ANCIENT GLACIERS. 113 



Throughout the Tertiary period a warm climate had 

 prevailed over British America, Greenland, and indeed 

 over all the lands in proximity to the north pole, so far as 

 explorers have been able to penetrate them. The vege- 

 tation characterizing these regions during the Tertiary 

 period indicates a temperature about like that which now 

 prevails in North Carolina and Virginia. Whatever may 

 be said in support of the theory that the Glacial period 

 was produced by astronomical causes, in view of present 

 facts those causes cannot be regarded as predominant ; at 

 most they were only co-operative. The predominant 

 cause of the Glacial period was probably a late Tertiary 

 or post-Tertiary elevation of the northern part of the 

 continents, accompanied with a subsidence in the cen- 

 tral portion. Of such a subsidence in the Isthmus of 

 Panama indications are thought to be afforded by the 

 occurrence of late Tertiary or, more probably, post-Terti- 

 ary sea-shells at a considerable elevation on the divide 

 along the Isthmus of Panama, between the Atlantic and 

 Pacific Oceans. Of this we shall speak more fully in a 

 later chapter. 



Fixing our thoughts upon what is known as the Lau- 

 ren tian plateau, which, though now in the neighbourhood 

 of but two thousand feet above the sea, was then much 

 higher, we can easily depict in imagination the begin- 

 nings of the great " Laurentide Glacier," which eventually 

 extended to the margin already delineated on the south 

 and southwest in the United States, and spread north- 

 ward and eastward over an undetermined area. Year 

 after year and century after century the accumulating 

 snows over this elevated region consolidated into glacial 

 ice and slowly pushed outward the surplus reservoirs of 

 cold. For a long time this process of ice-accumulation 

 may have been accompanied by the continued elevation 

 of the land, which, together with the natural effect of the 

 enlarging area of ice and snow, would tend to lower the 



