All our known coal-fields have a basin-shape, while the angles of the • strata dip to a common centre. If non-conforming coal strata exist, it is due to local and subsequent causes,—denudation and unequal movement of the superincumbent strata.
Consequently, all our coal-fields must have arisen out of basins, lakes, or seas, and, of course, from or in the water. That the vegetation grew entirely in the water, however, is not evident ; the contrary would seem to be the rule, if we accepted the fossil flora of the coal-slates and rocks as evidence that they also formed coal ; since nearly all the species of fossil flora found in those rocks are of land origin. But in close proximity to the coal we find the remains of a gigantic vegetation, that evidently sprung from the deep mud of those shallow seas. In the coal itself we find no positive existence of vegetation,—no trace of leaf or stem. But the vast forms of the Sigillaria and its enormous base of roots, the Stigmaria, are profusely abundant in the fire-clays of all our coal-beds, and particularly in the lower series. Here also the towering Lepidodendron and the gigantic Calamite had their existence. They and their species form the chief re mains in the strata of the lower veins, and exist exclusively in the beds of fire-clay supporting the coal, while their stems and leaves and branches are found in abundance through the slates which immediately overlie the coal. We are therefore to infer that they formed the mass of our lower beds at least, if we are to concede that a pure vegetation formed our exist ing beds of coal, which is doubtful, as we shall show.
Our coal-beds are of vast extent, and we find some of the upper seams existing over 14,000 square miles, without a single break or discontinua tion of strata, while, in all probability, the lower seams will be found to underlie the entire Alleghany coal-field, without a positive discontinuity except where cut off by the streams and such local and subsequent causes.
Therefore, unless we admit that most of our coal vegetation took root deep in the water, we must assume that all the vast area of over 70,000 square miles was level and one vast marsh, which is in contradiction to all fact and in violence to every natural process that we can conceive; and those who advocate this and claim an exclusive arborescent or land vegetation as producing coal must summon to their aid earthquakes and prodigies of Nature for every coal-seam existing, in order to reconcile their theory with the facts.
We have no reasoeto believe the sea which gave growth to the ancient flora was deep : on the contrary, it was a shallow sea, and from the mud of its bottom—our present fire-clays—sprung up the long, grape-vine-like Sigillaria, and Lepidodendra, and the gigantic Calamites.
In that tepid water they soon reached the surface, and spread out, a vast Sheet of luxuriant vegetation. The waters were impregnated with hydro carbon, and its surface, loaded with the heavy carbonic acid gas which gave such prodigious energy to the vegetable growth, came in contact with its vapors. We cannot imagine a more favorable condition for an unlimited growth, since no change of atmosphere, no rigid winter, came to check its vast increase, and the superabundance of carbon prevented its decay. Instead of 1000 years being required to form a coal-bed (3) three feet thick, it did not require as many months,—perhaps not as many weeks.
This vegetation was not solid wood, such as we now find in our forests, but rank fronds or sappy vines, full of carbon or resinous and oily juices, containing, in fact, more of the solid matter of coal than our most solid trees of to-day.
There could scarcely be a limit to the size of the coal-bed that might thus be formed but its own weight or a subsidence of the basin; and thus we can readily account for the benches in our large coal-beds, none of which are over three or four feet thick. The immense amount of vegeta tion gradually sinking under water, as it formed, and becoming heavy with water, carbon, and bitumen, would eventually sink to the bottom, but immediately the vines shoot up again to the surface, and the process is repeated, each time forming either a thin slate or a band of imperfect coal, as circumstances might determine. Sometimes even the surface of this mass of vegetation might be covered with a layer of ashes, soot, &c., from the discharge of distant volcanoes,* and produce the same result. It might be interesting to calculate the immense amount of vegetation it would require to form a vein of coal equal to the Mammoth.