BASIN RANGES 



337 



the highly heated interior is steadily cooling by radiation, and con- 

 tracting. The crust must follow the shrinking interior, and is 

 thereby crowded into a smaller space, which sets up irresistible 

 lateral pressure, to which the crust must give way, even though it 

 were far more rigid than steel. A withered apple has a wrinkled 

 skin because the fruit has shrunk from loss of water, and the skin, 

 crowded into a smaller space, is folded and wrinkled. 



Various objections have been urged against the contraction 

 theory, chiefly on the ground that the cause is insufficient to do 

 the work demanded of it. Those objections cannot be com- 

 pletely answered because of our ignorance of the quantitative 

 factors involved in the problem, but the fact remains that no 

 other suggestion explains the facts of mountain structure so well. 



Fig. 136. 



■The Charleston Mountains, Nevada. One of the Basin Ranges. 

 (Photograph by Merriam.) 



There are certain mountain ranges which have an exceptional 

 structure and must have had a correspondingly different mode of 

 origin. In the Great Basin which lies between the Sierra Nevada 

 and the Wasatch Mountains, are a number of parallel mountain 

 ranges with a prevalent north and south trend, which are collec- 

 tively called the Basin Ranges. These mountains are not folds 

 of very thick strata, but tilted fault blocks, which have been 

 made by normal faults, each upthrow side standing as a great 



