18S1.] ^^l^ [Spencer, 



course in the region of its south-western extremity, and the few deeper 

 holes to have been produced hy some receding cascade from the adjacent 

 shore to which there appears to be a transverse deep channel south of the 

 mouth of Gooseberry river. 



Again, Prof N. H. Wincliell calls attention to the depression in the low 

 country between the Chocolate river (east of Marquette), and Train bay 

 (near the Pictured rocks), as the only place where there could have been 

 connection between the basins of Lakes Superior and Michigan. It may 

 be remarked that some of the deeper soundings put in towards this portion 

 of the coast, whilst, to the westward and eastward, the present lake bottom 

 slopes more gradually. The soundings, however, that are near the sliore, 

 show a rock}'' bottom, excepting north of Laughing Fish point (Sable 

 river), and along a narrow channel north of the mouth of Chocolate river. 

 The lake is very shallow for some distance westward of the St. Mary's 

 river. 



LaTce Michigan. This lake may be said to consist of a broad long plane, 

 the northern half having a mean depth of about 600 feet, whilst the 

 soundings in the southern half are not much more than half that measure- 

 ment. The deepest sounding recorded is 870 feet, in the latitude of the 

 southern end of Green bay. Throughout the whole length, the lake ap- 

 pears to be traversed by a deep channel, and in the northern end by more 

 than one. Although the pitch of the bottom from the shore line is more or 

 less gradual — generally less than 40 feet in a mile— yet, along the eastern 

 side there is a precipitous escarpment extending for a considerable distance, 

 which in one place suddenly descends, in a horizontal line of little over a 

 mile, from 17 to 93 fathoms, or 456 feet, and increases 60 feet more in the 

 distance of another mile. 



The conspicuous channels in the submerged plane extend far northward to 

 near the end of the lake. An interesting sounding east of the mouth of 

 the Manistique river shows a depth of 448 feet, at a distance of two miles 

 from the shore, whilst all the adjacent depths do not exceed 11 fathoms- 

 This appears to be a continuation of the deep soundings, ten miles to the 

 southward, but the surrounding lake bottom is covered with drift to a great 

 depth, wherever the Niagara limestones have been removed. It is more 

 than probable that this great depth is in a rock-bound channel of an ancient 

 water course, which elsewhere has been filled with drift. It seems proba. 

 ble that it was a portion of a buried channel extending through the valley 

 of the Manistique lakes to the depression in the country south of Lake Su- 

 perior, alluded to above, and formed a Preglacial connection between the 

 valleys of Lakes Superior and Michigan. Prof Winchell regards the val- 

 ley between the two lakes along the Chocolate and White Fish rivers (the 

 latter emptying into Little Bale De Noc), as indicating the ancient con- 

 nection. This route seems less favorable, as both Little Bale de Noo and 

 Green bay are shallow compared with Lake Michigan, for the greatest depth, 

 which is near an outlet through Portes des Mortes, is only 32 fathoms, 

 whilst generally the bay does not exceed 100 feet. 



PKOC. AMEK. I'HILOS. SOC. XIX. 108. 3n. pkikted makcu 30, 1881. 



