THE GREAT CONGLOMERATE.
The formation next in order of deposit in the waters of the ancient sea is the great Conglomerate, which is the floor and base of the coal measures. This formation, like all the strata of the Appalachian basin, excepting the lime, is in vast preponderance in the east, particularly in the vicinity of the anthracite coal-fields. Here it is over 1000 feet in thickness along its lower border, but thins off rapidly in a western direction. On the western outcrop of the Wyoming coal-field it is about 100 feet thick; and on the eastern escarpment of the Alleghany, in the same direction west, it is not over 30 feet; while throughout the Western coal-fields it ranges from 10 to 15 feet.
There are several theories regarding this extraordinary formation. It is composed of a conglomeration of pebbles, chiefly of a white quartzose character, but is made up otherwise of a great variety of rocks in pebble shape, cemented by arenaceous and argillaceous material of all ages except the primitive. Prof. Rogers thinks it the drift or wash of some ancient current, sweeping with great force from the southeast, or "towards a point a little west of north," evidently the ocean-tides.
There are circumstances which substantially support this conclusion, such as the general water-worn character of the diversified and imbedded pebbles, the intervening strata of shale and sandstone, and the want of uniformity in the strata and composition of the conglomeratic rocks. It evidently could not have been of volcanic origin, since there appears to be little or none of the pure igneous rocks in the mass, but all came from the subsequent sedimentary formations.
The conglomerate must have been formed immediately after the carbon iferous limestone, and perhaps partly cotemporaneous with it. The com motion of the tides along the eastern coasts, and the different conditions of the water, prevented the formation of lime, or precipitated it among the enor mous mass of shales and conglomerate which has taken its place in the East. The carboniferous lime is 200 feet thick at Pittsburg, while the Umbral red shale is 3000 feet thick at Pottsville, and intermediate the same proportions will hold good, since both the shale and the lime thin in the same ratio. Thus, if the limestone contains, on an average, 45 per
cent. of carbonate,—which it does not,—the Umbral would absorb the whole with an imperceptible proportion of 3 per cent. of the carbonate of lime.
We also notice the absence of bituminous matter in the formations of the East, which evidently could not have been for the want of carbon; but the conditions which then and subsequently existed were dill'erent from those of the West, and, consequently, not only one but all formations have felt the resulting change.
The only reasonable theory in contradiction to the foregoing, in regard to the formation of the conglomerate, is that they are, or may be, water crystals or concretions; but the evidence against this theory seems to be conclusive, and we need not present the arguments of the theorists.
We have now, step by step, from the deepest formations which have filled the Appalachian basins, arrived at the upper and most magnificent, which crowned the whole and finished this stupendous creation of Nature.
We have seen how the vast and unfathomable gulfs of the ancient sea were filled by the molten lava which poured from the long chain of granite and volcanic hills that formed its steep and gigantic coast-line on the east, and perhaps from volcanoes in its centre; how the vapors of combustion —the escaping carbon—were collected and stored in the limestones and shales and sealed for future use; how the great basin was filled and levelled by later and finer deposits in preparation for the life and vegetation which was to follow.
It will be necessary to review at some length the form and structure of the Appalachian basin as prepared for the production of mineral coal, since the natural process here resembles, and was, in fact, the same as that which produced coal elsewhere; and a plain statement of the circumstances which come beneath our knowledge and comprehension in this case, will present a general exposition of the formation and origin of all the coal of the Carboniferous era.