Aye of the Earth. 359 



Although the numerous analyses which go to hnild np this 

 result are not of equal value, there are certain satisfactory 

 features in the computation. 



It is explained by Clarke that in the wonderfully detailed 

 analyses of the Mississippi by Dole and Stabler, taken along 

 with their work on other great rivers of N. America, and with 

 the observations of Forbes and Skinner for Colorado, data 

 have been obtained for the United States which are not likely 

 to be much altered by any future analyses. Twenty-two 

 river-basins enter into the mean for the United States, giving 

 a mean denudation factor of 79 tonnes. For the rest of 

 N.America an estimate only is possible; but, for reasons 

 given, Clarke concludes that " if we assume that six millions 

 of square miles of N. America lose 79 metric tons in solution 

 per square mile per annum, and that the composition of the 

 saline matter so transported is that found for the United 

 States alone, we shall not be very far from the truth. * J 

 Possessing, thus, a standard based on the drainage of a 

 great continent, we feel confidence in our criticism of other 

 data. The quantity of water thus dealt with is rather more 

 than one fourth of that supplied by the entire drainage areas 

 of the earth. 



It will be seen from the tables given by Clarke, that the 

 mean denudation factor of 68'4 tonnes is in good agreement 

 with the standard result from N. America, nor is it very 

 largely departed from by the factors derived from other 

 continents. 



There can, I think, be little doubt that the results arrived 

 at by Clarke and Sollas are not likely to be seriously disturbed 

 in the future. It is most improbable that they require 

 amendment to the extent of 50 per cent. This being so, we 

 conclude that the uncorrected estimates of the age of the 

 ocean as based on solvent denudation is of the order of 

 100 million years. It remains now to consider the legitimate 

 corrections to be applied to this figure. 



At the present moment the most important aspect of this 

 method of evaluating the age of the ocean is involved in its 

 degree of reliability as affording a maximum value of the 

 time elapsed since solvent denudation began. This point I 

 shall, therefore, specially consider. 



The errors affecting the crude result found by dividing 

 the sodium of the ocean by the annual river supply, and 

 tending to make this estimate too small, are: — 



(a) Under-estimation of the sodium now in the ocean. 



(b) Neglect of sodium which at seme period in the past 

 may have been in the ocean, but is now removed from it. 



2 B 2 



