Age of the Earth. 361 



on excessive estimates of the occluded sodium chloride in 

 such rocks, taken in conjunction with their rate of removal 

 by denudation, this source of supply to the rivers is less than 

 1 per cent. Clarke reconsiders the question and finds the 

 allowance would not be more than 1 per cent. Three per cent, 

 is regarded by Clarke as a maximum deduction for sodium 

 artificially supplied in modern times to the rivers. 



Oceanic salt deposits are not very abundant over the 

 surface of the earth, being generally confined to particular 

 formations. That they seriously affect the river analyses of 

 all the great rivers of the world is in the highest degree 

 improbable. In any case if we deduct all the chlorinated 

 sodium from the river supply we must include also all sea- 

 derived sodium. If we effect this calculation, we obtain an 

 age of about 150 million years. I do not think it will be 

 disputed that this figure is in its nature excessive. 



There remains the possibility (d) that the assumed uni- 

 formity of past and present conditions is illusory: in other 

 w T ords, that special conditions now exist tending to bring 

 about an abnormally great river supply of sodium. 



The present is admittedly a period of large land exposure. 

 This, however, involves a fact which must be held in mind. 

 At the present time the land area actually draining into the 

 ocean is about 39*7 millions of square miles. The total land 

 area is, however, rather over 55 millions of square miles. It 

 follows that about 30 per cent, of the land area contributes 

 nothing to the ocean. The areas which are classed as 

 " rainless," that is which have less than an annual rainfall 

 of 10 inches and have no run-off, amount to one-fifth of the 

 whole. Under such circumstances transgression of the ocean 

 upon the land simply results in the diminution or disappear- 

 ance of the great continental desert regions. It has been shown 

 by Murray that it would require a vertical depression, rela- 

 tively to the ocean, of 600 feet in order to reduce the existing 

 land area by 26"7 per cent. Penck on the same data concludes 

 that a submergence of 200 metres would reduce the area 

 29 per cent. A submergence of nearly 1500 feet is required 

 to diminish the land area 50 per cent. 



It is for geologists to judge whether world-wide trans- 

 gressions of these magnitudes obtained for any long periods 

 in the past. So far as I know, Palaeography would not 

 support such transgressions. A recent study of the Palaeo- 

 graphy of N. America by C. Sehuchert (Bull. Geo!. Soc. Am. 

 vol. xx. 1910) leads to the conclusion that the mean area of 

 that continent throughout the past has been about 8/10ths of 



