CHAPTER XIX. 



THE PLEISTOCENE OR GLACIAL PERIOD. 



Though it derives its systematic name from the fact that its life 

 constitutes the closing stage of the transition from the great past to 

 the present, the distinguishing feature of the Pleistocene period is 

 its phenomenal glaciation. Ice-sheets spread over six or eight million 

 square miles of the earth's surface where not long before mild climates 

 had prevailed. Were it not for this great ice deployment, and for 

 its profound effects on the conditions under which man has developed, 

 this period would more properly be joined to the Pliocene, the two con- 

 stituting a single period of great land relief and oceanic restriction. 

 The time assigned the Pleistocene is much shorter than that of the 

 average geologic period. It appears that the later periods, as a rule, 

 are shorter than the earlier ones, due to our magnifying the import- 

 ance of events that are near to us. The Pleistocene expresses this 

 more markedly, perhaps, than any other period. The importance 

 of the Pleistocene period has, however, been greatly increased by 

 recent investigations, not only in respect to its length, but also in 

 respect to its diversity and its bearing on human evolution. 



General Distribution of the Glaciation. 



More than half of the area of the Pleistocene glaciation lay in North 

 America, and more than half of the remainder lay in Europe. The 

 glaciation was, therefore, pronouncedly localized, as was that of the 

 Permian period, and probably also that of the still earlier Cambrian 

 or pre-Cambrian. But the whole world felt its effects; even in the 

 tropical regions, glaciation occurred on mountains where it did not 

 exist before and does not now exist, and on mountains now glaciated 

 the ice descended to levels 5000 feet or more below its present limit. 



The southern hemisphere was affected as well as the northern, 



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