THE PENNSYLVANIAN PERIOD. 543 



97th meridian in Indian Territory, 1 and nearly to the 100th meridian 

 in Texas. 2 



The distribution of the Pennsylvanian system in the eastern part 

 of the continent, as it now appears at the surface, is in some ways in 

 sharp contrast with the surface distribution of older systems. The com- 

 monest position for the outcrops of the Cambrian, Silurian, Devonian, 

 and Mississippian systems severally is around the outcrops of older 

 systems. But this rule does not seem to be followed in the case of 

 the Pennsylvanian, the outcrops of which appear to exhibit no ten- 

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

 cover areas between the outcrops of older formations. Thus in Michi- 

 gan, the Pennsylvanian strata occupy an area completely surrounded 

 by older formations. 



This difference in surface distribution does not betoken any real 

 difference in the distribution of the system. The Ordovician formations 

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

 consin, 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 

 again been submerged, the Ordovician system would not have been 

 covered, and its outcrops would now have appeared at the surface 

 not in zones about the exposed parts of earlier systems, but in broad 

 areas between the outcrops of older formations; that is, these out- 

 crops would have corresponded, in principle, with the surface distri- 

 bution of the Pennsylvanian, so far as it appears at the surface. 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 if so covered, 

 the overlying formations have been almost wholly removed. 



The matter may be looked at in another way. The several land 

 areas, which served as centers for the distribution of sediments, such 

 as Appalachia, the Adirondacks, northern Wisconsin, etc., became, 

 on the average, larger and larger as time went on; that is, the several 

 land areas grew as the result of uplift, or withdrawal of the sea, in 



1 Gould, Am. Jour. Sci , Vol. XI, p. 185, 1901; and Taff, 22d Ann. Rept., p. 373, 

 Pt. III. 



2 Taff, idem, p. 402. 



