PHYSICAL CONDITIONS OF THE GLACIAL PERIOD. 300 



42,000 years ; but it was probably much slower than this on an average ami 

 there must have been a long stationary period when these beaches were 

 forming, so that the estimate of Dr. James Croll, Dr. James Geikie and 

 others of 80,000 years as the time which has elapsed since the glacial period 

 in Great Britain and the inhabited parts of North America need not be con- 

 sidered excessive. 



The Cause of Glaciation. — In regard to the formation of the vast quanti- 

 ties of land-ice of the glacial period, it is a common error to suppose that its 

 accumulation was due to intense cold alone. The production of glaciers was 

 due to the same causes then as now, namely, a warm ocean with high land 

 so situated that the air coming from the water laden with moisture might pass 

 over the cold land and precipitate the vapor upon it in the form of snow. 

 There is reason for believing that the Laurentian area of Canada and the 

 northern part of the Appalachian region were much higher in the glacial 

 period than now. The great precipitation of snow which took place over 

 these areas may have been due to an extension at that time of the Gulf of 

 Mexico over part of the Mississippi valley. The Gulf Stream, perhaps of 

 greater volume then than now, would eddy round the enlarged Gulf, giving 

 an immense evaporating surface, and, passing round the southern part of the 

 Appalachian range, would flow northwestward close to this continent, being 

 protected from the Arctic current by the dry land which would take the 

 place of the now submerged banks of Newfoundland. If the weather circles 

 or ellipses travelled in courses corresponding to those which prevail at the 

 present time, we should thus have the most favorable conditions for the 

 rapid accumulation of ice all over the area which has been glaciated. 



The Causes of Changes in Level. — What caused the depression of the land 

 at the close of the drift period ? The suggestion that it may have been due 

 to the weight of the ice itself bending down the crust of the earth is worthy 

 of consideration, although this explanation would be more obvious had the 

 depression taken place while the weight was upon it, and not after its 

 removal. The subsequent elevation, which is still going on, may be the 

 slow return of the outline of this part of the earth's surface to its normal 

 curve. It is generally accepted that ice acts as a semi-fluid, and therefore 

 it must be subject to hydrostatic laws. Many facts in geology go to show 

 that rocks, too, on a large scale have manifested a sort of plasticity without 

 having undergone igneous softening. May not the whole globe of the earth 

 slowly follow these laws, even if its interior be not in a liquid condition? 

 The slightest sensible pressure on any part of its surface would be followed 

 by an effort to regain its perfect equilibrium. But the elevation of the land 

 above the general level of the ocean, which is still in progress in north polar 

 regions, may be something more than a mere upheaval of part of the crust 

 of the earth. It may be, as Dr. Croll supposed, an actual retiring of the 



