THE MISSISSIPPIAN PERIOD 



145 



mented by many deep well sections, proves that Mississippian 

 strata underlie nearly the whole Mississippi Basin except the Gulf 



border, and Wisconsin and Minnesota. 



Fig. 85 

 Map showing the surface distribution (areas of outcrops) of Mississippian 

 and Pennsylvanian strata in North America. The areas in British Columbia 

 include some other Paleozoic rocks as well as some early Mesozoic rocks. 

 (By W. J. M., data from Willis, U. S. Geological Survey.) 



Lower Mississippian Rocks in the East. — The Pocono sand- 

 stone, also including some conglomerate, shale, and thin beds of 

 coal, extends from northern Pennsylvania to Virginia in the Appala- 

 chian district. Its thickness varies from about 2000 feet in Penn- 

 sylvania to about 100 feet in the south. As judged by numerous 

 terrestrial fossils, the Pocono appears not to be a typical marine 



