116 LAND MAMMALS IN THE WESTERN HEMISPHERE 



posed of late immigrants from the Old World. Migrants 

 from North America likewise made their way to Europe. 



The upper continental Oligocene of the interior has re- 

 ceived the peculiar appellation of the John Day, from the river 

 of that name in eastern Oregon, a large part of which was 

 buried to a depth of 3000 or 4000 feet in stratified volcanic 

 ash and tuff. This great mass of finely divided volcanic 

 material was derived from the craters of the Cascade Moun- 

 tains to the westward ; a long-continued series of eruptions 

 would be needed to form such thick accumulations at such 

 a distance from the sources of supply. The John Day evi- 

 dently succeeded the White River very closely in time, but 

 is marked by the disappearance of almost all the European 

 migrants. This fact, together with the absence of any new 

 immigrant genera, is evidence that the connection had again 

 been broken and it was not renewed until after a considerable 

 lapse of time. 



There are many reasons for believing that the Oligocene 

 cUmate marked the beginning of the very long and gradual 

 process of refrigeration which culminated in the glacial con- 

 ditions of the Pleistocene epoch, but the change was sUght 

 and probably chiefly affected the far north. The climate, 

 however, remained notably warmer than the present one of 

 the same extra-tropical latitudes, as is abundantly proved by 

 the fossils. The Atlantic coast, as noted above, was bathed 

 in warm waters, the plants of the Alaskan Oligocene point 

 to temperate conditions and the vegetation of Europe was 

 subtropical, palms growing in the north of Germany. The 

 change which was distinctly to be noted in the Great Plains 

 region of North America was probably due rather to the ele- 

 vation and increased altitude of the western interior than to 

 general climatic alteration. Crocodiles are very rare indeed 

 in the White River beds and those that have been found all 

 belong to dwarf species, while none are known from the John 

 Day. Unfortunately, hardly anything has been ascertained 



