POTSDAM PERIOD. 



173 



the juxtaposition of the Potsdam and Azoic near Carp Biver. south 

 of Lake Superior ; the former rock (on the right in the view), 

 slightly inclined sandstone ; the latter (on the left), quartzite in 



Fig. 207. 



,\| 



a nearly vertical position, which position it had received before 

 the deposition of the sandstone. 



Through New York and the greater part of the Mississippi basin 

 the strata have usually a gentle dip or are nearly horizontal. Along 

 the Appalachian chain, east of Lake Champlain and the Hudson, 

 in the region of New England, of the Alleghanies in Pennsylvania, 

 and of the Cumberland Mountains in Tennessee, they are up- 

 turned at all angles to verticality. 



1. Potsdam epoch. — a. Interior Continental basin. — The Potsdam sand- 

 stone in New York varies from a hard quartzite, as at Keeseville, N.Y., and 

 Adams, Mass., to a friable sandstone. Much of it is a good building-stone, 

 as at Potsdam, Malone, and elsewhere, N.Y. The colors are gray, drab, 

 yellowish, brownish, and red. In the West, in Michigan, Wisconsin, and 

 Minnesota, the rock is often so soft as to crumble in the fingers. This want 

 of firmness in one of the most ancient of rocks shows how ineffectual are ordi- 

 nary waters, even through the lapse of ages, in causing solidification. At some 

 localities it consists of a clean white sand and crumbles readily, making a 

 good material for glass. The rock is sometimes a conglomerate, especially 

 in its lower part, for ten or twelve feet ; on the north side of the Azoic in the 

 northwest part of Clinton co. and part of St. Lawrence co., near De Kalb, 



