GEOGRAPHICAL DEVELOPMENT OF THE AMERICAS 115 



be traced, filled with the consolidated sands and gravels of 

 the old rivers. The country was very flat and the divides 

 between the streams very low, so that in seasons of flood 

 great regions were converted into shallow, temporary lakes, 

 in which were deposited the finer silt and mud, but were dry 

 for most of the year. The volcanic activity which had gone 

 on so impressively in the Bridger Eocene was renewed in 

 White River times, as is proved by thick beds of pure volcanic 

 ash, which must have been carried long distances by the wind, 

 for they occur far from any volcanic vent. 



The White River fauna is more completely known than 

 that of any other Tertiary formation of this continent. The 

 first discovery of these fossils was made more than 70 years 

 ago and since then oft-repeated expeditions have brought to 

 light an astonishing number and variety of mammals. Not 

 only are these beds remarkable for the immense quantity of 

 material which they have yielded, but also for its complete- 

 ness and beauty of preservation, a most unusual number of 

 skeletons having been obtained. The mammals demonstrate 

 that the land-connection with the Old World had been re- 

 established, for many European genera, which could not have 

 been derived from an American ancestry, are found in the 

 White River beds. At the same time, there was no such 

 proportion of forms common to both continents as there had 

 been in the Wasatch-Sparnacian stage of the lower Eocene, 

 each having many genera and even families which did not 

 extend their range into the other. The reason for this remark- 

 able and, at first sight, inexplicable difference between the 

 lower Eocene and the lower Oligocene is probably to be found 

 in climatic changes, in consequence of which relatively fewer 

 genera were able to take advantage of the reopened connection, 

 which lay far to the north. The White River mammals, like 

 those of the Recent epoch, are thus divisible into two groups 

 or elements, one set indigenous and descended from ancestors 

 which are found in the American Eocene, and the other com- 



