IMPORTANCE OF THE CORDILLERAN " WHITE SILTS." 249 



the underlying silt of the glacial Lake Agassiz, which however in some 

 portions, as near the deltas of the Sheyenne and the Assiniboine, occupies 

 large areas. 



Principal Glacial Lakes of Canada. 



British Columbia, Athabasca, and the Northwest Territory. — Light-colored 

 eilt deposits, distinctly stratified and of considerable thickness, which seem 

 to me referable in some districts to glacial lakes and in others to river-floods 

 supplied from the melting ice-sheet, are reported by Dr. G. M. Dawson in 

 many basins of the British Cordilleran region. His interpretation of their 

 origin, however, is by a marine submergence since tha latest glaciation of 

 the region. No fossils, either of the sea or of fresh water, are found, though 

 they are abundant in postglacial marine beds of the St. Lawrence valley, 

 on the southwestern side of Hudson's bay, and in Greenland and Grinnell 

 land ; but lakes of only' moderate size temporarily bordering the ice-sheet 

 during its departure would probably be destitute of life, and this would 

 certainly be true of rivers produced by the glacial melting. These deposits 

 occur, up to heights 2,300 to 2,700 feet above the sea, in the basin of the 

 Kootanie and upper Columbia, on the interior plateau of British Columbia, 

 on the northward extension of the great plains crossed by the Peace river, 

 and in the upper valleys of the Stikine, Liard, and Yukon rivers.* 



On the last named river and the Lewes, tributary to it, Russell refers the 

 formation of silt beds fully 200 feet thick, and of higher terraces, to a 

 glacial lake, named by him Lake Yukon, 150 miles long from north to 

 south, with a maximum width of about ten miles and depth of between 500 

 and 600 feet ; and he suggests that this lake was probably caused by a de- 

 pression of the upper part of the Yukon basin by the weight of the ice- 

 sheet. The mouth of Lake Yukon, at its northern end, was near the north- 

 western boundary of the ice-sheet at its maximum extension, the whole lake 

 being within the area that was ice-covered, as is known by the limits of 

 glacial drift and strise, which are first found in ascending the Yukon near 

 the Pink rapids, approximately |in latitude 62° 20' north and longitude 

 136° 15' west, about 160 miles east of the line between British America and 

 Alaska.f 



No other portion of the Dominion of Canada presents a more interesting 

 or more difl[icult problem in Quaternary geology than these " White silts," 

 as they are denominated by Dawson ; and much further field-work and 

 study will be needed to demonstrate the conditions of their deposition in 

 each of the numerous basins in which they are found. But I believe that ulti- 



* Reports of the Geological and Natural History Survey of Canada; Trans. Royal Society of 

 Canada, vol. VIII, sec. iv, 1890, pp. 3-74, with five maps; American Geologist, vol, VI, Sept. 1890, 

 pp. 153-162; Nature, vol. XLII, Oct. 30, 1890, pp. 650-653. 



t Bull. Geol. Soc. Am., vol. 1, pp. 140, 146-8, 544. 



