480 GEOLOGY. 



Igneous rock has occasionally had a significant influence on modern 

 vegetation, without regard to the age of the lava itself. The unwooded 

 tract shown in Fig. 564 corresponds somewhat accurately with a dike 



Fig. 534. — A basic dike, cutting crystalline schists, is the cause of the absence of 

 trees in the central part of the area shown. Bighorn Mountains, southwest of 

 Buffalo, Wyo. (Kiimmel.) 



of basic rock which affects the crystalline schists of the Bighorn Moun- 

 tains. 



Changes of Level During the Pleistocene. 



The very considerable changes of level which marked the closing 

 stages of the Pliocene have been mentioned, and many of them doubt- 

 less continued into the Pleistocene. 



Certain minor war pings of later date, such as those which affected 

 the basins of Lakes Bonneville and Lahontan during the Pleistocene 

 have also been noted, but such changes are probably but a meager 

 index of the crustal warpings of the period. Specific data on this 

 point are less abundant than could be desired, for the phenomena of 

 erosion and deposition which followed the elevation of the Ozarkian 

 or Sierran epoch are not readily differentiated from the similar phe- 

 nomena resulting from later elevation. Nevertheless evidence of Pleis- 

 tocene changes of level, as distinct from late Pliocene, are not wanting, 

 especially near the coasts and about the shores, of the Great Lakes. 

 From the evidence at hand, it appears that deformative movements were 

 wide-spread both in the western mountains and in the area covered by 



