472 



HISTORICAL GEOLOGY 



Pennsylvanian or Upper Carboniferous 



The Pennsylvanian system is generally separated from the Mississip- 

 pian by an unconformity, the Mississippian strata in some parts of the 

 central United States having been gently folded, faulted, and eroded 

 before the deposition of the Pennsylvanian sediments. A few seas 



persisted in Utah 

 and Arizona in which 

 sedimentation con- 

 tinued throughout 

 the period without 

 interruption, but 

 such areas are rare. 

 During the emergent 

 condition of the 

 continent the sur- 

 face rocks were 

 weathered, leaving 

 a residual layer of 

 insoluble quartz and 

 clay. In the east 

 this easily remov- 

 able material was 

 carried into a long, 

 narrow sea, formed 

 by the down-warp- 

 ing of the eastern 

 part of the old Ap- 

 palachian trough, 

 where it was worked 

 over and sorted by 

 the seas to form 

 the conglomerate 

 (Pottsville) of the basal Pennsylvanian. As this trough was 

 weighted down by the sediments carried into it by streams, it 

 sank intermittently. For long intervals this area was slightly above 

 the sea level, and the sediments which then accumulated were 

 continental and not marine; at other times the sea encroached on 

 the land and formed immense, shallow seas which, upon being further 

 shallowed by sediment from the land, formed vast, fresh, and 



Fig. 448. — Map showing the probable outline of 

 North America during a portion of the Upper Penn- 

 sylvanian. Continental deposits are shown in solid 

 black. (Modified after Schuchert.) 



