SHORT DURATION OF LAKE AGASSIZ. 241 



proportion between them being surely not less than ten to one ; and Lake 

 ^Michigan has a similarly greater amount of beach deposits, which upon 

 a large area about its south end are raised by the wind in conspicuous 

 dunes. This contrast, indeed, suggests that the duration of Lake Agassiz 

 and the recession of the ice-sheet from Lake Traverse to the lower part 

 of the Nelson River may have been included within less than one thousand 

 years. 



Likewise, as Mr. Tyrrell remarks, beach ridges of larger size tnan 

 those of Lake Agassiz, and composed of coarser shingle, occur on each of 

 the three great lakes of Manitoba, although these lakes are far smaller than 

 their glacial predecessor and therefore surely have less powerful waves. 



Comparison with Lakes Bonneville and Laliontan. — During the first high 

 stage of Lake Bonneville, a fine, laminated, olive-gray clay, which weathers 

 to a pale-yellow color, was spread throughout all the lower parts of its 

 basin, ascending also in the shallower bays toward the upper shore-lines. 

 In two typical deep sections this member of the lacustrine sediments has 

 an exposed thickness, respectively, of 90 and 100 feet, but its base is not 

 seen. Again, during the second rise of this lake it deposited a similarly 

 widespread stratum of light-gray or cream-colored marl or calcareous clay, 

 weathering nearly white, and passing upward into a fine sand; and typical 

 sections show that this marl and associated sand range from 20 to 50 feet 

 or more in thickness. In like manner, Lake Lahontan during its two high- 

 water periods deposited over all its bed fine marly clays, which in the earlier 

 flood stage attained a thickness of more than 100 feet, their base not being 

 exposed by the deepest sections, and in the later stage an average of 60 to 

 75 feet. These thick sediments occupying- the basins of the Pleistocene 

 lakes of Utah and Nevada indicate, like their great amount of shore ero- 

 sion and correspondingly massive beach deposits, that the term of existence 

 of these lakes during each of their high stages, and especially the first, far 

 exceeded that of Lake Agassiz. No such lacustrine beds are generally 

 spread over the basin of this glacial lake, which upon large tracts, even of 

 its lower portion, as on and near the Red River, near the lower Assiniboine 

 between Poplar Point and Winnipeg, and adjacent to Lake Manitoba, Shoal 

 Lake, and Lake Winnipeg, consists of till, with frequent bowldei's, the 

 MON XXV 16 



