THE GREAT CONGLOMERATE.
This peculiar rock seems to be the base-rock or floor of all the true coal formations, or those of the great Carboniferous era, throughout the world, wherever such coal is found. We do not find any great, reliable, or extensive coal-field in any other condition. An exception can scarcely be made to the Great Australian coal-field, or the coals of the subcarboniferous period in England, since neither of those coal formations is practically developed or definitely assigned to a prominent position; while the first is still in a doubtful status as to its relative age or position.
In the vicinity of Pottsville the conglomerate is over 1000 feet thick, a vast heterogeneous mass of firmly-cemented quartzose nodules, from the size of a pin to that of a hen's egg, diversified by intervening strata of coarse sandstones, shales, and sometimes thin veins of coal. It is an extremely persistent deposit, and is found everywhere below the coal, throughout the bituminous as well as the anthracite fields.
At the eastern extremity of the Southern field, near Mauch Chunk, the conglomerate is 950 feet thick; at Nesquehoning, a few miles farther west, and on the opposite side of the field, it is 792 feet thick; at Tamaqua it is 803 feet; at Pottsville, 1030 feet; at Lorberry Gap, 675 feet; at Yellow Springs, 660 feet; and at Bear Gap, Wiconisco, 460 feet. But at the latter place it is found doubled between the north and south basins, as shown by figure 93.
In the Lehigh detached basins it is generally about 700 feet; at Mahanoy, near Ashland, 800 feet; and at Shamokin, 630 feet. At
Solomon's Gap, in the Wyoming region, it is 170 feet thick, and at Nan ticoke, 60 feet ; but here the underlying coarse sandstones sometimes increase it to nearly double those dimensions.
In the Broad Top coal-region, which lies about 30 miles east of the margin of the Great Alleghany coal-field, and a little west of the Wyoming formation, the conglomerates appear to be 100 feet thick, and, inclusive of the coarse sandstones between the coal and the red shales, 250 feet.
In Sullivan county is found the most eastern and northern coal-basin of the great Western bituminous formations; and here the conglomerate is a coarse, massive rock about 30 feet thick; while the sandstones between the conglomerate and the red shales are frequently from 300 to 500 feet thick, containing one or two seams of imperfect limestone.
The conglomerate proper continues to depreciate to the west, and frequently consists of a thin plate of fine-grained sandstone, only ten (10) feet thick, but so set with quartzose pebbles as to be unmistakable in character. Yet accompanying this conglomerate plate are frequently large and massive strata of coarse sandstone, which belong properly to the mill stone grit. In the Great Central coal-field in Illinois this millstone grit, which is synonymous with our conglomerate, is 300 feet thick, according to Prof. Wilber, of Illinois.