CHAPTER V 



THE GEOGRAPHICAL DEVELOPMENT OF THE AMERICAS IN 

 CENOZOIC TIMES 



I. Tertiary Period 



In the interior regions of western North America the transi- 

 tion from the Mesozoic to the Cenozoic was so gradual that 

 there is great difficulty in drawing the line between them and 

 therefore, as might be expected, there is much difference of 

 opinion as to just where that line should be drawn. From 

 one point of view, the matter is of no great consequence ; but 

 from another, it is of the utmost importance, for, unless the 

 events in different continents can be approximately syn- 

 chronized, it will often prove a hopeless undertaking to trace 

 the course of migration of the various mammalian groups and 

 determine their place of origin and primary home. Until 

 a definitive answer can be given to the question as to when 

 the Cenozoic era began, many significant points must be left 

 in doubt, and much remains to be done in the geology of the 

 Far West before that definitive solution can be reached. 



1. Paleocene Epoch 



So far as North America is concerned, the best available 

 evidence points to the conclusion that we should regard the 

 Fort Union, Puerco and Torrejon as the most ancient of the 

 Cenozoic formations (see Table, p. 17), though retaining so 

 many features of Mesozoic life that a separate division of the 

 Tertiary, the Paleocene epoch, is made for them. Such a sepa- 

 ration is not the common practice in this country, where it is 

 more usual to employ the terms "Lowest" or ''Basal" Eocene. 



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