122 LAND MAMMALS IN THE WESTERN HEMISPHERE 



valleys and plains in a fiery sea of iriolten rock. In Oregon 

 these lavas rest upon the upper Oligocene (John Day stage) 

 and middle Miocene beds are deposited upon them, which 

 fixes their date sufficiently. In the Yellowstone Park was 

 piled up a huge mass of volcanic products, lava-flows and beds 

 of ash and tuff, to a thickness of several thousand feet. The 

 ash-beds have preserved the petrified forests, with their tree- 

 trunks still standing one above another; one locality in the 

 Park shows seven such forests, each ' one killed and buried 

 by a great discharge of ash and then a new forest established 

 and growing upon the surface of the accumulation. In the 

 tuffs are leaf-impressions which permit identification of the 

 plants. 



In the latter part of the Miocene and at its close there were 

 important crustal movements, which affected all the Pacific 

 coast mountain ranges, though this epoch was no such time of 

 mountain making in America as it was in the Old World. 

 The principal elevation of the Coast Range^in., California and 

 Oregon was due to these movements, and the Sierras and the 

 plateaus of Utah and Arizona were increased in neight. On 

 the Atlantic side the Florida island was joined to the mainland 

 and thus the present" shape of the continent was almest'^exactly 

 gained. " .^^ 



The Miocene climate of North America, as indicated by 

 the plants of Florissant,- the Yellowstone Park and Oregon, was 

 distinctly milder than at present, a southern vegetation of 

 warm-temperate character extending to Montana and perhaps 

 much farther north, but it was not so warm as it had been 

 in the Eocene, and palms are not found in any of the localities 

 mentioned, nor do crocodiles occur in any of the northern 

 Miocene formations. In Europe the climate of the early 

 Miocene was considerably warmer than in North America, 

 the vegetation of central and western Europe being very much 

 like that of modern India. This difference between the two 

 sides of the Atlantic was probably due, in large part, to the 



