STUDIES OF SPECIAL LACUSTKAL HISTORY. 103 



The views of Dawson and Spencer are set forth in the pubheatiuns 

 mentioned in the foUowing footnote,^ and shituhl be attentively studied 

 by all who luidertake to read the history of the Laurentian basins from 

 the original records in order that their conclusions may be fairly tested. 



Lake Agassiz. 



At the time the remarkalde eluinges described al)Ove were taking place 

 in the Laurentian basin, there were corres})onding revolutions in the 

 geogra[)hy i>f the region to the northwest which now drains to Lake 

 A\'iiniepeg and thence through Nelson riA^er to Hudson ba}-. 



li will be readily seen on glancing at a ma}) of Canada, that if a 

 glacier of the continental type should advance southward from the Hud- 

 son bay region, the drainage Avould be obstructed and a lake formed over 

 the cotuitry of mild relief surrounding Lake Winnepeg and tlie Lake of 

 the Woods, and extending southward through tlie Red River, valley, far 

 into ^Minnesota. Such a lake would discharge southward, and contribute 

 its surplus waters to the Mississippi. Should the hypothetical glacier re- 

 ferred to advance until it occupied all of the Winnepeg basin, the lake 

 about its southern margin would be obliterated, and there w onld be free 

 drainage to the Gulf of Mexico. Should the glacier then retreat to the 

 north of the divide now separating the waters flowing southward to the 

 Gulf of ]Mexico from those flowing northward to Hudson bay, a lake would 

 be born about the margin of the ice, and would increase northAvard as the 

 ice retreated. When a channel leading nortliAvard was uncovered and 

 rendered available as an outlet for the lake, the ponded waters would have 

 their level lowered and their area contracted. 



The study of the Pleistocene records in the Red River valley and 

 thence northward in jNLanitoba, has shown that changes very similar to 

 those postulated al)Ove actually occurred. 



The evidence of the former existence of a large lake in the Red River 

 valley Avas observed as far back as 1823 by Keating, the geologist of the 

 first scientific expedition to that region. Subsequent contril)utions to this 

 investigation have been made by several observers, and nota1)lv by 



' .r. W. Dawson, "The Canadian Ice Age," Montreal, IS'.i;]; ,1. \V. SiK'ncer, "'riii' De- 

 formation of Iroquois IJeach and Birtli of Lake Ontario," in Am. .lour. Sei., ser. ."i, vol. 4O, 

 1890, pp. 44;]-4.')l ; .T. \V. Spencer, "Deformation of tlie Algoiuiuin lieach and the Birth of 

 Lake Huron," in Am. Jour. 8ci., ser. .'J, vol. 41, 1891, pj). 12-21 ; .7. W. Spencer, "I'osl- 

 I'lei.stocene Subsidence versus Glacial Dams," in (Jeol. Soc. Am. Bull., vol. 2, 18!)1, jip. 

 4(55-474. 



