THE PENNSYLVANIAN PERIOD 623 



an. 1 In other regions, as in Ohio, a fourfold division was formerly 

 made, as shown in the table above, but the uppermost is now gener- 

 ally referred to the Permian. 



Distribution and outcrops between the Appalachians and the 

 Rockies. The distribution of the unburied part of the Pennsyl- 

 vanian system is shown in Fig. 435; also the areas where the system 

 is believed to exist, though concealed, and where it is thought to 

 have been removed by erosion. The system is probably concealed 

 in areas to the south of those shown on the map, and its equivalent- 

 doubtless underlies much of the sea. 



The surface distribution of the system in the eastern part of 

 the continent is in some ways in sharp contrast with the surface 

 distribution of older systems. The commonest position for the 

 outcrops of the preceding Paleozoic systems severally is around the 

 outcrops of older systems. 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 older formations. 

 Thus in Michigan, the Pennsylvanian strata occupy an area com- 

 pletely surrounded by older formations. 



This difference in surface distribution does not betoken any new 

 principle in the distribution of the system. The Ordovician forma- 

 tions come to the surface, among other places, in New York, Ohio, 

 Wisconsin, Missouri, and the Black Hills. Beneath the surface, 

 the beds outcropping in these several localities are believed to be 

 continuous, though concealed by younger formations. It will be 

 remembered that most of the eastern interior, and perhaps most 

 of the west as well, became land at the close of the Ordovician 

 period. Had it never been submerged again, the Ordovician system 

 would not have been covered, and its outcrops would now have 

 appeared at the surface in broad areas between the outcrops of 

 older formations; that is, its outcrops would have corresponded, 

 in principle, with the surface distribution of the Pennsylvanian. 

 After the deposition of the Pennsylvanian system, the surface where 

 it is now exposed was elevated, relatively, and except for the glacial 

 drift, was either never deeply covered by later deposits, or the 

 overlying formations have been removed. Some of the separate 



1 Geol. Surv. of Iowa. 



