THE GREAT CONGLOMERATE.
This is the fourth great sand-rock in the palozoic formations, and is the immediate floor or base of the true coal measures. It is deposited on the mud of the umbral red shales in the east, where it is of great thick ness, and is composed generally of a conglomeration of pure white water worn quartz pebbles, from the size of a pin-head to that of a pigeon's egg, cemented together by natural processes.
This rock is 1000 feet thick on the eastern and southern edges of the anthracite coal-fields, but gradually thins in a northwestern direction to a mere plate of coarse-grained sandstone,—occasionally presenting conglome rate pebbles,—from ten to thirty feet in thickness, and extending from Pennsylvania to Alabama, Missouri, and beyond.
This great bed-rock of the coal-fields is, to a certain extent, indestructi ble. It hardens in the atmosphere, and yields not to the action of water. There are but few rocks which are harder and more tenacious, or which have yielded less to the abrasion of the denuding forces. We quote still further from Prof. Lesley's manual:— "This much, however is certain, and should excite our admiration as one of those curious coincidences which may well bear the name of Provi dence, and be received as evidences of the forethought of benevolence, that we are indebted to this enormous local eastward thickening of the conglomerate proper and the conglomerates sandstone above it, for our anthracite treasures.
"Had the rocks beneath the anthracite coal been the mere thin sheets of sand which they are westward, weakened still further by intercalations of clay and coal, their outcrop edges never could have withstood the rush of denuding waters, and protected as they did the mineral fuel within their gigantic folds. What now are groups of long, slender, united, or closely parallel coal-basins, would have been, but for this protection, wastes of red sandstone, or deep lakes in the olive shales of No. VIII., like those of the north. The comparatively little coal that has been hardly left in these small basins would have gone the way of all that vast original deposit the debris of which lies buried under the profoundest bottoms of the Atlantic, together with the immensely greater ruin of the formations underlying and preceding it."