STJBCARBONIFEROUS PERIOD. 307 



mits of the Eocky Mountains is regarded as belonging mainly to 

 the Carboniferous period, and not to the Subcarboniferous. 



In the Appalachian region of Pennsylvania, the rocks are sand- 

 stones and shales, and are divided into two groups. The Lower 

 consists mainly of sandstones, and is thickest and most varied in 

 composition in the vicinity of the southeastern anthracite coal 

 region, in which, at Pottsville, the thickness is 1800 to 2000 feet. It 

 diminishes rapidly in thickness to the west. The Upper is composed 

 mostly of shales. It attains its maximum thickness in the same 

 region as the Lower, being 3000 feet thick on the Lehigh, at Mauch 

 Chunk. It thins out to the northward, and becomes somewhat 

 calcareous to the southwest. (Rogers.) In Virginia the shales 

 become still more calcareous, and a great formation of Subcarboni- 

 ferous limestone commences which extends into Alabama. 



Going westward from Pennsylvania into Ohio, the fragmental 

 deposits diminish in thickness, and become reduced to a com- 

 paratively thin group of arenaceous beds. 



There are thin workable seams of coal in some of these Subcar- 

 boniferous beds of Pennsylvania and Virginia, and also valuable 

 beds of clay iron-ore. 



(a.)Interior Continental basin. — The Subcarboniferous limestone of the Mis- 

 sissippi valley — especially in Illinois and Iowa — is divided, according to Hall, 

 into five distinct groups, each having its characteristic fossils : — 



1. The Burlington limestone (500 feet thick in Missouri), overlaid in Iowa by 

 cherty beds (60 to 100 feet). 



2. The Keokuk limestone (40 or 50 feet at Keokuk). 



3. The Warsaw limestone (50 to 100 feet). 



4. The St. Louis limestone (250 feet thick), overlaid by ferruginous sandstone 

 (200 feet). 



5. The Kaskaskia limestone. 



The Burlington limestone is made up to a considerable extent of Crinoidal 

 remains, and has afforded many fine species. The Warsaw limestone is some- 

 times called the Lower Archimedes limestone, from the species of Archimedes, 

 which is common in it near Warsaw and elsewhere. The Keokuk and Warsaw 

 limestones are not separated in Missouri by Swallow, who makes the united 

 thickness in some places 200 feet : he names the whole the Archimedes lime- 

 stone. The St. Louis limestone is partly a brecciated rock, but generally a light- 

 gray, fine-grained limestone, and is remarkable for the fine species of Ilelonites 

 (fig. 536), and also the coral Lithostrotion Canadense (figs. 521, 522). The Kas- 

 kaskia limestone has been styled the Upper Archimedes limestone: it contains 

 many Crinids, especially of the genera Poteriocrimis, Zeacrinus, and Scaphio- 

 crinv.s (Hall). The Burlington limestone extends two hundred miles farther 

 north in the Mississippi valley than the succeeding limestones. 



To the south, in Kentucky and Tennessee, the subdivisions of these limestones 

 disappear, and cannot be well made out even by the fossils. 



