4.16 GEOLOGICAL SUCCESSION OF ANIMALS. 



became much colder at the end of this period, causing the 

 polar glaciers to advance south, much beyond their previous 

 limits. It was this ice, either floating as 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 to- 

 wards the south, rounded and polished the hardest rocks, and 

 deposited the numerous detached fragments brought from dis- 

 tant localities, which we find everywhere scattered about upon 

 the soil, and which are known under the name of erratics, 

 boulders, or grey heads. This phase of the earth's history 

 has been called, by geologists, the Glacial or Drift period, 

 and is represented by the second circle of the frontispiece. 



685. 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 subsi- 

 dence 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. 



686. 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 or- 

 ganization, it is impossible that they could have migrated 

 from other countries, we conclude that they were created at a 

 more recent period than our marine animals. 



687. Among the 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 Masto- 

 don,* the remains of which are found in the uppermost strata of 

 the earth's surface, and probably the very last large animal which 



* The gallery of fossil remains in the British Museum contains a fine 

 skeleton of the Mastodon, a splendid specimen of which, disinterred at 

 Newburg, N. Y., is now in the possession of Dr. J. C. Warren, in Boston ; 

 the most complete skeleton which has ever been discovered. It stands 

 nearly twelve feet in height, the tusks are fourteen feet in length and 

 nearly every bone is present, in a state of preservation truly wonderful. 



