METHODS — GEOLOGICAL 19 



it is practicable to determine whether the way of intermigra- 

 tion was open or closed, because separation always led to greater 

 differences between the faunas of the two continents through 

 divergent evolution. 



Correlation with South America is exceedingly difficult and 

 it is in dealing with this problem that the widest differences of 

 opinion have arisen among geologists. Through nearly all 

 the earlier half of the Tertiary period the two Americas were 

 separated and, because of this separation, their land mammals 

 were utterly different. Hence, the lack of elements common 

 to both continents puts great obstacles in the way of establish- 

 ing definite time-relations between their geological divisions. 

 Only the marine mammals, whales and dolphins, were so far 

 alike as to offer some satisfactory basis of comparison. When, 

 in the later Tertiary, a land-connection was established between 

 the two continents, migrations of mammals from each to the 

 other began, and thenceforward there were always certain 

 elements common to both, as there are to-day. In spite of the 

 continuous land between them, the present faunas of North and 

 South America are very strikingly different. South America 

 being, with the exception of AustraUa, zoologically the most 

 peculiar region of the earth. 



In the following table of the South American Cenozoic, the 

 assignment of the ages to their epochs is largely tentative, 

 especially as regards the more ancient divisions, and repre- 

 sents the views generally held by the geologists of Europe and 

 the United States ; those of South America, on the contrary, 

 give an earlier date to the ages and stages and refer the older 

 ones to the Cretaceous instead of the Tertiary. 



CENOZOIC ERA (South America) 

 Quaternary period 



Recent epoch 



Pleistocene epoch — Pampean Beds, 

 BraziUan caverns 



