VERTICAL SECTION BROAD MOUNTAIN BASIN.
Figure 97 represents a section through the centre of the Broad Moun tain or New Boston basin. We gave the thickness of the strata in a former page ; the total depth is 860 feet; and this depth we assume as a medium basin, and the most favorable for the formation of coal, not merely because this basin presents a maximum thick ness of coal-beds, but because all other basins of the same dimensions present the same favorable condi tions. We may instance the eastern extremity of the Southern coal-field on the Lehigh, particularly the Summit Hill basin, most of the detached Lehigh basins, the Mahanoy basins, and the Mine Hill basins. Others might be brought in evidence; but these are enough. The original depth of those basins can only be found from the number of seams they contain, and all those in which the upper red-ash coal seams do not exist must have been originally comparatively shallow basins. Their present depth is not a safe criterion: the depth through the measures does not offer conclusive evidence of original depth, since the top of the water may have been many feet above the upper measures.
In the Pottsville basin the water-line may have been near the summit of the Sharp Mountain ; and, though we have good reason to say that range was not originally as elevated as at present, it must still have been as high as the Broad Mountain at that time, since the water of the anthracite coal-fields must have had a general level, as the identical seams must have had a cotemporaneous existence. There fore the Pottsville basin may have been more than 1000 feet deeper than the Broad Mountain,—or the difference between the level of the Mammoth and the Sandrock,—since the intervening seams were formed after the Broad Mountain basin had been filled.
When or how those basins were depressed or elevated, no one can say. The contracting forces may have been gradually doing their work during the formation of coal, as we should infer; but their most violent efforts were exerted subsequently, as the crushed and inverted strata testify.
We offered a theory on the formation of coal in the early chapters of this work, and we think the formations of this basin offer abundant proof that the commonly accepted arborescent vegetation, or the marsh and bog theories, are incorrect and erroneous, if it does not prove ours to be correct. It would be far beyond the probability, if not the possibility, for the slow growth of bog formation to form the grand column of Coal represented in figure 98, or the Mammoth of this basin. According to the slow growth of bogs, as de veloped within the last five hundred years in Ireland and elsewhere, it would require a million years to form the magnificent bed of coal here illustrated; and according to the theory of arborescent and marsh vegetation, as advocated by Professor Rogers and others, it would not only require an equal lapse of time to produce a sufficient growth, but it would be an impossibility for any land-growth of vegetation to form a solid bench or vein of coal equal to the 12-foot bench. of this great bed. It would require a thousand feet of massed and packed vegetation to form this 12 feet of coal; and we need not say that such a growth could not be sustained on the land or above water.
A vegetation capable of producing such a vast mass of coal must not only have taken root in the depth of the basin, and grown up through it, but the successive growths of years must have accumulated and settled in it. We have no doubt that the increase of growth was on the surface of the water, but that yearly in crease receded beneath the water and formed the mass, which could not increase in any other manner from simple vegetation. But, as before stated, we do not credit the theory of the formation of coal from vegeta tion alone. The carbon oils, which at that day must • have been in excess, particularly in the coal forma tions, were perhaps the most active agent in preserving the vegetation and in increasing and cementing its bulk, as set forth at large in the early chapters.