followed by an Arctic ice cap which, according to the Canadian 

 glaciologists, originated in North America from three distinct 

 centers of maximum accumulation and flowed outwards in all 

 directions. "One of these centers of maximum accumulation 

 and distribution lay to the north of the St. Lawrence river, and 

 on the highlands of Labrador, sending its ice-mantle southward 

 over the Maritime Provinces, New England, and the Middle 

 States, as far west as the Mississippi river." This ice sheet is 

 known as the Laurentide glacier. 



As this Laurentide ice-sheet advanced conditions of extreme 

 cold were felt far beyond its edge. Thus the loftier mountain? 

 of New England, Washington and Ktaadn, feeling its chilling 

 influence, became centers of ice accumulation. These mountain 

 floras were consequently early forced down the mountain slopes 

 into the plains below, uniting with the migrating Arctic forms 

 from the far north. The nature of those pre-glacial alpine 

 species is mere conjecture. However, they, as well as the accom- 

 panying lowland forms, doubtless exerted a modifying influence 

 on the Arctic species. Yet the fact of their migration in unison 

 precludes any marked modification of their mutual relations. 



From these mountain centers, as general glaciation advanced, 

 extensive ice-sheets flowed out in all directions, coalescing with 

 each other and finally with the Laurentide glacier, and, united, 

 advanced over New England even to the sea. This ice sheet 

 was thousands of feet in thickness at its maximum, no mountain 

 peaks, with possibly the exception of Mt. Washington, rising 

 above the vast mer de glace. Before this all life retreated, many 

 species of plants doubtless to the sea and extermination. This 

 advance continued ; the ice-sheet reaching at its greatest develop- 

 ment to latitude 40, about the middle of New Jersey. At this 

 time our Arctic flora was doubtless enjoying a congenial climate 

 along the Gulf. 



After an extended period of this general glaciation, warmth 

 gradually returned, according to Prof. Chamberlin, by the 

 re-establishment of the CO 2 blanket, thus restricting radiation. 

 With this return the ice-sheet gradually retreated, closely fol- 

 lowed by the Arctic life. This retreat in New England was pre- 

 sumably one of continuity, yet in Wisconsin there is strong evi- 

 dence of five glacial and four interglacial stages, representing 



13 



