48 SCIENCE PROGRESS 



salts. Hence if it is true, as has been stated, that we now live 

 in a period of exceptionally high continental elevation, we must 

 infer that the average supply of salts to the ocean by the rivers 

 of the world is less than over the long past, and that, therefore, 

 our estimate of the age of the earth as already given is 

 excessive. 



There is, however, one condition which will operate to 

 unduly diminish our estimate of geologic time, and it is a con- 

 dition which may possibly obtain at the present time. If the 

 land is, on the whole, now sinking relatively to the ocean level, 

 the denudation area tends, as we have seen, to move inwards. 

 It will thus encroach upon regions which have not for long 

 periods drained to the ocean. On such areas there is an 

 accumulation of soluble salts which the deficient rivers have 

 not been able to carry to the ocean. Thus the salt content of 

 certain of the rivers draining to the ocean will be influenced not 

 only by present denudative effects, but also by the stored results 

 of past effects. Certain rivers appear to reveal this unduly 

 increased salt supply : those which flow through comparatively 

 arid areas. However, the flow-off of such tributaries is relatively 

 small and the final effects on the great rivers apparently un- 

 important — a result which might have been anticipated when 

 the extremely slow rate of the land movements is taken into 

 account. 



The difficulty of effecting any reconciliation of the methods 

 already described and that now to be given increases the interest 

 both of the former and the latter. 



The Age by Radioactive Transformations 



Rutherford suggested in 1905 that as helium was con- 

 tinually being evolved at a uniform rate by radioactive 

 substances (in the form of the alpha rays) a determination 

 of the age of minerals containing the radioactive elements 

 might be made by measurements of the amount of the stored 

 helium and of the radioactive elements giving rise to it. The 

 parent radioactive substance is — according to present know- 

 ledge — uranium or thorium. An estimate of the amounts of 

 these elements present enables the rate of production of the 

 helium to be calculated. Rutherford shortly afterwards found 

 by this method an age of 240 millions of years for a radioactive 

 mineral of presumably remote age. Strutt, who carried his 



