138 Straining of the Earth resulting from Secular Cooling, 



If we make e'/e equal to zero, or /3 equal to infinity, in 

 these expressions, we get 



W=6VWeV« 3 , . . . . ". . . (11) 

 and h=3eYa*/2 *J (*)<?. (12) 



This value of h agrees with that obtained by the Rev. 0. 

 Fisher on the supposition that the coefficient of dilatation is 

 constant*. 



- The mean value of the coefficient of dilatation deduced 

 from Adie's experiments t on six rocks at temperatures 

 varying from about 50° to 208° F., is '0000057. Taking this 

 as the surface-value e, and as before V equal to 7000° F., 

 and j3 to '1, we find that after a lapse of 100 million years 

 the total volume of solid rock is about 6,145,000 cubic miles, 

 and the mean thickness of the layer formed by spreading this 

 over the whole earth is '03120 mile or 164*7 feet. If the 

 coefficient of dilatation were constant and equal to the value 

 above-mentioned, the corresponding figures would be 184,500 

 cubic miles and 4*95 feet. 



Alleged Insufficiency of the Contraction Theory. — If the co- 

 efficient of dilatation increases with the temperature, instead 

 of being constant, it thus appears that the result is a con- 

 siderable increase in the depth of the surface of no strain, 

 and in the total amount of rock-folding due to the cooling of 

 the earth. If the conductivity increases with the temperature, 

 there will be a further increase of both quantities. It is 

 possible, moreover, that besides their variation with the tem- 

 perature, both the conductivity and the coefficient of dilata- 

 tion may be greater in the material which composes the 

 earth's interior than they are in the surface rocks. Mr. Rudski 

 has also pointed out t that the depth of the surface of no 

 strain will be much greater if initially the temperature, in- 

 stead of being uniform, increased with the depth, and there is 

 some reason for supposing that this may have been the case. 

 If, then, we regard the contraction theory as a theory of the 

 formation of mountain-ranges only, and not necessarily of 

 the continental masses, we may, I think, conclude that 

 calculations as to its alleged insufficiency are at present 

 inadmissible. 



* Phil. Mag. vol. xxv. 1888, p. 17. 



t Edinb. Roy. Soc. Trans, vol. xiii. 1836, pp. 354-372. 



% Phil. Mag. vol. xxxiv. 1892, pp. 299-301. 



