36 



TWELFTH REPORT. 



of the ic-e from the vicinity of the Straits of Mackiuao and the Ste. 

 Marys River, a single large lake, known as Lake Algonquin (Fig. 7), 

 occupied the basins of the present Lakes Superior, Michigan, and 

 Huron, aiid part of the intervening territory. Sometime during the 

 life of Lake Iroquois the ice uncovered the Trent valley in Ontario 

 and g'ave an outlet for the waters of Lake Algonquin into this lake 

 in the Ontario basin. Prior to this the discharge had been either 

 through the Chicago Outlet or the St. Clair Outlet, or perhaps through 

 both of these outlets. The change to the Trent Outlet appears to have 

 been due to lower altitude along that line than either of the other out- 



Nipi s s in£ Great LaKes 

 with I solo a s e s 



Sc ale 



-: ''• " " -^M,ig.9 



Fig. 8. Nipissiug Great Lakes and Cliamplaiii Sea. Numerals aud isobases give altitude 

 above sea level of the highest exposed shore. Isobases of Michigan and Huron 

 basins by Taylor and Goldthwaite. 



lets possessed. Uplift in the region bordering the Trent outlet, how- 

 ever, eventually brought the lake up to a level high enough to resume 

 discharge through one or both of the former outlels, and to 

 cause the abandonment of the Trent outlet. The altitude of Lake Algon- 

 quin when brought up to a redischarge through the southern outlet 

 or outlets Avas about 25 feet above the level of Lakes Huron and Michi- 

 gan, its beach at the southern end of Lake Huron being ()Oij to (>07 feet 

 above sea level. In the Lake ^lichigan l;asin it came back to the level 

 of the third beach of Lake (Miicago which is 20 to 25 feet above Lake 

 Michigan. 



The conditions on tlie north side of Lake Su])erior have not been 



