244 BIOLOGICAL LECTURES. 



rado. They overlie the strata of the upper Bridger stage 

 (Washakie substage) and clearly precede the White River in 

 time, though not separated from the latter by any great interval. 

 Ordinarily the Uinta beds have been called Upper Eocene, but 

 it would perhaps be better to refer them to the Lower Oligo- 

 cene, for they are to be correlated with the Paris Gypsum, 

 which the best French opinion now refers to the base of the 

 Oligocene. It will be convenient to repeat here a part of the 

 table of American fresh-water Tertiary formations given in a 

 previous volume of these lectures. 



fjohn Day. 



Oligocene J White River. 

 [ Uinta. 



Eocene ^ 



f Bridger. 



Wasatch. 



Torre jon. 

 I Puerco. 



The correlation of the Uinta with the Paris Gypsum is made in 

 spite of very marked differences in the mammals which are 

 found in the two regions, for the differences are geographical 

 rather than geological, and are doubtless to be explained by the 

 existence of barriers which made migration between the two 

 continents difficult, though not impossible. In the Wasatch 

 the mammalian faunas of Europe and North America were 

 remarkably similar and clearly indicate that the two continents 

 were connected by land areas, which allowed the freest migra- 

 tion between one region and the other. In the Bridger there 

 appear to have been some obstacles raised in , the way of this 

 free interchange of terrestrial mammals between the northern 

 continents, though the correlative fauna of Europe is still too 

 incompletely known to show the exact amount of difference. In 

 the Uinta, however, the materials for comparison are abundant 

 and prove that intermigration was opposed by such difficulties 

 that only a few forms were able to overcome them. By White 

 River times these obstacles, of whatever nature they may have 

 been, were removed and once more the mammalian faunas of 

 the two continents contained a large number of genera common 



