204 GEOLOGICAL SUCCESSION OF ANIMALS. 



like icebergs, or, as there is still more reason to believe, 

 moving along the ground, like the glaciers of the present 

 day, that, in its movement towards the South, rounded and 

 polished the hardest rocks, and deposited the numerous 

 detached fragments brought from distant localities, which 

 we find everywhere scattered about upon the soil, and 

 which are known under the name of erratics, boulders, or 

 greyheads. This phase of the earth's history has been 

 called, by geologists, the Glacial or Drift period. 



496. After the ice that carried the erratics had melted 

 away, the surface of North America and the North of Europe 

 was covered by the sea, in consequence of the general 

 subsidence of the continents. It is not until this period 

 that we find, in the deposits known as the diluvial or pleis- 

 tocene formation, incontestable traces of the species of ani- 

 mals now living. 



497. It seems, from the latest researches of Geologists, 

 that the animals belonging to this period are exclusively 

 marine ; for, as the northern part of both continents was 

 covered to a great depth with water, and only the summits 

 of the mountains were elevated above it, as islands, there 

 was no place in our latitudes where land or fresh-water 

 animals could exist. They appeared therefore at a later 

 period, after the water had again retreated ; and, as from 

 the nature of their organization, it is impossible that they 

 should have migrated from other countries, we must con- 

 clude that they were created at a more recent period than 

 our marine animals. 



498. Among these land animals which then made their 

 appearance, there were representatives of all the genera 

 and species now living around us, and besides these, many 

 types now extinct, some of them of a gigantic size, such as 

 the Mastodon, the remains of which are found in the upper- 

 most strata of the earth's surface, and probably the very 



