Spencer.] diO [March 18, 



Green bay is separated from Lake Michigan by a Niagara escarpment 

 facing the westward, and rising two or three hundred feet above the 

 waters. There appears not to have been any closer connections between 

 these two basins at any previous time than at present, excepting when the 

 waters were at a higlier level. We are told that from Green bay for 400 

 miles to the Mississippi river, a broad, low depression occurs in the coun- 

 try and may have been a former outlet for Lake Superior. This valley is 

 filled with drift even if it ever had a sufficient depth.* 



Grand Traverse bay has a considerable depth in both of its branches, 

 especially in the eastern. Here we find depths to 612 feet, whilst its north- 

 ern mouth is now filled, so that it does not exceed 120 feet. 



The north eastern portion of the basin of Lake Michigan has a general 

 depth of less than 100 feet, but with deeper channels running through it. 

 Many of the soundings about the Straits of Mackinac show a rocky bottom 

 at no great depths. The channel between the 10-fathom contour margins 

 is not much more than a mile and a half wide, and though generally shal- 

 lower, contains a hole 252 feet deep. In proceeding outward, the deepest 

 channel passes northward of Mackinac island, having a depth not exceed- 

 ing 216 feet, and a width of less than a mile. 



Again, a depression of the country extends from near Chicago, on Lake 

 Michigan, towards the Mississippi river, which, in some places, is known 

 to be tilled with drift to a depth of more than 200 feet, according to Dr. 

 Newberry. This is along the Illinois river, whose valley is from two to 

 ten miles wide ; whose mouth is 200 feet lower than Lake Michigan ; and 

 whose upper streams, near Chicago, are only a few feet higher than tlie 

 neighboring lake. 



Lakes Huron and St. Clair. Of these water basins we can make four 

 divisions. The first section may be made to include the shallow basin 

 south of a line drawn from Thunder bay, or Presqu' He, to Kincardine, in 

 Canada, and Lake St. Clair. The second basin comprises the deep chan- 

 nels of Lake Huron, and extends northward to the Manitoulin islands and 

 the Indian peninsula ; the third, the north channel between the Manitoulin 

 islands and the Huronian hills, to the northward ; the fourth, Georgian 

 bay proper. 



The first of these divisions is represented by shallow water, seldom thirty- 



* Since writing the above, I have fortunately been able to see General War- 

 ren's Report on the Transportation Route from the Mississippi river to Green 

 bay {via the Wisconsin and Fox rivers). In this report we find that the bottom- 

 of the valley alluded to In the text has a inaximum height of 208.8 feet above 

 Green bay, and also that Lake Winnebago (on Fox river) is 169 feet above the 

 same water. This small lake discharges by the Fox river, which flows over 

 hard limestones down a series of rapids. Therefore Green bay never discharged 

 its waters into the Mississippi river, and this depression in the country between 

 the Great river and Lake Michigan (the Green bay portion) was not a former 

 outlet of Lake Superior, since it was within about 200 feet of the present level. 

 This fact strengthens the probable correctness of the suggestion that Lake Su- 

 perior emptied into the northern end of Lake Michigan directly. Also, Green 

 bay has evidently the character of a flord. The outlet of Lake Michigan could 

 only have been by the low country along the Illinois river. 



