STRUCTURE OF MOUNTAINS 



265 



regions of the Rockies, an elevation to be termed a moun- 

 tain must rise to a striking height above the generally 

 elevated surface, which is itself nearly everywhere more 

 than 4000 feet above the sea. 



Structure of Mountains. Mountains are the results 

 of deformations in the earth's crust, due to causes not 





LOFTY MOUNTAINS 

 The high Sierras. 



fully understood. The crust of the earth has been folded, 

 pushed up, crumpled and in many ways distorted so that 

 some portions have been elevated to great heights above 

 sea level. 



All lofty mountains have been elevated in comparatively 

 recent geological time, but this of course means millions 

 of years ago. If mountains now lofty were geologically 

 old, they would long ago have been worn down, or eroded, 

 by winds, rain, streams, avalanches, and glaciers. The 



