236 GEOLOGICAL SUCCESSION OF ANIMALS. 



their previous limits. It was this ice, either floating like ice- 

 bergs, 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 frag- 

 ments brought from distant localities, which we find every 

 where scattered about upon the soil, and which are known 

 under the name of erratics, boulders, or graylieads. 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 sub- 

 sidence of the continents. It is not until this period that 

 we find, in the deposits known as the diluvial or pleistocene 

 formation, incontestable traces of the species of animals 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 conclude 

 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- 



