CHAPTER XXI 

 THE PENNSYLVANIAN (UPPER CARBONIFEROUS) PERIOD 



FORMATIONS AND PHYSICAL HISTORY 



The most distinctive feature of this system in North America 

 is its content of coal in the central and eastern part of the United 

 States. It includes the Pottsville conglomerate (Millstone grit) 

 below, and the Coal Measures above. 



THE POTTSVILLE CONGLOMERATE (MILLSTONE GRIT) 



The lowest formation of the system in the Appalachian region is 

 generally sandstone or conglomerate, having different names in 

 different regions. From its conglomeratic phase in the east, it 

 grades into sandstone at the west, and with local conglomeratic 

 phases, the sandstone persists over the interior. It has not been 

 recognized in the western part of America. Over wide areas it is 

 unconformable on the Mississippian system, as already noted, and 

 in places it appears to be a true basal conglomerate. In some places 

 it is made up partly of cherts derived from the Mississippian lime- 

 stone, showing that the latter had undergone prolonged decay be- 

 fore the deposition of the conglomerate. In other places the pebbles 

 of the conglomerate seem not to have had a local origin. This 

 argues for wide-spread emergence before the epoch of the Millstone 

 grit, for on land, streams shift materials great distances. Locally 

 as in Illinois, the formation is oil-bearing. 



At various points in the east the formation contains thin bods 

 of coal showing the local beginnings of the conditions which existed 

 later over wide areas, and in the southern Appalachians it is ;m 

 important source of coal. 



The formation varies in thickness from a maximum of some 

 1,500 feet in the Appalachians, to less than 100 feet in some pur; 



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