SUMMARY 145 



The second section deals with the origin of the radioactive elements. 

 The distribution of elements in the universe as revealed by the spectro- 

 scope is discussed in connection with Mr Clarke's evolutionary hypothesis. 

 The distribution, taken in its relations to the periodic law, points to the 

 truth of the hypothesis that elements are evolved, those of highest 

 molecular weight being the yoiingest and confined to cooling stars or 

 planetary bodies. This conclusion is in line with Mr J. J. Thomson's 

 corpuscular theory of matter and evolution is the converse of the devolu- 

 tion established by the Curies and Messrs Rutherford and Soddy. If 

 uranium existed in the nebulae, the mystery of the universe would be 

 almost hopelessly deepened. Eegarding uranium as an endothermic com- 

 pound, it must be stable at certain elevated temperatures and pressures. 

 Its association with pegmatites indicates in a general way the conditions 

 of stability, and it is pointed out that the artificial production of radio- 

 active substances is not a hopeless aspiration. 



The bearing of radioactivity on the age of the earth is next considered. 

 The more important estimates — geological, chemical, astronomical, and 

 physical — are assembled, including a new calculation of the age of a 

 cooling globe made for this memoir, but published in detail elsewhere. 

 This agrees well with the determination by other methods and gives about 

 60 million years as the age. From it may be' deduced a gradient of 1° 

 centigrade in 42.3 meters, while the best normal value of the gradient aa 

 derived from Mr Koenigsberger's discussion is 1° centigrade in 38 

 meters. The difl^erence, about one-tenth of the latter, is offered as a first 

 approximation to the terrestrial heating effect of radioactive substances. 

 The theory that the age of minerals can be inferred from the ratio of 

 uranium to helium or to lead is discussed and rejected. 



A concluding section is devoted to the distribution of radioactivity in 

 depth. The total heating effect is independent of this distribution, which, 

 however, would affect the rate of increase of temperature in mines or 

 wells, and something may possibly be learned of it by such means. For- 

 mulas for the temperatures due to various distributions have been given 

 by Mr Koenigsberger and Mr Strutt. It is shown how that for a uni- 

 formly radioactive shell the highest surface gradient compatible with 

 tidal stability is 1° Fahrenheit to 57 feet, while if the gradient due to 

 radioactivity is a tenth of that due to cooling, the uniformly radioactive 

 shell is a little less than 10 kilometers in thickness or, say, between 6 and 

 13 kilometers. The ocean has scarcely any radioactivity, while the 

 abysmal red clay is intensely active. A plausible hypothesis is offered for 

 the accumulation of radioactive matter beneath water or wet rocks. The 

 intimate association of radium and granitic rocks leads to the considera- 



XIV— Bull. Geol. Soc. Am., Vol. 19, 1007 



