CHAPTER XVIII 

 THE PENNSYLVANIAN PERIOD 



THE system which contains the most important deposits of 

 coal in both the United States and Europe is called in America 

 the Pennsylvanian. Because of the great value of the coal 

 beds, this division of the old Carboniferous has received more 

 attention than the earlier and later portions. 



Land interval at the beginning. At the close of the Mis- 

 sissippian, a large part of the United States emerged from the 

 sea, and the fact is recorded by an extensive unconformity. 

 Sediments continued to accumulate in certain low or sub- 

 merged regions, for example in Arizona and Utah, and there 

 we find transitional formations; but in the eastern interior 

 especially, land conditions prevailed. The long-continued 

 weathering and erosion of the land removed part of the Mis- 

 sissippian rocks, and locally uncovered still older formations. 

 As the limestones crumbled and decayed, a residual layer of 

 clay, with bits of flint which had formed part of the original 

 rock, was left upon the surface. These insoluble grains and 

 nodules, worked over by the currents of the rivers and the sea 

 of the ensuing period, contributed to the formation of the 

 basal part of the Pennsylvanian system. 



Marine conditions in the West. In the Southwest, where 

 changes of geography had been slight, the interior sea had 

 been maintained. Early in the Pennsylvanian it extended 

 itself over a much larger part of the West. In its clear waters 

 limestone was deposited in Nevada, while shales and sand- 

 stones are found in Arizona and Montana. 



Corals, crinoids, and other marine invertebrates (Figs. 396 

 and 397) flourished in these waters, as in the preceding age, 

 although the number of fossils which have been found is far 



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