THE PENNSYLVANIAN PERIOD 



159 



satisfactorily separated. The accompanying map (Fig. 93) 

 shows the surface distribution of the Pennsylvanian in eastern 

 North America. Two points of difference as compared with the 

 older systems in this portion of the continent are worthy of mention 

 as follows: (1) The Pennsylvanian rocks occupy distinctly larger 



Fig. 93 

 Map showing the surface distribution (areas of outcrops) 

 of Pennsylvanian rocks in eastern North America. 

 These are also essentially the great areas of Penn- 

 sylvanian coal. (By W. J. M., data after Willis, U. S. 

 Geological Survey.) 



(surface) areas than the rocks of any older Paleozoic system, and 

 (2) "the commonest position for the outcrops of the preceding 

 Paleozoic systems severally is around the outcrops of the older sys- 

 tems. But the outcrops of the Pennsylvanian exhibit no tendency 

 to a similar concentric distribution. Rather do they seem to cover 

 areas between the outcrops of the older systems" (Chamberlin 



