BEITISH COLUMBIA AND LAKE YUKON. 207 



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 interpre- 

 tation of their origin, however, is by a marine submergence since the 

 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 Bay, and in 

 Greenland and Grinnell Land ; but lakes of only moderate size tempora- 

 rily bordering the ice-sheet during its departure would probably be desti- 

 tute of life, and this Avould 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 Stikiue, 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 10 miles and depth of bet\'\een 

 500 and 600 feet ; and he suggests that this lake was probably caused by 

 a depression 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 northwestern 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 striae, which a" e first found in ascending the 

 Yukon near the Rink Rapids, approximately in latitude 62° 20' north and 

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

 America and Alaska." 



No other portion of the glaciated area of this continent presents a 

 more interesting or more difficult problem in Pleistocene geology than 

 these "white silts," as they are denominated by Dawson; and much fur- 

 ther field work and study will be needed to demonstrate the conditions of 



'Reports of the Geol. and Nat. Hist. Survey of Canada; Trans. Royal Society of Canada, Vol. 

 VIII. sec. 4, 1890, pp. 3-74, with five maps; Am. Geologist, Vol. VI, Sept., 1890, pp. 153-162; Nature, 

 Vol. XLII, Oct. 30, 1890, pp. 650-6.53. 



- Bulletin 0. S. A., Vol. 1, pp. 140, 146-148, 544. 



