496 GEOLOGY. 



ing ice well to the northward in the intervening stages of deglaciation. 

 Whether there was intermigration with Eurasia by the northeastern or 

 northwestern routes during the interglacial intervals, is not positively 

 determined, but it is not improbable. The great proboscidians, the 

 mammoth and mastodon, and the bear, bison, reindeer, and musk-ox, 

 were characteristic members of this group. With these, in the mid- 

 latitudes, were mingled several types on the verge of extinction in 

 North America, such as the horse, tapir, llama, and sabre-tooth cat. 



A second prominent feature was a southern group consisting of 

 gigantic sloths, armadillos, and water-hogs, whose forebears had 

 come from South America when the isthmian route had been opened 

 in the Pliocene. There is perhaps room for question whether these 

 southern giants ever lived in the mid-latitudes after the first ice inva- 

 sion, though remains referred to the Pleistocene have been found as 

 far north as Pennsylvania and Oregon. If these really fall within 

 the glacial period proper, there must have been a northern migration 

 in some one or more of the mild interglacial epochs. 



The boreal group. — As in the Pliocene, the proboscidians dominated 

 the fields and forests in mid-latitudes. A leading form was the 

 mammoth (Elephas primigenius or columbi) which ranged from the 

 southern states and Mexico northward probably to a fluctuating line 

 determined by the stages of glaciation. In interglacial stages, and 

 at the close of the glacial period, it seems to have ranged far to 

 the north, for remains have been found in Canada and Alaska. Siberian 

 species which have been kept in cold storage in underground ice or 

 frozen earth, show that the mammoth, there at least, was covered 

 with wool and hair and was obviously adapted to a cold climate. It 

 is not improbable that the southward range to Mexico represents the 

 mammoth's exceptional migration in front of the ice invasions rather 

 than a permanent occupancy of such low latitudes, for the mam- 

 moth is said to have been limited in its southerly range in Europe. 

 The Elephas survived the glacial period in America, and its tusks and 

 skeletons are not infrequently found in beds of peat and muck that 

 have accumulated in the shallow basins on the surface of the late Wis- 

 consin drift, in the northern United States and Canada, indicating 

 its presence there some time after the ice left the country finally. 



The mastodon also ranged widely over the Northern States and 

 into Canada, as well as southward into the Southern States. Not 



