378 



PALEOZOIC TIME. 



of the contrast between the two regions may be gathered from a 

 comparison of the section of the New York rocks, on p. 131, with 

 the general section of the formations in the Mississippi valley here 

 presented. 



In the Lower Silurian of this section the Calciferous beds are 

 mainly of limestone, as well as the Trenton and the greater part of 



Fig. 618. 



Permian 



Carboniferous 



SCBCARBONIFEROUS \ pi 



( Hamilton 

 1 U. Helderberg 

 Niagara 



Hudson River 

 Trenton 



Potsdam 



■s^=r—- Permian. 



Coal Measures. 



Coal Conglomerate. 

 Subcarboniferous limestone. 



Waverly sandstone (= Chemung 



and Subcarboniferous). 

 Black shale. 



Cliff limestone. * 



Blue limestone and shale. 



Trenton limestone; Galena lime- 

 stone; Black River limestone. 



Lower magnesian limestone (= 

 Calciferous). 



Potsdam sandstone. 



Section of the Palaeozoic rocks in the Mississippi basin. 



the Hudson. The Upper Silurian contains little but limestone ; the 

 Lower Devonian and the Subcarboniferous are also limestone. 

 Moreover, many limestone beds intervene in the Coal measures ; 

 and west of the Mississippi, over a considerable portion of the 

 Rocky Mountain slope the Carboniferous beds are mainly lime- 

 stones. 



The rocks of the northern border of the Interior Continental 

 basin towards the Azoic contain a much smaller proportion of lime- 

 stone than those of the central portion. 



The contrast between the Appalachian region and the Interior will become 

 more apparent from a few general sections. The first here given is from the 

 State of Pennsylvania, which lies within the Appalachian region ; it is from 

 the Geological Report of H. D. Rogers; the second is a section of the Michi- 

 gan rocks, by A. Winchell, lying on the northern side of the Interior basin; 

 the third, of Iowa, which is also on the northern side, by Hall; trie fourth and 



