THE PENNSYLVANIAN PERIOD 161 



in the Mississippi Basin they have remained in almost horizontal 

 position. 



Pennsylvanian Rocks in the East. — Rocks comprising this 

 system in the eastern part of North America are partly of marine 

 and partly of non-marine origin with the latter (including coal) 

 unusually prominently developed. 



The following extracts from a paper by D. White concisely 

 describe the Pennsylvanian rocks of the Appalachian province 

 (also see Fig. 95): "The Pottsville, like the succeeding formations, 

 is composed of sandstones, shales, and clays (including fire clays, 

 coals, and limestones), but it contains a larger proportion of sand- 

 stones and arenaceous shales than the later formations . . . The 

 Pottsville is thickest in the southern exposures, where, near the 

 eastern outcrops, it probably exceeds 7500 feet. In the north- 

 western bituminous area ... it measures locally less than 200 

 feet. . . . The Pottsville contains all the workable coals south of 

 the Kentucky-Tennessee state line. 



The Allegheny, next succeeding the Pottsville, is a thin forma- 

 tion characterized by a larger proportion of coal, shale, limestone, 

 and iron ore. In the bituminous districts . . . the Allegheny 

 ranges generally between 250 and 350 feet in thickness near 

 the northern outcrop, though it thins southwestward to 160 feet 

 in northeastern Kentucky. 



The Conemaugh, which succeeds the Allegheny, is generally 

 marked at its base by sandstone or conglomerate. It is especially 

 characterized by sandstones, shales, and limestones, intermingled, 

 particularly in the western area, with red and green shales, clays, 

 and sandstones. It contains less coal than any of the other 

 Pennsylvanian formations of the Appalachian trough. 1 



The Monongahela is distinguished by its relatively large pro- 

 portion of coal and limestone, the latter composing over one- 

 third in some districts. The formation . . . averages about 325 

 feet or less in thickness. Its coals, including the great Pittsburg 

 coal at its base, are of notable thickness and value." 2 



The four distinct subdivisions of the system above described 

 are generally not recognized as such in the Mississippi Basin, but 

 various local names are there given to the subdivisions of the 



1 A maximum thickness of 800 to 900 feet for the Conemaugh is shown in 

 western Pennsylvania and Maryland. 



2 D. White: U. S. G. S., Professional Paper 71, pp. 431-432. 



