108 LAND MAMMALS IN THE WESTERN HEMISPHERE 



western Colorado and eastern Utah to the Uinta Mountains, 

 around the eastern end of which it passes in a narrow band and 

 then expands again over southwestern Wyoming. A second 

 area is in the Big Horn Basin of northwestern Wyoming and 

 southern Montana, and probably two small areas in southern 

 Colorado are of the same date. The Wasatch beds are richly 

 fossiliferous and have yielded a most interesting and important 

 series of mammals, which were far more advanced than those 

 of the Paleocene ; and, at first sight, the student is tempted 

 to believe that they must be of very much later date. A more 

 critical examination shows that this appearance of a great 

 lapse of time between the Paleocene and the Wasatch is decep- 

 tive ; the more advanced and characteristic of the Wasatch 

 mammals were obviously not the descendants of ancestors 

 in the North American Paleocene, but were altogether new- 

 comers to this continent, immigrants from some region which 

 cannot yet be identified. On the other hand, a considerable 

 number of the old, indigenous types still persisted, and these, 

 when compared with their Paleocene ancestors, are found not 

 to have changed so much as to require a very great length of 

 time, geologically speaking, for the degree of development 

 involved. This is the earliest recorded one of the great waves 

 of mammalian migration which invaded North America 

 down almost to our own time. 



The same wave of migration extended to Europe, and that 

 there was a broad and easy way of communication between 

 that continent and North America is plain, for the similarity 

 between the Wasatch mammals and those of the corresponding 

 formation in France, the Sparnacian, is remarkably close. 

 At no subsequent time were the mammalian faunas of North 

 America and Europe so nearly identical as during the Wasatch- 

 Sparnacian age, which is especially remarkable when the dis- 

 crepancy is noted between the vast stretches of the Wasatch 

 (150,000 square miles) and the very limited areas in France. 



If, as is probable, the Ural Sea was in existence at that time, 



