636 HISTORICAL GEOLOGY. 



1. SUBCARBONIFEROUS PERIOD. 

 ROCKS— KINDS AND DISTRIBUTION. 



The Subcarboniferoiis period, like several other periods of the Paleozoic, 

 is noted for extensive limestone formations with thin shales and sandstone 

 over the Central Continental Interior, or the area of the Mississippi basin ; 

 for sandstones and shales, with little limestone, along the Eastern Interior 

 region, especially its northern bay-like portion ; and, like all the preceding 

 periods after the close of the Lower Silurian, for no deposits yet known over 

 the Atlantic continental border south of the latitude of New York. The 

 peculiarities of the Eastern Interior are attended by another distinctive 

 feature: The limestones of the Mississippi basin abound in fossils, especially 

 Crinoids, Brachiopods, and Corals ; and, owing to the Crinoids, they are often 

 called Crinoidal limestones ; while the f ragmental rocks to the eastward con- 

 tain fewer fossils, and almost all of these are of different species from the 

 western, except where limestone occurs in the series. Owing to the wide 

 differences in the rocks and fossils, there is much difficulty in bringing the 

 beds of the two distant regions into parallelism. 



The rocks of the lower of the two groups in Pennsylvania, the Pocono, 

 are mainly beds of hard gray sandstone and conglomerate; and those of the 

 upper, the Mauch Chunh, reddish shales and shaly sandstones. In south- 

 western Pennsylvania a thin bed of siliceous limestone makes the top of the 

 Pocono, and a similar layer occurs also in the upper shales. 



The enduring Pocono sandstone is 800 feet thick near Pottsville, Pa. It 

 extends northeastward, capping at many points the high northern plateau of 

 the state ; and it also stretches southwestward, making the summit, in 

 Bedford County, of the Alleghanies, where it is 1400 feet thick — holding its 

 place against denuding agencies. It is supposed, by Lesley, to constitute 

 some hundreds of feet of the higher peaks of the Catskills. The overlying 

 Mauch Chunk shale is a fragile rock and was easily swept off by denuding 

 waters from the Pocono floor. Its thickness is stated to be 3000 feet at 

 Pottsville. The two formations thin down to 600 feet, in southwestern, and 

 300 feet in northwestern, Pennsylvania. 



The thickness of the limestone layers of the Eastern Interior increases 

 in West Virginia ; and in the southwest counties of Virginia becomes rather 

 abruptly over 2000 feet thick. Earther south, in Tennessee and Alabama, 

 siliceous beds and cherty limestones make the chief parts of the formation^ 

 and they once covered the Silurian limestone basin of central Tennessee. 

 Some thin beds of coal occur in the upper formation, and one in southwest 

 Virginia, near New Piver, is worked. 



In Ohio, about 600 feet of shale and sandstone are overlaid in some parts 

 by 15 to 20 feet of limestone. In Michigan, the beds are chiefly shales and 

 limestones, with less than 70 feet of limestone in the upper part. 



The limestones of the Mississippi basin, with the included shales and sand- 



