MOUNTAIN BUILDING 



35 



Given a sufficiently large increase in the value of the 

 coefficient of expansion with rise in temperature and 



S.L 



5O Miles 



1200 



Area, a/* Depostion 



FLoiv c&icl Rise of 



II 



FIG. 2. Diagram drawn to scale to illustrate the successive stages in 

 the formation of a mountain chain. to 50 represents a horizontal 

 distance of fifty miles : S.L. sea-level. 



1. The first stage previous to the deposition of sediment. The dotted 



lines are subterranean geotherms. 



2. The second stage after the deposition of five miles of sediment in 



the course of about three millions of years. The sinking of the 

 sea-floor over a.a.a. pari passu with deposition is accompanied by 

 compression of the underlying crust, and by a flow of superheated 

 rock in the direction of the arrow. The removal of this rock leads 

 to a rise of the subterranean geotherms, below the area of deposition. 

 The lowest line in this figure should be bent downwards below the 

 area of deposition. 



3. The third stage. The contraction of the interior of the earth due 



to cooling for three millions of years or so has set up stresses 

 throughout the crust. Some of these are relieved by thrusting. 

 The continental land with the superficial crust moves along these 

 towards the ocean, and so produces folding of the sediments. 

 Liquefaction results wherever the superheated rocks are relieved 

 of some part of the overlying pressure, and molten magma is 

 injected into the mountain chain. 



