i 5 4 HUXLEY MEMORIAL LECTURES 



they do not agree very well among themselves, 

 are concordant in assigning a very much greater 

 age to the rocks. If the radioactive estimate is 

 correct, then we are now living in a time when 

 the denudative forces of the Earth are about 

 eight or nine times as active as they have been on 

 the average over the past. Such a state of things 

 is absolutely unaccountable. And all the more 

 unaccountable because from all we know we 

 would expect a somewhat lesser rate of solvent 

 denudation as the world gets older and the land 

 gets more and more loaded with the washed-out 

 materials of the rocks. 



Both the methods referred to of finding the 

 age assume the principle of uniformity. The 

 geologist contends for uniformity throughout the 

 past physical history of the Earth. . The physicist 

 claims the like for the change-rates of the radio- 

 active elements. Now the study of the rocks 

 enables us to infer something as to the past his- 

 tory of our Globe. Nothing is, on the other 

 hand, known respecting the origin of uranium or 

 thorium the parent radioactive bodies. And 

 while not questioning the law and regularity 

 which undoubtedly prevail in the periods of the 

 members of the radioactive families, it appears to 

 me that it is allowable to ask if the change rate of 

 uranium has been always what we now believe it 

 to be. This comes to much the same thing as 

 supposing that atoms possessing a faster change 

 rate once were associated with it which were 



