306 Nezv York State Museum 



deposits of New York, southern Michigan and On- 

 tario are very important sources of salt. In New York 

 State beds of salt occur aggregating 50 to 100 feet in 

 thickness and beds of pure salt have been found with 

 a thickness of 40 to 80 feet. 



The small, shifting nature of late Silurian seas has 

 been touched upon above. Corals showing an Arctic 

 origin occur and coral reefs form again in some areas. 

 The Cobleskill limestone of New York marks an east- 

 ward extension of this reef fauna, although reef condi- 

 tions are seldom found. The Cobleskill includes many- 

 corals of types characteristic of formations of this age 

 in Michigan, belonging to a northern fauna. A per- 

 sistent Middle or Southern Atlantic fauna of small 

 brachiopods, Leperditias, pelecypods and Tentaculites 

 occupied the southern part of the Appalachian trough 

 and extended up into eastern New York (Manlius). 

 In the northern end of the Atlantic trough (Nova Sco- 

 tia) there was an Atlantic fauna, essentially that of the 

 Upper Silurian of England. 



The Silurian of western North America is not well 

 known and apparently is poorly developed in the 

 United States. In the Cordilleran area of Canada there 

 was a deep trough in which between 2000 and 3000 

 feet of dolomites and limestones were deposited, and 

 in the southern Cordilleran sea 200 or 300 feet of Silu- 

 rian limestones were deposited in the Great Basin area 

 (Nevada, Utah), also a considerable thickness of Silu- 

 rian graptolite shales in Idaho. The Middle Silurian 

 is represented in western Texas by a moderate thickness. 



Besides New York State other important regions for 

 the Silurian are Wisconsin, Michigan and Ohio in the 

 interior; eastern Canada, especially the Island of An- 



