44 SCIENCE PROGRESS 



We may arrive at a probable age lying between the major 

 and minor limits. If, first, we take the arithmetic mean of these 

 limits, we get 117 millions of years. Now this is almost cer- 

 tainly excessive, for we here assume that the rate of covering 

 of the primary rocks by sediments was uniform. It would not 

 be so, however, for the rate of supply of sediment must have 

 been continually diminishing during geological time, and hence 

 we may take it the rate of advance of the sediments on the 

 primary rocks has also been diminishing. The average rate 

 of supply has therefore been greater than the mean rate. Now 

 we may probably take, as a fair assumption, that the sediment- 

 covered area was at any instant increasing at a rate pro- 

 portionate to the rate of supply of sediment ; that is, to the area 

 of primary rocks then exposed. On this assumption the age 

 is found to be 8y millions of years. 



The Age by the Sodium of the Ocean 



I have next to lay before you a quite different method. I 

 have already touched upon the chemistry of the ocean, and on 

 the remarkable fact that the sodium contained in it has been 

 preserved, practically, in its entirety from the beginning of 

 geological time. 



That the sea is one of the most beautiful and magnificent 

 sights in Nature all admit. But, I think, to those who know 

 its story its beauty and magnificence are ten-fold increased. Its 

 saltness is due to no magic mill. It is the dissolved rocks of 

 the earth which give it at once its brine, its strength, and its 

 buoyancy. The rivers which we say flow with "fresh" water 

 to the sea nevertheless contain those traces of salt which, 

 collected over the long ages, occasion the saltness of the ocean. 

 Each gallon of river water contributes to the final result ; and 

 this has been going on since the beginning of our era. Consider 

 the mighty total of the rivers : 6,500 cubic miles of water in the 

 year ! Yet vast as it is, how little in the overwhelming 

 magnitude of the ocean ! 



There is little doubt that the primeval ocean was in the 

 condition of a fresh-water lake. It can be shown that a primi- 

 tive and more rapid solution of the original crust of the earth 

 by the slowly cooling ocean would have given rise to relatively 

 small salinity. The fact is the quantity of salts in the ocean 

 is enormous. We are only now concerned with the sodium; 



