PALEOZOIC TIME — UPPER SILURIAN". 



553 



Michigan, and thin out in Wisconsin. They also cross Pennsylvania south- 

 westward. They have not been observed in Missouri, Iowa, or elsewhere in 

 the Mississippi valley. They are absent from the Black Hills of Dakota, 



790. 



791. 



Sections illustrating the relations of the Onondaga beds. HaU. 



and nothing definite is known of their occurrence over the Kocky Mountain 

 region, or the Great Basin, or in California, or any part of the Pacific Coast 

 region. 



The group is 100 to 200 feet thick south of Albany in the Helderberg 

 Mountains, 800 in Onondaga County, central New York, 1500 at Ithaca, 

 1600 in central Pennsylvania, 600 in northern Ohio, and only 100 in southern 

 Ohio. 



The two formations, the Salina and Water-lime, are not consecutive 

 strata, but more or less cotemporaneous, the Water-lime being thin where 

 the Salina beds are thickest. 



Salina Group. 



The rocks of the Salina group are mostly reddish shales or marlytes, with 

 little limestone, which is usually dolomyte ; or alternations of shales with thick 

 beds of limestone. In either case, gypsum and rock salt are often present. 



The outcrop of the formation extends as a narrow belt across New York 

 State, extending from the Helderberg Mountains south of Albany, westward, 

 passing just north of Sharon Springs, Syracuse, and Batavia to the Niagara 

 Eiver above the Falls, where the thickness is but 300 feet. From this belt it 

 dips southward beneath the higher beds of the Upper Silurian and Devonian, 

 becoming 1000 feet below the surface in 25 miles nearly south of Batavia, 

 aiid 1500 feet in 33 miles. At Syracuse the thickness of the formation is 

 about 600 feet ; at Ithaca, ■ 30 miles south of the belt, it is 1230 feet. In 

 western Ontario, Canada, on Lake Huron, about Goderich, the thickness is 

 over 1400 feet, the lower 600 feet consisting of limestone, shale, and salt, 

 and the rest of dolomyte ; and to the south, near Cleveland, Ohio, there are 

 750 feet of shale, limestone, and rock salt beneath 800 feet of dolomyte. 



Salt springs occur in many parts of New York, west of Syracuse and 

 Tully. Those around Onondaga Lake led, first in 1825, to the sinking of 

 wells 70 feet to 75 feet deep at Salina, for the manufacture of salt by evapo- 

 ration. Eock salt appears to have been first discovered in New York, in 

 Bristol, Ontario County, at a depth of 1200 to 1300 feet ; but the discovery 



