UMBRAL RED SHALES.
We have now reached the red shales of our coal-fields, so well known, and so easily distinguished by their peculiar color and thin, soft, and gene rally friable nature. This must have been a pure red mud, as it now forms the softest of rocks, and could only have been the shore deposits of a quiet sea.
This soft red shale of No. XI. encircles our coal-fields between the vespertine, No. X., and the next overlying or great conglomerate, No. XII. of the true coal measures. It is generally cut down by the stream in low, deep valleys, with the vespertine white rocks towering in mountain form on one side, and the great conglomerate in almost equal bulk on the other. Occasionally these red shale valleys are widened out by the undu lation of the strata, and present fertile and cultivated valleys amid the wild and barren mountains of X. and XII. We may note, for instance, the Quakake, Nescopeck, Catawissa, Mahantango, and Lykins valleys, as surrounding the anthracite coal-fields; while others of more or less note exist around the Alleghany spurs and the detached coal-basins on the waters of the Susquehanna.
This deposit thins rapidly in a western direction. Though it is 3000 feet thick on the Schuylkill and Lehigh, south of the anthracite fields, and 1000 at Broad Top and on the New River, in Southwestern Virginia, it is only 200 feet thick in the eastern escarpment of the Alleghany, along the head-waters of the Juniata, and is lost to view before it reaches the Alleghany,and Monongahela Rivers.
This disappearance is owing more to a metamorphism than a deprecia tion. We lose sight of the red shale, but it is simply a change from the red mud of the eastern shore of the Appalachian Sea to the mountains or great carboniferous limestone of its interior basins. The limestone pre dominates and increases invariably towards the centre of the Appalachian formations; while the sandstones, shales, slates, and coarse conglomerates of its eastern margin depreciate in the same direction, and all those formations become finer in grain as they become changed in bulk and character. This Umbral limestone, which usurps the place of the Umbral shale, is very thin on its northeastern edges; commencing in the middle of the red shales but a few feet thick, it increases in thickness in a south western direction as rapidly as the red shale diminishes. But this great carboniferous limestone—identical, undoubtedly, with the British carbon iferous lime—does not appear to diminish in the same proportion towards the western margin of the ancient or Appalachian Sea It is found in the Black Hills and the Rocky Mountains, and even farther west, to the plateaus of Sonora and the Sierra Nevada, of California, giving positive evidence of the existence of coal in those great western unexplored regions: