42 SCIENCE PROGRESS 



so. In the case of every substance but one only, the ocean 

 continually gives up again more or less of the salts supplied 

 to it by the rivers. The one exception is the element sodium. 

 The great solubility of its salts has protected it from abstraction, 

 and it has gone on collecting during geological time, practi- 

 cally in its entirety. This gives us the clue to the denudative 

 history of the earth. 1 It is the secret of the sea. 



The process is now simple. We estimate by chemical 

 examination ol igneous and sedimentary rocks the amount of 

 sodium which has been supplied to the ocean per ton of 

 sediment produced by denudation. We also calculate the 

 amount of sodium contained in the ocean. We divide the 

 one into the other (stated, of course, in the same units of 

 mass), and the quotient gives us the number of tons of sedi- 

 ment. The most recent estimate of the sediments made in 

 this manner affords 56 x io 16 tonnes. 2 



Now we are assured that all this sediment was transported 

 by the rivers to the sea during geological time. Thus it follows 

 that if we can estimate the average annual rate of the river 

 supply of sediments to the ocean over the past we can calculate 

 the required age. Now the land surface is at present largely 

 covered with the sedimentary rocks themselves. Sediment 

 derived from these rocks must be regarded as, for the most 

 part, purely cyclical ; that is, circulating from the sea to the 

 land and back again. It does not go to increase the great body 

 of detrital deposits. We cannot, therefore, take the present 

 river supply of sediment as representing that obtaining over 

 the long past. If the land was all covered still with primary 

 rocks we might do so. It has been estimated that about 25 per 

 cent of the existing continental area is covered with archaean 

 and igneous rocks, the remainder being sediments. 3 On this 

 estimate we may find valuable major and minor limits to the 

 geological age. If we take 25 per cent, only of the present 

 river supply of sediment, we evidently fix a major limit to the 

 age, for it is certain that over the past there must have been 



1 Trans. R.D.S., May 1899. 



2 Clarke, A Preliminary Study of Chemical Denudation (Washington 1910). 

 My own estimate in 1899 (loc. cit.) made as a test of yet another method of 

 finding the age, showed that the sediments may be taken as sufficient to form 

 a layer vi mile deep if spread uniformly over the continents ; and would amount 

 to 64 x io ,G tons. 



3 Van Tillo, Comptes Rendues (Paris), vol. c.xiv. 1893- 



