down in deep and others in shallow water, while the shallower beds were once more sunk to greater depths, and overlaid by newer sediments. In this way the Carboniferous system consists, in most regions, of several series of strata, and in the British Islands these are generally arranged and named as follows :—
1. Upper or true coal-measures.
2 Millstone grit or sandstone series.
3. Carboniferous or mountain limestone.
4. Lower coal-measures or carboniferous shales.
Although these several series have evidently been deposited in waters of various depths and under somewhat different geographical conditions—the lower being apparently more estuarine, the mountain limestone being more marine, the millstone grit more littoral, and the upper more terrestrial—still there is a great family resemblance, so to speak, between them, and, with the exception of the coal-seams, they are all strictly sedimentary, and bear in their structure and texture abundant evidence of the aqueous agencies concerned in their formation. In the sandstones and grits— often ripple-marked, rain-pitted, and worm-burrowed—we
trace the sands of open and exposed shores ; * in the shales and fire-clays and ironstones, the muddy deposits of deeper waters; and in the limestones, which also vary much in composition and character, the shell-beds, the coral-growths, and zoophyte drifts, both of the brackish estuary and of the outer ocean. Of course, among sediments so varied we may expect to find every degree of admixture—sandstones pure, quartzose, and compact; sandstones flaggy, laminated, and clayey; and sandstones calcareous and coaly. Shales so purely argillaceous as to be termed fire-clays, shales calcareous, shales bituminous, and shales so sandy as to pass into flaggy sandstones. So also it is with the limestones; some so pure as to contain scarcely a trace of earthy matters, and others so mingled with earthy impurities as to be altogether unfit for economical purposes. The ironstones, too, which were merely the ferruginous muds of the Carboniferous sea (chemically aggregated by the union of the carbonic acid given off by decaying vegetation, and the iron held in solution in the waters), appear as "clay-bands" or claycarbonates, as "black-bands" mingled more or less with coaly matter, or as stony impregnations too poor to be worked to advantage.
Eespecting the coals, which we separate from the strictly sedimentary beds, there are also many varieties both as to composition and structural peculiarity. Where the vegetable mass has evidently accumulated on the spot, as peatmoss, swamp-growth, and forest-growth, the seam is generally pure, and spread over a considerable area, with great regularity as to thickness and quality. Surprise has been frequently expressed at the uniform thickness which many coal-seams maintain over extensive areas. Growth of the vegetable mass in situ is no doubt the main cause, but we must not lose sight of the fact that, when in a semi-plastic state of bituminisation, the pressure from above would have a tendency to spread out the seam, and insensibly equalise its thickness. On the other hand, where interruption to this growth has taken place either from periodical inundations or otherwise, the seam contains layers of earthy matter, and is more or less impregnated throughout with such impurities. Again, where the seam has arisen from drifted vegetation, it is still less regular in thickness, and often so earthy and impure as to pass into a bituminous or coaly shale. These bituminous or coaly shales, now coming so largely into use in the Scottish coal-fields for the distillation of paraffin and paraffin oil, are indeed of very various origin and composition. Some of the richer sorts are merely compressed and mineralised vegetable muds that have arisen from long maceration and decay; others, during this long decay in shallow water, have got so largely mingled with the remains of minute crustaceans (cyprides, &c), as to be partly of animal origin; * and some again of the poorer sorts are little else than thick clayey silts, irregularly intermingled with vegetable and animal debris. In this way the various purities of coal can be readily accounted for, while difference of mineral aspect and quality may have arisen partly from the nature of the vegetation, partly from
* Several of the thick-bedded sandstones of the British coal-fields have evidently arisen, in the first instance, from zEolian or wind-blown sands, like those that form the "links" and "dunes" of the present day. Their whole internal arrangement points to this mode of aggregation, though they have, of course, been subsequently submerged and planed down on their upper surfaces by the action of water. Illustrative examples may be seen along the eastern shores of Fife, and especially between Crail and St Andrews.
* One of the most remarkable we have examined is the "Grey Shale" of West Calder, raised for the extensive paraffin works of Young & Company, and which derives its name from the colour imparted to it by the calcareous cases of these minute organisms. In some places the seam, which is upwards of two feet thick, is literally a mass of these remains.
the rapidity with which it was entombed, partly from the porous or retentive character of the imbedding strata, and partly also from the degree of mineralisation the respective seams have undergone. Hence the soft caking coals, which fuse together in burning; the hard, slaty, splint coals, which burn dry and open; the coarse cubic coals, which also burn open and leave much earthy ashes; the compact, lustrous, cannel coals, used chiefly in the making of gas, and other varieties well known in one or other of our British coalfields. There is no great difficulty, we repeat, in accounting for the varieties of palaeozoic coals, if we only make allowance for difference in the nature of the vegetation, in the modes of its accumulation, the length of time it was exposed to maceration and decay, the retentive character of the covering stratum, and the intensity of mineralisation which these different conditions would induce. Of course all this implies long ages of growth and decay, repeated emergence and submergence of the land, but in the main a gradual subsidence to permit that vast accumulation of sediments—sandstones, shales, ironstones, and limestones, to the thickness of many thousand feet—which constitute the bulk of the Carboniferous system.
Besides the strictly sedimentary strata, the coal-measures are also in some districts largely made up of igneous products, which intermingle with them as masses of basalt and greenstone, beds of trap-tuff, and other vulcanic discharges. Of course these discharges must have taken place in or near the seas of deposit,—now as overflows of lava, now as showers of ashes, and again as the mingled products of volcanic eruption. Just as insular and submarine volcanoes are at the present day mingling their eruptive matters with the sediments of the adjacent seas, so in the old coal period similar agencies were at work, and the results are now the interstratified greenstones and trap-tuffs, the bent and fractured strata, and the filling-up of the rents and fissures with igneous rock-matter. We say " interstratified greenstones," for some observers speak of "intrusive greenstones," as if, during the vulcanic paroxysms, such igneous rocks had been forced for miles in every direction between the separated strata! There are, no doubt, intrusive masses among the strata of every formation, but these are generally limited in extent, irregular in form, and bake or harden alike the immediately underlying and overlying beds. The interstratified greenstones, on the other hand, affect only the strata on which they rest, and to regard such widespread lava-like sheets as intrusive, is simply an absurdity. Indeed, almost every feature of the vulcanism of the period has been perfectly preserved to us, and the imagination has little difficulty in recalling the broad bays and estuaries of the Carboniferous ocean, studded with their cones and craters of eruption—here ejecting showers of dust and ashes, there discharging floods of molten rock-matter, and ever and again the whole shaken and fractured by earthquake energy and convulsion. Part of this vulcanism was contemporaneous with the deposition of the sediments, as proved by the interstratified greenstones and trap-tuffs, but part also happened long subsequent to the solidification of the strata, as seen in the faults and dykes of injected matter that intersect the whole thickness of the system; but whether contemporaneous or long subsequent, it forms one of the most remarkable features in the coalformation, engaging the closest study of the geologist, and exercising all the ingenuity of the miner and engineer. If the reader could picture to himself the district in which he resides fractured by earthquake convulsion—here a portion thrown many fathoms up, there another portion thrown many fathoms down, and the rents between filled with solidified lava— he would have before him precisely