42 C. R. VAN HISE 



Fisher calculates that the radial contraction of the globe 

 has been .65 miles since its temperature was 4000 F., and 

 1.9 miles if its temperature were ever 7000 F. These cal- 

 culated contractions are so slight that it is not worth while 

 to calculate the surficial contraction which would result from 

 them. 1 Certainly, if Fisher's conclusion is approximately cor- 

 rect, loss of heat by secular cooling is not even an important 

 cause for crustal shortening. 



Darwin, 2 as a result of a discussion of the strains of the crust 

 resulting from secular cooling, concludes that an earth 8000 

 miles in diameter would contract so that "in 10,000,000 years, 

 228,000 square miles of rock would be crumpled up and piled on 

 top of the subjacent rocks." 



The variation in the estimates above given is so great that 

 the question not unnaturally arises as to whether the truth may 

 not be far from any of them. Indeed Darwin says, with refer- 

 ence to his estimate, that "the numerical data with which we have 

 to deal are all of them subject to wide limits of uncertainty." 



All of the foregoing calculations as to the amount of heat 

 lost by the earth are based upon the hypothesis that the earth 

 has not had a higher average temperature than 7000 F. during 

 geological time, and also on the hypothesis that the entire loss 

 of heat is by conduction. If the present temperatures deep 

 within the earth are to be measured by many thousands of 

 degrees, as some believe, the amount of heat lost would be much 

 greater than calculated, and the resultant contraction corre- 

 spondingly important. Also the process of cooling would have 

 been much more rapid if convectional currents assisted, by means 

 of which the hotter material comparatively deep within the earth 

 continued for a long time to be brought near or to the surface. 

 It has been customary to consider the heat lost through convec- 

 tion as so small as to be negligible, and all calculations upon the 

 amount of heat lost by secular cooling have ignored this quan- 

 tity. 



1 Physics of the earth's crust, by Osmond Fisher : London, 1881, p. 72. 



2 Note on C. Davison's paper on the straining of the earth's crust in cooling, by 

 G. H. Darwin : Phil. Trans. Roy. Soc, Vol. CLXXVIII, Pt. A, 1887, p. 249. 



