110 LAND MAMMALS IN THE WESTERN HEMISPHERE 



The third of the Eocene stages of the interior is the Bridger 

 of southern Wyoming and northeastern Utah, where it lies 

 upon the Green River shales, but overlaps these shales both 

 eastward and westward, extending out upon the Wasatch. 

 The Bridger beds are largely made up of volcanic ash and dust 

 deposited partly upon the land and partly in shallow or tem- 

 porary lakes. The frequency with which the remains of fishes, 

 crocodiles and fresh-water shells are found indicates deposition 

 in water, and the large crystals of gypsum which are abundant 

 in certain localities show that the water became salt, at least 

 occasionally. From the immense mass of volcanic debris, it 

 is evident that volcanic activity broke out at this time on a 

 much greater scale than had been known in that region since 

 the Cretaceous period. Two different horizons, or substages, 

 are distinguishable in the Bridger, lower and upper, each of 

 which has its distinct mammalian fauna, though the two are 

 very closely allied. Their difference from the contemporary 

 mammals of Europe is very great, hardly any genera being 

 common to the two continents. So striking a difference in- 

 dubitably points to a severance of the land-connection, a sever- 

 ance which, as was shown above, probably took place during 

 the Wind River stage, for its effects would not be immediately 

 apparent ; time would be required for the operation of diver- 

 gent evolution, the fauna of each continent developing along 

 its own lines, to make itself so strongly felt. Had the connec- 

 tion never been renewed. North America, on the one hand, and 

 Eurasia on the other, would to-day be utterly different from the 

 zoological point of view, instead of containing, as they do, 

 a great many identical or closely similar animals of all classes, 

 a likeness due to subsequent migrations. 



The fourth and last of the stages referred to the Eocene 

 is the Uinta, the geological position of which is the subject 

 of much debate; almost as good reasons can be brought 

 forward for placing it in the Oligocene as in the Eocene, so 

 nearly is it on the boundary line between those two epochs. 



