and Deposits of Flooded Rivers. 43 



orologic objections of Woeikof, but also from the recurrence 

 of intense glacial conditions during the second and third 

 epochs of glaciation, long after the earth's eccentricity had 

 diminished to its present long minimum stage. 



The western margin of the drift spread upon the northern 

 part of the great plains, in Alberta, belonging to the first 

 Glacial epoch, overlies moraines of the Rocky Mountain gla- 

 ciers ; and it seems most probable that this earliest epoch of 

 glaciation brought the maximum extension and thickness of 

 ice in British Columbia, where Dr. Dawson finds that it over- 

 topped mountains from 5,000 to 7,200 feet above the sea.* 

 Continuous land-ice, broken only by the projecting highest 

 Cordilleran ranges, then extended, if my interpretation of the 

 origin of the drift deposits of our interior area is true, from 

 New England, Newfoundland and Labrador, to Vancouver 

 Island, the upper part of the Yukon basin, and the Arctic Sea 

 east of the Mackenzie River. But the division of this ice- 

 covered area by the main Rocky Mountain range, rising, at 

 least in portions of its course, above the mer de glace, and 

 containing glaciers in its alpine valleys, may well be recog- 

 nized by the names Laurentide and Cordilleran, proposed by 

 Dr. Dawson respectively for the ice-sheet of the northeastern 

 part of our continent and for that covering British Columbia. 

 The thickness of the Laurentide ice-sheet of the first Glacial 

 epoch, along its belt of maximum development, was probably 

 3,000 to 6,000 feet over central Newfoundland and Labrador, 

 increasing to 10,000 or 12,000 feet on the Laurentide highlands 

 and in the basin of James Bay and over the south part of 

 Hudson Bay, but thence decreasing to 8,000 or 7,000 feet in 

 the region of Reindeer and Winnipeg Lakes, and farther west 

 to only 2,000 or 1,500 feet at the Cypress and Sweet Grass 

 hills ; while the central part of the Cordilleran ice-sheet, 

 according to Dawson, was from 2,000 to 6,000 feet thick, lying 

 on a very uneven mountainous country. I have estimated the 

 area of North America covered by these ice-sheets as about 

 4,000,000 square miles, and the probable average thickness of 

 the ice about 3,600 feet, or two-thirds of a mile.f 



The ice-sheet of the second Glacial epoch had an ascending 

 slope of twenty to thirty feet per mile from its southern 

 border to Mt. Katahdin, the White Mountains, and the Adiron- 

 dacks, where its altitude was about one mile above the present 

 sea level, with a thickness of 4,000 to 5,000 feet above the 

 land surface. Its greatest thickness, estimated by Dana to 

 have been 11,000 feet, was on the Laurentide highlands, be- 



* Geological Magazine. Ill, vol. vi, pp. 350-2, Aug., 1889. Am. Geologist, 

 vol. vi, pp. 153-162, Sept., 1890. 



f The Ice Age in North America, p. 579. Am. Geologist, vol. iv, pp. 165-174 

 and 205-216, Sept. and Oct., 1889. 



