of the Earth's Crust in Cooling. 577 



be 300 times its actual value *, and the earth would be 

 getting hotter instead of colder. There are many objections 

 to this view, and it seems that the amount of radioactive 

 matter per unit volume must decrease so rapidly with depth 

 that the total is insufficient to supply more than about f of 

 the present loss of heat from the surface t- At the same 

 time the ratio of the amounts of uranium and lead in 

 minerals gives valuable direct evidence concerning the age 

 of the mineral, and hence a minimum estimate of the age of 

 the earth J, which is very much greater than that derived 

 on Lord Kelvin's hypothesis. From a review of the evidence 

 obtained by several different lines of investigation, Holmes 

 has found that the observational data can be satisfied 

 exceedingly well if the age of the earth be taken to be about 

 1600 million years, and if the rate of liberation of heat 

 per unit volume decrease exponentially with the depth. The 

 interval of time concerned is so much greater than that 

 found by Lord Kelvin that his theory of cooling requires to 

 be revised so as to take into account our most recently 

 acquired knowledge, and at the same time the contraction 

 theory, which depends on it, needs similar revision. This is 

 the principal object of the present paper. 



The level of no strain is found now to be at a considerably 

 greater depth than the older determinations gave, and at 

 the same time the amount of compression at the surface is 

 much increased. For both reasons the volume of crumpled 

 rock is increased. On the basis of the exponential distri- 

 bution of radioactive matter the available compression is 

 133 kilometres ; in other words, enough to shorten every 

 great circle of the earth by this amount. Actually, folding 

 is not uniformly distributed over the earth, but nearly 

 confined to certain definite lines of weakness, as one would 

 naturally expect. In some places, as in the case of the valleys 

 of East and Central Africa, a tension is actually indicated. 

 Some great circles thus show very'little crumpling, and others 

 are free to be folded to a greater extent than would be pos- 

 sible if the distribution of mountains were uniform. When a 

 numerical estimate is made of the amount of compression >-.. 



required to produce the known mountain ranges, that found 



* A. Holmes, ' The Age of the Earth,' 1913, p. 129. 



t Holines, " Radioactivity and the Earth's Thermal History," Part II, 

 Geol. Mag. March 1915, p. 109. 



\ Holmes, ' The Age of the Earth,' p. 157 ; or " Radio-activity and 

 the Measurement of Geological Time," Proc. Geol. Assoc, vol. xxvi. 

 pp. 289-309 (1915). 



Phil. Mag. S. 6. Vol. 32. No. 192. Dec. 1916. 2 R 



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