Cenozoic Floras Around the Northern Pacific 75 



suggestion of the Mascall and other related floras which occur 

 in tufFaceous deposits overlying the Columbia lava, and which 

 are referred to the Upper Miocene. In these later floras, Sequoia 

 holds a relatively unimportant position; the dominant types are 

 Abies, Acer, Aesculus, Arbutus, Betula, Platanus, Populus, and 

 Quercus (both broad-leafed and evergreen types). In such 

 florules as that from Trout Creek in southeastern Oregon" and 

 from the Blue Mountains of eastern Oregon,"^ there is definite 

 indication, in the occurrence of Abies, Amelanchier, Picea, and 

 Sorbus, of an altitude of several thousand feet at the site of 

 deposition. The general character of the Upper Miocene floras 

 has been shown to resemble closely that of the forest now living 

 on the borders of the redwood belt'^ under more exposed and 

 less humid conditions than those of the Bridge Creek flora. The 

 stratigraphic position of these Upper Miocene floras is commonly 

 so well marked, separated as they are from the sediments bear- 

 ing the Lower Miocene Bridge Creek flora by the thick series of 

 basaltic flows of Columbia lava, that there is litde possibility of 

 error in establishing their relative age; and the Upper Miocene 

 vegetation is always of a less mesophytic type than that of the 

 Lower Miocene. The habitat, involving in all probability a more 

 exposed topographic setting, was also characterized by less rain- 

 fall and greater extremes of temperature. These physical changes 

 appear to have been caused by the incipience of the orogeny 

 which has subsequendy produced the Cascade Range and re- 

 lated structural units in the northern part of the Great Basin, 

 together with renewed uplift of the Sierra Nevada. The effect of 

 these mountains on the climate and vegetation of the areas to 

 the east is too well known to merit description in this paper. It 

 need only be mentioned that they cut off from the interior of 

 western North America the atmospheric circulation which had 



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