GEOLOGY. 255 



upon the rate of tiow of Muir glacier, which emboiicbes into the liead 

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

 stream is about 3,o00,(K)0 square feet (5y()00 feet wide by about 700 feet 

 deep), and the mean flow is about 40 feet i)er day (70 feet in the cen- 

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

 glacier is uow retreating, but it has evidently oscillated considerably 

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

 and buried beneath its ground moraine and aqueoglacial 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 hypothetic ice-caps, first, ui)on the volume 

 of the oceans from which thej' 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 temperate conviction 

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

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

 called by Lyell). This view has recently rec.eived strong support from 

 a new quarter: Within the last decade the Canadian geologists have 

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

 tend from the Laurentide plateau southwesterly toward Lake Superior 

 and the Ked River, westerly toward Lake Winnipeg, and northerly and 

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

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

 northeastern United States was overflowed. These Canadian obser- 

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

 tled forever the fate of the ice-cap hypothesis. During the past two 

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

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

 Geological Survey of Canada have ascertained that the stria? of the 

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

 ern Rockies 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 

 rejecLod in Canada. 



The variouK American observations upon glacial strife have just been 

 assembled, gri^phically depicted upon a remarkably instructive nuip, 

 and discussed at length by the foremost living authority upon glacial 



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

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



