496 C. SCHUCHERT PALEOGEOGRAPHY OF XORTH AMERICA 



in the Appalachian trough of early Pennsylvanic time. Later in the 

 same period this emergence was greatly accentuated, and forever obliter- 

 ated the Mississippian sea. 



Pennsylvanic-Permic period or system. — Pottsville transgression (see 

 map, plate 83). — This was a transgression of the Gulf of Mexico and 

 the southern Pacific as seen in the widespread Cordilleran, Sonoran, and 

 Mississippian seas. It also affected, but less persistently, the Appalachian 

 sea. The eastern distinctly aggrading seas, however, were so near sealevel 

 that they often became brackish and fresh-water marshes, which was par- 

 ticularly true of the Appalachian area. Loading went on, and every now 

 and then the continental region subsided sufficiently to permit an exten- 

 sive areal invasion of marine waters, but the rivers constantly pushed their 

 loads farther and farther seaward, so that there resulted an alternation of 

 continental deposits interspersed with limited marine zones. The latter 

 are rarely met with in the southern and extreme northern Appalachian 

 areas, more often in the medial Appalachian region, while in the eastern, 

 but more especially in the western, portion of the Mississippian sea the 

 marine deposits are more frequently dominant. Worthen 127 gives a com- 

 posite section of the "Upper Coal Measures" of northern Illinois adjoining 

 the upper course of the Kaskaskia and Wabash rivers, in which there are 

 95 zones having a united thickness varying from 1,072 to 1,722 feet. In 

 these deposits there are 19 fossiliferous marine zones alternating with 17 

 coal beds. The latter vary in depth from 6 inches to 9 feet, averaging 

 about 2 feet and 6 inches, while the marine zones of shale or limestone 

 vary from 1 foot to 25 feet, averaging 6 feet in thickness. The inter- 

 mediate zones are shale and sandstone. 



In late Pennsylvanic time normal marine conditions again prevailed in 

 the western areas of the Mississippian sea, and finally in the north this 

 region also passed into coal-marsh conditions, while in the south the 

 retreating sea of early Permic time deposited red shales and vast quanti- 

 ties of gypsum. In the Cordilleran sea the conditions throughout were 

 those of a normal marine sea. 



The alternation of marine and continental deposits is particularly char- 

 acteristic of the Pennsylvanic in nearly all lands of the northern hemi- 

 sphere, from England to Moravia, Carinthia, probably in Spain, northern 

 China, and the United States {see Suess, II : 243). 



The Pennsylvanic was a time "of extraordinary duration. ... In 

 the middle of the Coal-measure period [of Europe] a number of great 

 folded ranges were both elevated and worn down by denudation, and the 



127 Worthen : Geological Survey of Illinois, vol. f>, 1875, pp. 2-5. 



