CHAPTER XX 



THE PENNSYLVANIAN (UPPER CARBONIFEROUS) PERIOD 

 FORMATIONS AND PHYSICAL HISTORY 



This system includes the Pottsville conglomerate (Millstone grit) 

 below, and the Coal Measures above. Its most distinctive feature, 

 far as North America is concerned, is its coal. 



The Pottsville Conglomerate (Millstone Grit] 



The lowest formation of the system in the Appalachian region is 

 sandstone or conglomerate, having different names in different 

 regions. From its conglomeratic phase in the east, it grades into 

 sandstone in the interior. It has not been recognized in the western 

 part of America. Over wide areas it is unconformable on the Mis- 

 sissippian system, as already noted. Locally as in parts of Illinois, 

 the formation is oil-bearing. At various points in the east it con- 

 tains thin beds of coal, and in the southern Appalachians, some 

 thicker beds. 



The formation varies in thickness from a maximum of some 

 1,500 feet in the Appalachians, to less than 100 feet in some parts of 

 western Pennsylvania. It is so firmly indurated that the outcrops 

 of its tilted beds have become ridges in many places. 



The Coal Measures 



Above the Pottsville conglomerate and its equivalents in the 

 central and eastern parts of the continent, lie the formations known 

 as the Coal Measures. They consist of a succession of alternating 

 beds of shale, sandstone, conglomerate, limestone, coal, and iron ore. 

 The succession differs greatly in different regions, but shale perhaps 

 recurs more frequently than other sorts of rock, and in thicker beds. 

 Both the coal and some of the iron ore are in layers interstratified 

 with the other members of the series, and are to be looked upon as 

 strata of rock. Important as the coal and iron ore are from an 

 economic point of view, they make up but a small part of the Coal 

 Measures. There are many beds of coal in some regions, and some 



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