256 



ON THE HYDROLOGY OF THE BASIN 



ward to the head of Lake Michigan, obliging the waters of Wisconsin to feed Green Hay 

 before they can enter Lake Michigan. 



Curving round the head of lake Michigan, the Niagara rocks sweep eastward in a 

 narrow belt to the head of Lake Erie, thus closing a hydrographic, as well as a geological 

 circle, around the great peninsula of Michigan, with its isolated coal basin in its centre. 

 In troughs hollowed out of the concentric belt of soft Devonian shales, just inside this 

 circle of Niagara rocks, lie Lakes Michigan and Huron ; and in a third trough excavated 

 from the same shales, thrown eastward by the great Detroit and Cincinnati anticlinal, lies 

 Lake Erie, also. This anticlinal is a swell of the earth's crust, separating the Appalachian 

 coal area of Eastern Kentucky and Tennessee from the coal area of Western Kentucky and 

 Illinois, and casting off' the rock-dips gently east and Avest from its broad back. Com- 

 mencing in Tennessee, it crosses the Ohio in the region of Cincinnati, and the head 

 of Lake Erie into Upper Canada. But for this anticlinal Lake Erie would have had 

 no existence; and the other two lakes must then have emptied their waters by way of 

 Georgian Bay into Lake Ontario. 



The escarpment, limiting Lake Eric on the south, has already been described, and the 

 disposition of the head waters of the Ohio to form along its summit and flow south. In 

 like manner, but in a more remarkable degree, the belt of Niagara rocks, circling around 

 the head of Lake Michigan, cuts oft' the drainage into it. The head waters of the Illinois, 

 a tributary of the Mississippi, and of the Wabash, a tributary of the Ohio, start close to 

 its margin on their long career to the Gulf of Mexico. In fact, there is a marsh but five 

 miles back of Chicago, only seventeen feet above the level of the lake, and in wet 

 seasons its waters flow partly into the lake and partly down the Illinois, as will be de- 

 scribed more particularly in another place. Only at the head of Lake Eric can drainage 

 be said to enter, in any abundance, the Canadian Basin. Hci"e the Miami brings in the 

 waters of a belt of Lower Devonian country of no great size, lying along the anticlinal in 

 Northern Indiana. 



The basin of Lake Superior lies apart from the other great lakes, at the extreme north- 

 west limits of the formations which have been described. Its immense area,, and profound 

 trough, nearly eight hundred feet deep, excite new interest by their surroundings. Hol- 

 lowed out in part from the lowest Silurian rocks, it is the highest of the lakes. Its mineral 

 resources, copper and iron, belong to still older formations, which surround it on all sides 

 except the south. Laurentian and Huronian mountains support a back country of forests 

 and lakes of great extent, which pour their waters into all its shores, and offer commerce 

 with the unknown regions of the interior of the continent : in fact, the western end of 

 the great northern basin drained by the Ottawa, St. Maurice, and Sagucnay. 



All that can be said of this northern basin is, that it is a wilderness of small lakes, the 



