CHAPTER XX. 



THE HUMAN OR PRESENT PERIOD. 



The end of the Glacial period. — The termination of the Pleisto- 

 cene or Glacial period is usually placed at the time when the ice-sheets 

 disappeared from the lowlands in the middle latitudes of Europe and 

 North America. Notwithstanding this conventional usage, it is to be 

 noted that the ice-sheets had not then completely disappeared, and 

 have not even now, for about 10% of the recently glaciated area of 

 North America is still buried in ice. This lies chiefly in Greenland, 

 the most central and northerly of the areas of glacial radiation in the 

 permanent low-pressure area of the North Atlantic, and subordinately 

 in Alaska, in the northeastern portion of the permanent North Pacific 

 " low." These lingering residues of the last Glacial epoch signalize the 

 fact that a complete emergence from the characteristic features of the 

 Glacial period has not been reached. 



If the speculative conception that the deep-sea circulation was 

 actuated by evaporation in the low latitudes during most of the geo- 

 logical periods, and that this circulation was reversed by low polar 

 temperatures in the glacial periods only, be true, the reversal in this 

 circulation, when it shall take place, will constitute the really radical 

 limit of the Glacial period; for when dense, warm, saline waters shall 

 occupy the depths of the ocean, and emerge in the high latitudes, 

 giving them mild climates, glacial conditions will have disappeared 

 most effectually, and typical warm uniform climates, such as affected 

 most geological periods, will have returned. This is, of course, hypo- 

 thetical, and has its chief value perhaps in loosening the hold of our 

 too-fixed presumption that the present atmospheric and oceanic con- 

 ditions are normal for a late stage of the planet's history. 



Future glaciation. — It is not absolutely clear that there may not 

 be another recrudescence of glaciation before this series closes, but 



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