194 G. A. Shufeldt on the Subterranean Sources 
the. water-shed called the Summit, which separates the waters 
which flow into the St. Lawrence from those which flow into the 
Gulf of Mexico, and from the southern slope of this Summit, 
- flowing southward, is the Aux Plaines river, a tributary of the 
Illinois. So that Lake Michigan gets no water from Illinois, 
but a trifle from Wisconsin, and very little from Michigan. And 
_ yet the straits of Mackinaw carry off a large quantity of water 
from this lake, and Lake Michigan furnishes its due proportion 
of the great current which passes over the Falls of Niagara. 
the question arises, whence comes this great volume and mass of ~ 
ground streams and water courses. They know that the crust 
th 
map of North America and note particularly the point where 
the thirty-second degree of west longitude crosses the fort -fourth 
parallel of north latitude. Within a radius of five hun 
