166 



PALEOZOIC TIME. 





Fig. 216 is an ideal section of the rocks of New York, along a line 

 running soutkwestward from the Archaean on the north across the State 



Fig. 216. 



Silurian. 



to Pennsylvania. It shows the relative positions of the successive 

 strata, — bringing out to view the fact that the areas on the preceding 

 map are only the outcrops of the successive formations. This is all the 

 section is intended to teach ; for the uniformity of dip and its amount 

 are very much exaggerated, and the relative thickness is disregarded. 



A. LOWER SILURIAN. 



I. PRIMORDIAL PERIOD (2). 

 1. American. 



The Primordial or Cambrian Period in North America includes two 

 subdivisions, distinct in their fossils, according to present knowledge. 



(1.) The Acadian Epoch ; (2.) The Potsdam Epoch, so named from 

 Potsdam in northern New York, where they occur. 



I. Rocks: kinds and distribution. 



Primordial rocks probably cover the larger part of the continental 

 area outside of the Archaean, as they are the beds first formed after 

 the Archaean in the continental seas. But to a great extent they are 

 covered by later formations. They are now exposed to view for the 

 most part either (1) on the borders of the Archaean ; or (2) in moun- 

 tain regions, in which the upturnings of the rocks have brought them 

 to the surface. 



Those of the Acadian or earlier epoch have been observed on the 

 eastern border of the continent, at St. John in New Brunswick, at 

 Braintree near Boston, and in southeastern Newfoundland ; they are 

 shales, sandstones, and limestones. 



The Potsdam beds are mostly sandstones, but are in some regions 

 shale or slate and limestone. They outcrop along the borders of the 

 Archaean in New York, Canada (see 2 b, on map, p. 1 65), Wisconsin, 



