GEOLOGICAL EXPLORATIONS AND LITERATURE. 



79 



This table shows also what members of the diflferent series that have 

 been described as occurring in Michigan, Wisconsin and Minnesota, cor- 

 respond with each one of these groujiis. It does not appear that Prof Win- 

 chell would regard these several groups as necessarily separated by time-gaps 

 from one another, but merely as certain lithologically distinct Iiorizons, to 

 which all of the rocks of the lake Superior region may be refen-ed. In tlie 

 original tabulation, of which the above is a partial copy, Prof. Winchell 

 gives also four other cohnnns, one of which indicates the equivalency of 

 these six groups to the several divisions of the pre-Potsdam rocks of New 

 England as recognized by Emmons. The other columns headed respect- 

 ively Hunt, Brooks, and Irving, give the relations of these six groups to the 

 divisions recognized l)y the geologists named. As to the relation of any of 

 these rocks to any of the several groups recognized by Emmons and others 

 in the East, we do not propose now to say anything, 1)ut merely to restrict 

 ourselves to a few remarks with regard to equivalencies indicated in the table 

 for the lake Superior region itself. To begin with, we can by no means 



