AGE OF THE EARTH — JOLY. 273 



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 tons is in good agreement with the stand- 

 ard result from North 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 uncor- 

 rected estimates of the age of the ocean as based on solvent denuda 

 tion 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 relia- 

 bility 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) Underestimation of the sodium now in the ocean. 



(b) Neglect of sodium which at some period in the past may have 

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



ifi) Overestimation of the legitimate river supply of sodium. 



(d) Decreased river supply of sodium in the past. 



Of these possible sources of rror (a) may be at once dismissed. 

 The average depth and area of the ocean and its average chemical 

 composition are sufficiently well known to preclude the possibility 

 of any serious error. 



In considering (b) it is necessary to bear in mind the magnitude of 

 the quantities involved. The saline matter in the ocean would rep- 

 resent a volume of over 4,800,000 cubic miles on Clarke's estimation. 

 I have formerly pointed out that the rock salt alone would suffice to 

 cover the land area of the globe to a depth of 122 meters. In com- 

 parison with quantities so vast all the salt deposits known sink into 

 insignificance; nor is it likely that deposits adequate to enter into 

 consideration exist. 



The errors referred to in (c) must be of the nature of cyclic sodium — ■ 

 that is, sodium which circulates from the sea to the land and back 

 through the rivers to the ocean. Cyclic sodium exists in the form 

 of wind-borne spray, which, descending on the land with the rainfall, 

 augments that which is truly derived by denudation. In arid 

 regions it may settle as dust, to be, under special circumstances, 

 washed ultimately into the sea. Again, the sodium which the rivers 

 38734°— sm 1911 18 



