E. MINERALOGY AND GEOLOGY. 91 



iron ages, since it lived in the Hyrcanian Forest in the clays 

 of Julius Caesar. For the truth of M. Lartet's classification, 

 it was considered essential to show that these animals in- 

 vaded Europe in a definite succession ; and as evidence, of 

 this is wanting in the present state of our knowledge, it fol- 

 lows that the* chronological value of M. Lartet's classification 

 must be regarded as inadmissible. 18 A, August 25, 562. 



PHYSICAL GEOGEAPHY OF NOETH AMEEICA IX THE PLIOCENE 



PEEIOD. 



In a review by Mr. Boyd Dawkins of Professor Leidy's re- 

 cent great work on the fossil mammals of North America, 

 while discussing the distribution of animal life in America 

 during the pliocene period, he shows that it furnishes impor- 

 tant information in regard to the physical geography of the 

 continent at that period. Thus the absence of edentata, as 

 well as of the opossum, and of the South American forms of 

 rodents, implies that North America was separated from 

 South America by an impassable barrier this, of course, 

 being water. At that time the Isthmus of Panama probably 

 did not exist, so as to form a bridge connecting the two 

 lands, over which animals could cross. On the other hand, 

 however, the genera belonging to the basin of the Upper Mis- 

 souri indicate an unmistakable inroad of animal forms from 

 some other region. Thus the deer, the mastodon, the ele- 

 phant, the hipparion, and the horse, together with the wolf, 

 could only have been' driven from Europe and Asia, with 

 which there was evidently a connection during both the j3li- 

 ocene and miocene epochs. During the quaternary period 

 this separation from South America no longer existed, 'and 

 the South American forms seem to. extend northward to a 

 considerable distance in North America, thus showing the 

 period of the elevation of the Isthmus of Panama to ha-ve 

 been a portion of the post-pliocene or quaternary period. 

 With all this, however, no barrier seems to have existed be- 

 tween North America and what we now call the Old World, 

 since many forms continued to be common to both, such as 

 the bisons, horses, moose, mammoth, musk-ox, etc. ; and we 

 are therefore entitled to assume that North America was 

 separated from Asia at Behring's Straits during a compara- 

 tively recent period. From the evidence adduced in Dr. 



