THE ORDOVICIAN PERIOD. 



313 



Southern Wisconsin section. — It is not to be understood that the 

 same subdivisions hold for the Ordovician system in all regions, and 

 even if the same number of subdivisions is made elsewhere, it does 

 not follow that they are equivalent, severally, to those of New York. 



Fig. 132. — Exposure of the TJtica shale. Bluff at Van Tine's, on the west shore of 

 Grand Isle, Lake Champlain. (Perry.) 



In Wisconsin, Iowa, and Minnesota, for example, the formations com- 

 monly recognized, numbered in the order of their deposition, are as 

 follows : 



Upper Ordovician 5. Hudson River (Cincinnati) shale. 



Middle Ordovician. . . . 



/ 4. Galena limestone. 

 \ 3. Trenton limestone. 



Lower Ordovician. 



{? 



2. St. Peters sandstone. 



Lower Magnesian limestone. 



While there is here, as in New York according to the older classi- 

 fication, a fivefold division of the system, it is not certain that 

 any formation of Wisconsin is the exact equivalent of the formation 

 which occupies the corresponding position in the New York section. 



