The subsidence of one portion of the earth must either result from its contraction or condensation, or from volcanic eruptions : the latter cause we must presume to have been the prevailing one during this early period; and the existence of long lines of constantly active or intermittent vol canoes on either hand, to the east and west, gives ample evidence of the fact.
But whether the result was the effect of one or the other of those causes, the formation of the superincumbent strata would be much the same. In the one case it would be slow and slaty, or the sediment would be fine and argillaceous; while in the other and later it would accumulate rapidly and be coarse and arenaceous. The first, resulting from the debris of older strata brought in by rivers and floods in a comparatively quiet manner, forms slates, shales, and limestones ; and the second, resulting from violent commotions, would not only cause the accumulation of strata from the sources of the first, but would receive vast acquisitions from the floating ashes, sand, and dust of not very distant volcanoes.* The formation of the upper series of coal measures was evidently more slow than the lower series, as the coarse arenaceous rocks are but few in number, while the slates and shales are profusely abundant. This type, however, of the general character of the great Appalachian region has an exception in the isolated anthracite coal basins of Pennsylvania, since the coarse strata here exist to the top of the coal measures : still, even here we find the material finer in the late than the early formations. The anthra cite basins existed in the vicinity of active or intermittent volcanoes, and derived most of the rocky strata from these sources.
In this connection we may notice several singular facts, which somewhat impair the best theories of fossil coal vegetation. Of all the millions of specimens appertaining to a thousand or more species of fossil flora found in the coal measures, but few have been found in the coal. They are
generally found in the clays, slates, shales, and sandstones above or below the coal ; consequently, as those measures were formed from the debris of the land, the flora found in them would be of terrestrial growth. But it does not follow that the coal must be of the same vegetation, since the theory of cliV't cannot be entertained in the consideration of its production. The only species of vegetation found in such close connection with the coal as to warrant an assumption of their being the coal-producing flora, are the deep, water-rooted Lepidodendra, Calamites, &c., and their numerous species, growing up through the water and spreading on its sur thee, as before described.
But of all the fossil remains of the terrestrial flora we do not find one that has been changed to coal ; and the same may be said of such marine vegetation as exists in the strata not in immediate contact with the coal. All the numerous fossil remains of the ancient arborescent vegetation are solidified into sandstones or limestones, or partake of the character of the slates and shales in which they are found. A few of the larger trunks of the Sigillarice, Calamites, Sic., have been found coated with a thin film of coal ; but their bodies are always silicious or calcareous.
That these trees originally contained both carbon and bitumen there can be no doubt ; and we are led to infer that these constituents of all vege tation must have been expelled either by pressure or heat; and if expelled in the shape of oil or bitumen, the results might produce coal ; and thus, indirectly, the whole of the ancient vegetation would be economized in the formation of our mineral fuel; while the direct conversion of all the vege tation of the Carboniferous era into coal was an impossibility.