32 



GLACIAL DRIFT OF THE NORTHWESTERN STATES. 



the lake. The western half of Lake Superior occupies a synclinal basin in the 

 Potsdam sandstone, and its long axis coincides with the glacier movement pre- 

 cisely. 



Fiff. 11. 



Fac-Simile op a Slab of Niagara Lime-kook, polished and striated by .„„ „, 

 clay. Sheboygan Light-House, Wisconsin. 



vueath the red 



The tough trap rocks rise on both sides above the sandstone. As the continental 

 glacier pursued its course to the southwest, it was divided and resisted by the 

 trap ranges ploughing out for itself channels such as Keweenaw bay, Chefoimegon 

 bay, and the larger bay at the west end of the lake, which terminates at Superior 

 city. 



Cleveland, Ohio. June, 1864. 



PUBLISHED BY THE SMITHSONIAN INSTITUTION, 



WASHINGTON CITY, 



MAY, 1866. 



