POST-PLIOCENE PERIOD. 557 



Missouri region, are strata which are also believed to be refer- 

 able to the Pliocene period, and probably to its upper division. 

 They are from 300 to 400 feet thick, and contain land-shells, 

 with the bones of numerous Mammals, such as Camels, Rhino- 

 ceroses, Mastodons, Elephants, the Horse, Stag, &c. 



LIFE OF THE PLIOCENE PERIOD. As regards the life of 

 the Pliocene period, it is sufficient to indicate two general 

 considerations. In the first place, we have to notice that the 

 introduction upon the globe of existing species of animals was 

 carried on rapidly during this period. In the older Pliocene 

 deposits the number of shells of existing species is only from 

 40 to 60 per cent ; but in the Newer Pliocene the proportion 

 of existing species rises to as much as 80 to 95 per cent. The 

 Mammals still all belong to extinct species, but modern types 

 gradually supersede the more antique forms of the Eocene and 

 Miocene periods. In the second place, there is good evidence 

 to show that the Pliocene period was one in which the climate 

 of the northern hemisphere gradually became colder. In the 

 Miocene period, as we have seen, Europe possessed a climate 

 probably very similar to that now enjoyed by the Southern 

 States of the Union, and certainly very much warmer than its 

 present climate. In the Older Pliocene, northern forms, on 

 the other hand, predominate among the shells, though some 

 of the types of warmer "regions still survive. In the Newer 

 Pliocene, the Mollusca are almost exclusively such as inhabit 

 the seas of temperate or even cold regions. It might be 

 thought that the occurrence of Mammals such as the Elephant, 

 Rhinoceros, and Hippopotamus, would prove that the climate 

 of Europe and the United States must have been a hot one 

 during the later portion of the Pliocene period. We have, 

 however, reason to believe that many of these extinct quadru- 

 peds were more abundantly furnished with hair, and more 

 adapted to withstand a cool temperature, than any of their 

 living congeners. 



Amongst the Pliocene Mammals may be mentioned the 

 following, as comprising the more important forms : 



Mastodon Arvernensis. 

 Elephas meridionalis. 

 Elephas antiquus. 

 Rhinoceros megarhinus. 



Tapirus Arvernensis. 

 Machairodus ctiltridens. 

 Ursus Arvernensis. 

 Equus plicidens. 



Hippopotamus major. 



POST-PLIOCENE PERIOD. 



Later than any of the Tertiary formations are a series of de- 

 posits which are spoken of as Post-Tertiary or Quaternary, 

 and which are characterised by the fact that all the contained 



