THE PENNSYLVANIAN PERIOD 



377 



FIG. 396. A brachiopod 

 (Spirifer) from the Penn- 

 sylvanian limestone of Col- 

 orado. 



less. We are not to suppose, however, that the West was 



entirely submerged at this time. Reddish sandstones in the 



Black Hills of South Dakota and coarse red conglomerates 



in parts of Colorado were probably 



deposited upon land by streams. 



They contain no marine shells. 



These red strata are linked with 



the more widespread red beds of the 



Permian period and with the peculiar 



conditions of its climate, a topic 



discussed in later pages. 



Transformation eastward. In the 



Rocky Mountains the Paleozoic 



rocks have been exposed by the upturning of the beds (Fig. 



450). Traced eastward, they dip beneath Mesozoic strata 



which underlie the Great Plains, reappearing hundreds of 

 miles away in eastern Nebraska, Kan- 

 sas, and Oklahoma. Where they re- 

 appear the Pennsylvanian system is 

 notably changed. Marine limestones 

 are subordinate, and are interbedded 

 with shales, sandstones, and beds of 

 coal. Still farther east the coal be- 

 comes more abundant and the marine 

 Pennsylvanian strata correspondingly less conspicu- 

 ous. Evidently the eastern part of the 



country was not the site of a clear, open sea. 



Coal measures of the East. The Pennsylvanian rocks of 



eastern United States contain so many beds of coal that they 



are often called the Coal Measures. Only a small part (about 



2 per cent) of the 



total thickness of 



the system actu- 

 ally consists of 



COal. The section Fl( , 3 98- _ C ross section of Coal Measures. 

 (Fig. 398) shows heavy black lines represent coal seams. 



FIG. 397. A large spiny 

 brachiopod (Productus) 

 of the 

 period. 



The 



