of the Earth's Crust in Cooling. 



587 



by about 1*8 million square kilometres = 1*8 X 10 16 square 

 centimetres. 



Now if K be the relative compression at the surface, 

 the area crumpled is 



4ttc 2 {1-(1-K) 2 } = 8ttc 2 K. 



By substituting, we see that the value of K requisite 

 to account for all the mountain ranges of the earth is 

 1*8 x 10~ 3 , which is only about half of the amount (3'3 x 10" 3 ) 

 that the contraction hypothesis has been shown able to 

 produce. If we allow for possible folding in regions at 

 present submerged and for old ranges, now almost denuded 

 away, the agreement may be better, or the available com- 

 pression may even be insufficient ; but in any case the 

 theoretical and the observed compressions are of the same 

 order of magnitude, so that it seems highly probable that 

 the contraction hypothesis is adequate to account for a 

 very large fraction of the mountain-building that has taken 

 place, and perhaps for the whole of it *. 



VI. The influence of denudation and thermal blanketing. 



Suppose that by some means not yet specified, a large 

 area of the solid earth has been raised up to form a 

 continent, and at the same time other parts have been 

 lowered to form oceans. Then the influence of the atmo- 

 sphere causes the continent to be denuded , and the solid 

 outer layers of it come to be redeposited on the ocean bed. 

 Then the uniform law of cooling already considered is 

 disturbed in two ways. In the first place, it has been 

 shown that the stretching or crumpling of rocks contains 

 as a factor the quantity S — f \ 2 <f>'QC)d\, and the second 

 term of this is a large proper fraction of the first. If, 

 however, the upper layers, with their radioactive con- 

 stituents, are removed, then, instead of the limits of the 

 integral being and go, they will be X and oo , where 

 X is the depth of the rocks denuded away. Thus the 

 integral is much diminished in value, and consequently 

 denudation increases the factor as a whole. Hence a 

 greater strain will be thrown on the crust in continental 

 areas. Similarly, if the ocean bottom is considered, the 

 compression is seen to be diminished there. In the second 

 place, although convection currents in the ocean will cause 



* I am much indebted to Dr. Bonney for calling my attention to the 

 possibility of the comparison here made. 



