LAKES MICHIGAN, HURON, AND ERIE. 179 



systems, — of the Mississippi, St. Lawrence, and Saskat- 

 chewan, — as has been mentioned in a preceding page. 



If we trace the south side of the St. Lawrence basin 

 from this quarter, we find that already on the upper lake 

 it assumes a different aspect, being composed chiefly of 

 sandstone, and having less elevation than the north bank. 

 Lake Michigan runs far to the southward among the 

 silurian strata, entering as it were into the valley of the 

 Mississippi. So small is the elevation of the ground 

 between the Fox River which falls into Green Bay on 

 the west side of that lake, and the Wisconsin, a tributary 

 of the Upper Mississippi, that in times of flood a barge 

 may float readily from one stream into the other.* A very 

 moderate elevation in like manner separates the south end 

 of Lake Michigan from the Illinois, another affluent of 

 the Mississippi, so that a depression of 600 feet would 

 produce a communication between the waters of the Gulf 

 of the St. Lawrence and those of the Gulf of Mexico, 

 through the Illinois valley.f The basin of Lake Huron 

 is excavated in the silurian strata, and the great promon- 

 tory which divides it from Lake Michigan is said to be a 

 deposit of old red sandstone enclosing the extensive coal 

 measures of Saginaw. The lake shores are bold, but not 

 mountainous. 



Lake Erie has lower shores, and is the shallowest of the 

 series. Its bed and much of its northern margin is formed 

 of the corniferous limestone, one of the upper members of 



* Featherstonhaugh. 



f A depression of thrice that amount would carry the ocean to the 

 western border of the prairie islands, leaving, as insular ranges, the 

 summits of the Alleghanies and their continuation in Vermont, New 

 Hampshire, and Gaspe, with a few peaks in the hilly region of New 

 York, which lies between Lakes Ontario and Champlain ; while the 

 primitive masses on the north of Lake Superior would be mere reefs, 

 over the highest of which an agitated sea would dash its spray. 



n 2 



