o 4 4 



HISTORICAL GEOLOGY 



which joined Europe and Africa, and that at Gibraltar also disap- 

 peared. It seems probable that the separation of Japan and the 

 Philippine archipelago occurred in post-glacial times. 



(2) Glaciation. — The gradual refrigeration of the climate at the 

 close of the Tertiary culminated in the Pleistocene. The cause, or 

 causes, which produced this marked decrease in temperature will be 

 discussed later (p. 660). 



The lowering of the temperature resulted in the accumulation of 

 snow and ice to form great ice sheets, several hundreds to several 

 thousands of feet thick, which spread over 6,000,000 to 8,000,000 

 square miles of the earth's surface, especially in the northern hemi- 

 sphere. Although this is an important event in the world's history, 

 it should be remembered that it is not unique, since extensive glacia- 

 tion occurred in the Permian (p. 505) as well as in Pre-Cambrian times 

 (p. 398), and possibly at other periods. It should also be noted that 

 these earlier ice invasions have not been considered of sufficient im- 

 portance to form a basis for a further subdivision of these earlier 

 periods. Why, then, is the separation into Tertiary and Quaternary 

 made ? It is because the event was so recent (geologically) that 

 the evidences of glaciation are widespread and conspicuous, since 

 sufficient time has not yet elapsed to obliterate them by erosion and 

 weathering, and also because the indirect effect upon man has been 

 of great importance (p. 662). 



(3) Changes in Life. — The least important change between the 

 periods is in the life. As far as this is concerned, the Quaternary 

 might almost equally well be considered a continuation of the Plio- 

 cene. At the beginning of the period nearly all living species of 

 mollusks were in existence, and most of the species of living mammals, 

 but during the Pleistocene there was a gradual disappearance of many 

 mammals, such, for example, as the mammoth, mastodon, woolly 

 rhinoceros, and saber-toothed tiger. After the Tertiary there was no 

 longer a mingling of tropical and subtropical plants with temperate 

 and Arctic plants (p. 634) ; but, apparently as a result of the migra- 

 tions forced on them by the climate, they became adapted to special 

 habitats. 



REFERENCES FOR CHANGES AT THE CLOSE OF THE TERTIARY 



Chambi run and Salisbury, — Geology, Vol. 3, pp. 483-490. 

 Willis and Salisbury, — Outlines of Geologic History, pp. 265-275. 



