Age of the Earth. 363 



northern lands of the earth : may they not affect the river 

 discharge of sodium ? The answer is to be found in the 

 river analyses. It is sufficient to refer to the figures cited by 

 Clarke in his ' Data of Geochemistry.' There is no indication 

 of excessive supplies from northern rivers. 



I am not aware of any sources of error other than those 

 now considered. It would appear that solvent denudation 

 estimated in the only manner open to us assigns an age to the 

 ocean which at its probable maximum does not exceed luO 

 million years. Assuming that certain sources of error com- 

 bined to lower this age, for instance that more complete 

 knowledge will reveal a lesser sodium supply than has been 

 determined on existing data ; that the cyclic sodium should 

 be taken as somewhat more than we have assumed ; that 

 former fluctuations of land area on the whole produced an 

 effect on solvent denudation : assuming all this we might 

 be somewhat out in our reckoning. We have, however, 

 neglected all those sources of error tending to increase the 

 age unduly. Chief among these are the following: — 

 Primitive sodium existing in the ocean : marine solvent 

 denudation effected directly on the coasts and sediments : 

 sodium supplied with volcanic ejectamenta : sodium supplied 

 by submarine rivers and springs. For a discussion of 

 these sources of error I must refer to the several papers 

 cited above. It is generally conceded that any precise 

 evaluation of their effects is not possible; so that a considerable 

 margin must be left when considering the minor limit of the 

 age of the ocean by this method. They certainly produce 

 some effect as a set off to the corrections already dealt with. 

 It is against probability to add 50 per cent, to the value 

 already estimated as a major limit. We can only double it 

 by appealing purely and simply to the imagination for effects 

 of which we possess no indication, and the existence of which 

 is at variance with what we know. 



The age as determined is based upon the summation of the 

 sodium supplied by the rivers during geological time. This 

 integral can, obviously, give us no information as to the. 

 relative durations of the geological epochs. The latter 

 question can be approached in two ways. (1) By means of 

 the stratigraphical column or measured maxima of detrital 

 and chemical deposits, assuming that these were laid down at 

 an approximately uniform rate ; and (2) by the radioactive 

 method. I shall first consider the former method 



