128 G. F. BECKER RELATIONS OP RADIOACTIVITY TO COSMOGONY 



great part, to this cause. In fact it has been established that a relatively 

 thin la3"er of rock possessing the average radioactivity of surface rocks 

 would suffice to maintain the observed temperature gradients indeiinitely. 

 This line of reasoning is in part supported by the discrepancy which has 

 hitherto existed between the age of a cooling globe as found by Kelvin's 

 application of Fourier's method and the antiquity of the ocean as is 

 found from geological observations. 



Supposing the uniform initial temperature of the globe and the thermal 

 difiusivity of its substance known, the age by Kelvin's solution is inversely 

 proportional to the square of the surface temperature gradient. If the 

 age were accurately known from independent geological observations, the 

 corresponding gradient for a cooling globe could be inferred at once, and 

 a comparison of this with the observed gradients would show in how far 

 the theory of cooling fails to explain the facts. A brief review of the age 

 determinations, however, will show that they lack the precision requisite 

 to give such a comparison value as a means of determining the part 

 plaj'ed by radioactivity. 



During the last 15 or 20 years much work has been expended i;pon 

 geological estimates of the duration of post-Archean time. The accumu- 

 lation of strata is a gradual process proceeding jMii passu with the degra- 

 dation of land surfaces, though greatly affected by local conditions; and 

 had we the assurance that the maximum rate of accumulation in the past 

 were the same as the maximum rate at present, fairly accurate results 

 would, be attainable. The formations have also heen studied with especial 

 view to the rate of deposition, a variation in which might manifest itself 

 in a number of ways. The studies referred to, however, have not dis- 

 closed any indication that the limits of variation with time are great, 

 and to a certain extent they have been allowed for by Mr Walcott,^* whose 

 discussion of the data is more minute and comprehensive than any other 

 with which I have met. In 1893 he reached the conclusion that the 

 minimum estimate for post-Archean time is 25 to 30 million years, the 

 maximum 60 to 70 million years, and that 45 million is the most prob- 

 able. By a similar method Mr de Lapparent ^" in 1890 had estimated 67 

 to 90 million. In 1899 Sir Archibald Geikie ^^ stated that, so far as he 

 was able to form an opinion, 100 million 3^ears would suffice for the for- 

 mation of the stratified rocks, and in 1900 Mr SoUas ^'' arrived at 261^ 

 million, assuming a constant rate of deposition. On the other hand, Mr 



I 



»* Journal of Geology, vol. 1. 1S9.S. p. 675. 



== Bulletin Societg de Geologie de France, vol. 18. 1890, p. 351. 



" British Association for the Advancement of Science, 1899, p. 727. 



^ British Association for the Advancement of Science, 1900, p. 711. 



