ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 45 



consequent upon self-condensation, as a result of increased pres- 

 sures coming from the greater effectiveness of gravity as the 

 rotation period increased, additional heat within the earth would 

 be developed. Also condensation of the earth as a result of 

 change of physical state (see pp. 59—61) or in any other way, 

 would result in the development of heat. 



What the residual effect of these opposite neglected factors 

 is upon the loss of heat as ordinarily calculated, it is impossible to 

 say, and until the various estimates of the loss of heat approach 

 one another more nearly than they do at present, it is not worth 

 while to make a conjecture upon this subject. 



If it be true that the temperature of the interior of the earth 

 is much higher than premised in the calculations of heat lost 

 during secular cooling, and if the convectional movements of 

 magma and water are important means of refrigeration, it may 

 be that heat has been transferred to the surface from much 

 deeper within the earth than estimated by the physicists. Dut- 

 ton 1 states that below " 200 or 300 miles the cooling has, up to 

 the present time, been extremely little." Davison affirms that 

 below 400 miles the earth has not sensibly cooled. 2 These 

 figures are based upon the hypothesis that the loss of heat is 

 due wholly to conduction in a globe having a uniform initial 

 temperature of 7000 F. If this hypothesis is incorrect, the 

 hypothetical level of no lateral stress would be at a greater 

 depth than calculated by Reade, Darwin, and Davison,- — from 2 

 to 8 miles. 3 



Whatever the total loss of heat as a consequence of the 

 various positive and negative factors, if we assume a liquid earth, 

 it is certain that all of the resultant contraction is not available 



1 A criticism upon the contractional hypothesis, by C. E. DuTTON : Am. Journ. 

 Sci., Vol. VIII, 1874, p. 120. 



2 On the distribution of strain in the earth's crust resulting from secular cooling ; 

 with special reference to the growth of continents and the formation of mountain 

 chains, by Charles Davison: Phil. Trans. Roy. Soc, Vol. CLXXVIII, Pt. A, 1887, 

 P- 235- 



3 Estimates summarized in Manual of geology, by James D. Dana : 4th edition, 

 1895, pp. 384-385- 



