CONCLUSIONS 203 



earth at the compensation level and emitted at a comparatively low tem- 

 perature by the outer surface of the globe. To account for such a mech- 

 anism it is sufficient to assume that conductivity of certain areas, destined 

 to be occupied by continents, was lower than that of the remaining sur- 

 face of the globe. 



Very early in the history of radiology — the greatest achievement of 

 physics since the establishment of the second law of thermodynamics — 

 means were suggested for determining the ages of minerals. They seemed 

 plausible, but gave results agreeing very badly among themselves, and for 

 the most part many times as great as those deduced from study of the 

 earth as a cooling body or of the ocean as the depository of the sodium 

 extracted from continental rocks. 



Eecent advances in the study of atomic structure, and especially the 

 discovery of isotopic elements — that is, different elements occupying the 

 same place in the periodic table but inseparable by ordinary chemical 

 means — render the uranium-helium ratio and the uranium-lead ratio 

 practically valueless for the determination of the age of minerals. 



If it is granted that the compensation level is an eutectic level, and 

 this seems the only intelligible theory, the age of an earth heated both by 

 compression and by radioactivity can be computed. 



Geodesists assert that the compensation level is between 110 and 140 

 kilometers from the surface. The smaller depth would correspond to an 

 age so small as to be unacceptable to geologists. For a depth of 121 kilo- 

 meters the age would be 68 X 10^ years and one-seventh of the heat 

 emitted would be due to radioactivity. For a depth of 1-10 kilometers the 

 age would be 100 X 10^ years and 26 per cent of the heat lost would be 

 ascribable to radioactivity. Greater depths of the compensation level 

 seem incompatible with slight strain beneath that level, and can not be 

 accepted from the point of view of this paper, but data are given for an 

 earth no less than 1,314 X 10^ years old corresponding to the hypothesis 

 that two-thirds of the heat emitted by the globe is due to radioactivity. 



To some extent the results reached depend on the constants adopted, 

 but there is strong reason for believing these so nearly correct that a large 

 percentage change in the results is very improbable. 



It appears, then, that the position of the level of compensation is in- 

 compatible with any immense age for the earth, while the discovery of 

 isotopic elements throws us back on means not involving the uranium- 

 helium or the uranium-lead ratios for the determination of this age. In 

 particular the age of a radioactive earth which is also initially hot may 

 be computed from Fourier's law of conduction as readily as if it were not 



XV— Bull. Ckol. Sor. Am.. Vol. -JO. 1!»14 



