526 THE PLEISTOCENE EPOCH 



advance and retreat, yet never entirely disappeared, or whether 

 there were several Glacial and Interglacial ages, when the ice alter- 

 nately advanced and was completely melted away. There is much 

 to be said on both sides of this question, but the present tendency 

 among students of the Glacial epoch is to favour several distinct 

 Glacial and Interglacial ages, one of the strongest arguments for 

 which is the evidence given by the fossils of the return of mild 

 and even warm climates. Professor James Geikie accepts no 

 less than six Glacial and five Interglacial stages for Europe, and 

 Professor Chamberlin finds evidence for five Glacial and four Inter- 

 glacial stages in North America. 



American. — At the times of great expansion the ice-sheets 

 covered nearly all of North America down to latitude 40 N., an- 

 ticipating the conditions of modern Greenland, though on a vastly 

 larger scale. Three distinct centres or areas of maximum accumu- 

 lation of the ice have been identified in northern Canada, from 

 which the great ice-sheets flowed outward in all directions, though 

 each one of the sheets had its own episodes of advance and retreat, 

 so that the same region of country was overflowed, now by exten- 

 sions from one sheet, and again by those from another. One of 

 these centres of accumulation and distribution lay to the north of 

 the St. Lawrence River, and on the highlands of Labrador, send- 

 ing its ice-mantle southward over the Maritime Provinces, New 

 England, and the Middle States, as far west as the Mississippi 

 River. This is called the Laurentide Ice-sheet or Glacier. A 

 second centre was near the west coast of Hudson Bay, and from 

 this area the ice streamed outward in all directions westward 

 toward the Rocky Mountains, northward to the Arctic Ocean, 

 eastward into Hudson Bay, southward through Manitoba into the 

 Dakotas, Minnesota, and Iowa. This great ice-sheet has been 

 named the Keewatin Glacier, from the Canadian district of that 

 name. A third centre was formed by the Cordillera of British 

 Columbia, which for a distance of 1200 miles was buried under 

 a great ice-mantle that flowed both to the northwestward and 

 southeastward. The thickness of the ice in these vast flows was 

 very great ; over New England, the scorings on the mountain sides 



