of the Earth's Crust in Cooling. 



585 



Solving by successive approximation to find the level of 

 no strain, we find that the value of q corresponding to it is 

 •200, and hence the depth is 76 km. The depth is thus 

 not much different from that found on the hypothesis of 

 a homogeneous radioactive layer of uniform depth. The 

 value of K at the surface is 3*32 xl0~ 3 , so that the com- 

 pression is adequate to shorten the circumference by 

 133 kilometres. If the crumpled rock were spread uni- 

 formly over the surface, it would cover it to a depth of 

 170 metres. 



V. The amount of compression required to produce existing 

 mountains. 



In the cases of certain particular ranges, geologists have 

 succeeded in obtaining a direct quantitative estimate of 

 the amount of compression that must have been necessary to 

 produce the folding that is observed. In the Appalachians, 

 for instance, the width of the rocks, measured perpendi- 

 cularly to the chain, is estimated to have been shortened by 

 about 40 kilometres. Similarly, the compression in the 

 Rockies is 25 miles, in the Coast Range in California 

 10 miles, and in the Alps 74 miles*. The larger ranges, 

 particularly in Asia, have not been so exhaustively treated ; 

 but a rough idea of their importance can be obtained by a 

 comparison with the Rockies, or in the case of narrower 

 ranges, the Coast Range, whose geological age is about 

 the same. The Alps are probably abnormal, and have not 

 been used as a standard. The elevation of a continent or a 

 large tableland involves little crumpling within it, and no 

 great amount at the coast as long as the slope is there gradual. 

 Hence, in determining the amount of compression, we need 

 consider only the steep slopes of mountains. The amount 

 in any range is here supposed to be proportional to the 

 mean height t, and in the following table the latter has 

 been obtained roughly from the maps in Philips' Student's 

 Atlas. From this estimated compression combined with 

 the length of the range the area lost by folding is at once 

 found. The amount of compression found is of course 

 essentially provisional and must be revised when further 

 geological evidence is available. 



* These data are from Pirsson & Schuchert, ' Textbook of Geology/ 

 p. 361. 



t This would be exactly true if the strata were similarly folded in all 

 mountains. 



