114 MAN AND THE GLACIAL PERIOD. 



temperature around the margin and to increase still more 

 the central area of accumulation. 



The vigour of movement in any direction was deter- 

 mined partly by the shape of the valleys opening south- 

 ward in which the ice-streams would naturally concen- 

 trate, and partly by those meteorological conditions which 

 determine the extent of snow-fall over the local centres 

 of glacial dispersion. For example, the general map of 

 North America in the Ice period indicates that there 

 were two marked subcentres of dispersion for the great 

 Laurentide Glacier, the eastern one being in Labrador 

 and the western one north of Lake Superior. In a 

 general way the southern boundary of the glaciated re- 

 gion in the United States presents the appearance of por- 

 tions of two circumferences of circles intersecting each 

 other near the eastern end of Lake Erie. These circles, I 

 am inclined to believe, represent the areas over which a 

 semi-fluid (or a substance like ice, which flows like a semi- 

 fluid) would disperse itself from the subcentres above 

 mentioned. 



A study of the contour of the country shows that that 

 also, in a general way, probably had something to do with 

 the lines of dispersion. The western lobe of this glaciated 

 area corresponds in its boundary pretty closely with the 

 Mississippi Valley, having the Ohio Kiver approximately 

 as its eastern arm and the Missouri as its western, with 

 the Mississippi River nearly in its north and south axis. 

 The eastern lobe has its farthest extension in the axis of 

 the Champlain and Hudson Eiver Valleys, its western 

 boundary being thrown more and more northward as the 

 line proceeds to the west over the Alleghany Mountains 

 until reaching the longitude of the eastern end of Lake 

 Erie ; but this southern boundary is by no means a water- 

 level, nor is the contour of the country such that it could 

 ever have been a water-level. But it conforms in nearly 

 every particular to what would be the resultant arising 



