THE SILURIAN PERIOD. 



389 



material were shifted from time to time, as if by gentle changes of level 

 of land or water. 



The thickness of the beds of the Salina group is considerable, being 

 about 1500 feet near Ithaca, and still more in the mountains of Penn- 

 sylvania, where it attains its maximum development, though free 

 from salt. From central New York the formation thins both to the 

 east and west. 



The sequence of changes in the Salina region of New York is indi- 

 cated by the following table: 1 



.9 >> 



?'H J 



• G/ 



Kg 



F 



p. 



I E 





D 



Zei 



C 





B 



C W 



I A 



Niagara-Guelph fauna 



Waterlime with Eurypterus. . . . 



Gypseous shales 



Beds of rock salt. 



Gypseous shales 



Pittsford shale with Eurypterus. 



Guelph fauna 



Niagara (Lockport) fauna 



Cobleskill 



Salina 



Niagaran 



Associated formations. — Above the Salina proper of New York, 

 there is a thin (150 feet maximum) formation of limestone, the Water- 

 lime (Cobleskill, Rondout, Manlius, etc.), which is generally regarded 

 as a part of the Silurian system. The name Waterlime has reference 

 to the fact that the limestone is the source of hydraulic cement, though 

 it is by no means everywhere useful for this purpose, and many other 

 limestone formations are similarly used. The Waterlime is more 

 wide-spread than the Salina, extending westward through Ohio (where 

 it is often shaly) to Indiana (Kokomo) and to Wisconsin. Both its 

 distribution and its character show that the eastern interior was again 

 more generally submerged than during the deposition of the salt- 

 bearing series which preceded. In the Waterlime group, especially 

 its lower portion (Cobleskill), the Niagara-Guelph fauna reappears 

 (see p. 3S6), indicating a return of marine conditions, and of a 

 fauna which had been excluded for a time from this region because 

 of the salinity of the waters. The sequence of conditions during the 

 Salina is shown diagrammatically above, and the relations of the 

 Silurian and Early Devonian formations on opposite sides of the barrier 

 which separated eastern from central New York, are suggested by 

 Fig. 182. 



1 Hartnagel, Bull. 69, N. Y. State Museum, p. 1159. 



