ONONDAGA GROUP. 55 



the Straits of Mackinac, where it forms the island and the points of the main 

 land. The thickness on the peninsula of Michigan does not exceed 50 feet. It is 

 broken up in a ridge extending west from the west end of Lake Erie near the 

 southern line of Michigan, where it is much thicker, and again at Put-in Bay 

 Island, and at Sandusky and other places in Ottawa County, Ohio, and may 

 be seen on the western and south-west anticlinals, which pass through Wood 

 County, and as far south as Delaware and Pike. The thickness in Ohio has 

 not been accurately ascertained, but including the Waterlime, which is not sepa- 

 rable, the thickness is several hundred feet. It has been identified in Missouri, 

 varying from 10 to 75 feet in thickness. It does not occur in Wisconsin or 

 Iowa, and is unknown south of Pennsylvania in the Appalachian system. The 

 composition of the rocks indicates shallow water; but as there is no conglomerate, 

 it does not appear as a shore deposit. 



110. It is not very fossiliferous at any locality, and generally fossils are 

 extremely rare. In addition to the two species mentioned as common to the lower 

 part of it and the Guelph, Orthpceras sublceve, Euomplwtlus sulcatus, and Avicula tri- 

 quetra were early described from Wayne County ; but the indistinct forms of Spiri- 

 fera, Atrypa, and Cornidites remain without specific names. 



111. The Waterlime takes its name from the earthy, drab-colored limestone 

 used for making hydraulic cement, and is regarded by some as a distinct Group, 

 while the Canadian Geologists regard it as the lower member of the -Lower Helder- 

 berg. It has its characteristic minerals and fossils ; but, following the New York 

 Geologists, it is here treated as the upper member of the Onondaga. In New York 

 and Pennsylvania its thickness is from 30 to 300 feet, and is well-defined and recog- 

 nized by its mineral nature, its fossils and position. In Eastern New York a 

 brownish limestone, often mottled, containing corals, fragments of crinoids, and 

 small Orthoceras forms the base of it. All the species of Pterygotus belong to the 

 Waterlime, while Eurypterus remipes and Pterinea rugosa are characteristic of it in 

 New York. The species which has the greatest geographical distribution in the 

 Onondaga, is that peculiar form called Pleurodictyum problematicum. 



112. The whole Group contains more or less carbonaceous matter, and the 

 quarries usually smell of petroleum, and the limestone generally gives up the 

 odor when struck with a hammer. This Group is the source of a large part of the 

 gas supplied by the gas-wells of Ohio and Indiana. It is the chief source of the 

 salt manufactured in New York and in Michigan. On the St. Clair River, at Marine 

 City, rock-salt occurs in a mass, extending from 1,633 feet to 1,748 feet below the 

 surface, which is mined by forcing fresh water down into it to take up the salt, and 

 afterward pumping the brine and evaporating it. Thick masses of rock-salt have 

 been formed at various other places in this Group within the salt districts of New 

 York, Michigan, and Ontario. 



