20 



LAND MAMMALS IN THE WESTERN HEMISPHERE 



Tertiary period 



Pliocene epoch 



Miocene epoch 



Oligocene epoch 



Eocene epoch 



Monte Hermoso age 



Catamarca age 



Parana age 



Santa Cruz age 



Patagonian age 



Deseado age {Pyrothe- 

 rium Beds) 



Astraponotus Beds 

 I Casa Mayor age {Noto- 

 [ stylops Beds) 



The Pleistocene and Pliocene deposits are most widely dis- 

 tributed over the Pampas of Argentina, but the former occur 

 also in Ecuador, Brazil, Chili, and Bolivia. The other forma- 

 tions cover extensive areas in Patagonia, and some extend into 

 Tierra del Fuego. 



We have next to consider the methods by which past geo- 

 graphical conditions may be ascertained, a task which, though 

 beset with difficulties, is very far from being a hopeless under- 

 taking. As has already been pointed out, it is perfectly 

 possible for the geologist to determine the circumstances of 

 formation of the various kinds of rocks, to distinguish terrestrial 

 from aquatic accumulations and, among the latter, to identify 

 those which were laid down in the sea and those which were 

 formed in some other body of water. By platting on a map 

 all the marine rocks of a given geological date, an approximate 

 estimate may be formed as to the extension of the sea over the 

 present land for that particular epoch. It is obvious, however, 

 that for those areas which then were land and now are covered 

 by the sea, no such direct evidence can be obtained, and only 

 indirect means of ascertaining the former land-connections 

 can be employed. It is in the treatment of this indirect evi- 

 dence that the greatest differences of opinion arise and, if two 

 maps of the same continent for the same epoch, by separate 

 authors, be compared, it will be seen that the greatest dis- 



