POTSDAM PERIOD. 



171 



(p. 137), the strata of the Silurian and Devonian outcrop in suc- 

 cession on going from the Azoic (No. 1) southward. The numbers 

 on the areas render a comparison with the section and with the 

 tables beyond easy. The Silurian strata are lined horizontally ; the 

 Devonian, vertically; and the subcarboniferous beds, which appear 

 at the southern margin of New York State (No. 13) are cross-lined. 



The area 2, which is that of the Potsdam period, is divided into two portions, 

 one distinguished as a or 2 a, and the other as b or 2 b : the former is the area 

 of the rocks of the Potsdam epoch, and the latter of the Calciferous epoch, which 

 epochs are elsewhere in the work distinguished by the same numbering. Other 

 areas are similarly divided. As another example, the area 5 (Niagara period) is 

 divided into a, b, c, d, corresponding to 5 a, 5 b, 5 c, 5 d in the above table. The 

 areas of Nos. 7 and 8 in the series (the Lower Helderberg closing the Silurian, 

 and the first period of the Devonian) are not distinguished on the map from 

 No. 9. 



Fig. 206 is an ideal section of the rocks of New York, along a 

 line running southwestward from the Azoic on the north across the 



Fig. .206. 



Carboni- 

 ferous. 



£ 3 

 E2 a 



C 



era 





s m 



Devonian. 



Silurian. 



State to Pennsylvania. It shows the relative positions of the suc- 

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

 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. 



.1. POTSDAM OR PRIMORDIAL PERIOD (2). 



Epochs. — 1. The Potsdam epoch, or that of the Potsdam sand- 

 stone (2a). 2. The Calciferous epoch, or that of the Calciferous 



sandrock {2 b). 



