38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



out employing as a datum any observed superficial temperature gradient, 

 yields results which can hardly be forced above 70 million or below 55 

 million years. The weak point here is our ignorance of the depth of the 

 top of the diabase couche; but if Laplace's law of density holds true, the 

 limits would be about 65 and 55 million years. 



These three methods seem to be mutually confirmatory and to give 

 results which converge towards some value near 60 or perhaps 65 million 

 years. 



This being granted, it follows that radioactive minerals cannot have the 

 great ages which have been attributed to them. Only something like a 

 tenth of the heat emitted by the earth can be ascribed to radioactivity 

 plus all other exothermic chemical transformations; the remaining nine- 

 tenths is heat due to compression. 



