STATE GEOLOGIST. 4& 



members are found in the Lower Peninsula, and whose axis of 

 elevation lies north of the great lakes. At Marquette, Keewe- 

 naw Point, the Porcupine Mountains, and other localities, how- 

 ever, we find accessory axis of elevation, giving rise to dips in 

 various directions, which will be explained on some future occa- 

 sion. Lake Superior occupies a valley between the elevations 

 on the north and south shores, while the other lakes rest in 

 troughs, which have been excavated nearly along the outcrop- 

 ing edges of some of the softer formations. On the south, a 

 basin similar to that of lower Michigan, occupies the southern 

 part of Illinois ; while, passing east from Sandusky, in Ohio, 

 we begin to step over the north-western limits of another one, 

 which reaches to the Alleghanies, and in the other direction 

 stretches from New York to Alabama. Still further west, an- 

 other basin rests, with its northern border in Iowa, and its 

 southern in Missouri. 



A knowledge of these great undulations in the wide-spread 

 strata of the north-west, and of the effects of denudation of the 

 crests of the elevations, will aid materially, in connection with 

 the descriptions which follow, in giving definite ideas of the 

 geological structure underlying any particular portion of our 

 State. 



