GEOLOGY. 255 



uiiou tbe rsito of flow of Muir giiicier, wliicli cTiihoacbes into the liead 

 of the bay, are particularly interesting-. The cross-section of the ice 

 stream is about 8,50(),0()() scjuare feet (5,000 feet wide by about 700 feet 

 deep), and the mean How is about 40 feet per day (70 feet in the cen- 

 ter and 10 feet near the margin) in the month of August. The Muir 

 glacier is iu)W retreating, but it has evidently oscillated considerably 

 during recent times, and in some of its advances it lias encroached upon 

 and buried beneath its ground moraine and aipieoglacial deposits, 

 whole forests of full grown trees, whose remains occasionally appear 

 about the shores of the bay. 



It was an early notion that during glacial times great ice-caps formed 

 about the poles and extended far toward the equator; and during one 

 stage in the development of geologic science mathematicians sought to 

 compute the effects of these hyi)oth«5tic ice-caps, first, u})on the volume 

 of the oceans from which they were drawn, second, upon the isostatic 

 terrestrial crust on which they were heaped, and third, upon the cen- 

 ter of gravity of the earth as a whole. The students of more recent 

 years have, however, settled down to the more temi)erate conviction 

 that great polar Icecaps never existed, and that during the glacial pe- 

 riod the ice flowed radially from certain " centers of dispersion " (so 

 called by Lyell). This view nas recently received strong supi)ort from 

 a new (piarter: Within the last decade the Canadian geologists have 

 shown that the striaj which form the unmistakable trail of glaciers ex- 

 tend from the Laureutide plateau southwesterly toward Lake Superior 

 and the Red Kiver, westerly toward Lake Winnipeg, and northerly and 

 even northeasterly toward Hudson's Strait,* proving that these high- 

 lands were the center of dispersion for the mer de glace by which 

 northeastern United States was overflowed. These Canadian obser- 

 vatioiia nave continued until within the last year or two, and have set- 

 tled forever the fate of the ice-cap hypothesis. During the i>ast two 

 years, too, parallel observations have been made in the northwestern 

 part of the American continent: G. M. Dawson and other ollicers of the 

 Geological Survey of Canada Jiave ascertained that the striae of the 

 Mackenzie Valley extend in northerly directions, proving that the north- 

 ern Kockies also formed an independent center of ice dispersion, t Daw- 

 son's publication upon the glacial phenomena of Canada is of special 

 interest to American geologists in that it contains definite recognition 

 o" the glacial theory, essentially in the form long held in America but 

 rcyecLod in Canada. 



The yariua? American observations upon glacial striae have just been 

 assembled, griiphically depi(!ted upon a remarkably instructive map, 

 and discussed at length by the foremost living authority upon glacial 



* Anu. Rep. Geol. Survey of Canada, 1885, p. 14 dd. 

 t Geol. Mag. Decade 3, IbdS, vol. v, pp. 347-50, 



