ANCIENT GLACIERS. 121 



Illinois and Indiana ; thus accounting for the extensive 

 fine silt which has done so much over that region to 

 obscure the glacial phenomena. 



West of the Rocky Mountains. 



The glacial phenomena in the United States west of 

 the Rocky Mountains must be treated separately, since 

 American geologists have ceased to speak of an all-per- 

 vading ice-cap extending from the north pole. But, as 

 already said, the glaciation of North America has proceed- 

 ed from two definite centres of ice-accumulation, one of 

 which we have been considering in the pages immediately 

 preceding. The great centre of glacial dispersion east of 

 the Kocky Mountains is the region south of Hudson Bay, 

 and the vast ice-field spreading out from that centre is 

 appropriately named the Laurentide Glacier. The move- 

 ment of ice in this glacial system was outward in all 

 directions from the Laurentian hills, and extended west 

 several hundred miles, well on towards the eastern foot of 

 the Kocky Mountains. 



The second great centre of glacial dispersion occupies 

 the vast Cordilleran region of British Columbia, reaching 

 from the Rocky Mountains on the northeast to the Coast 

 Kange of the Pacific on the southwest, a width of four 

 hundred miles. The length is estimated by Dr. Dawson 

 to be twelve hundred miles. The principal centre of ice- 

 accumulation lies between the fifty-fifth and the fifty- 

 ninth parallel. From this centre the movement was in 

 all directions, but chiefly to the northwest and to the 

 south. The movement of the Cordilleran glaciers ex- 

 tended northwest to a distance of three hundred and 

 fifty miles, leaving their moraines far down in the Yukon 

 Valley on the Lewes and Pelly Rivers.* Southward the 



* See George M. Dawson, in Science, vol. xi, 1888, p. 186, and 

 American Geologist, September, 1890, pp. 153-162. 



