JOLY'S ESTIMATE 23 



into which it sometimes discharges by submarine 

 springs. 



Again, several considerations lead to the belief that the 

 supply of sodium to the ocean has proceeded, not at a 

 uniform, but at a gradually diminishing rate. The rate 

 of increase of temperature with descent into the crust 

 has continuously diminished with the flow of time, and 

 this must have had its influence on the temperature of 

 springs, which furnish an important contribution to river 

 water. The significance of this consideration may be 

 judged from the composition of the water of geysers. 

 Thus Geyser, in Iceland, contains 884 parts of sodium 

 per million, or nearly 160 times as much as Sir John 

 Murray estimates is present in average river water. A 

 mean of the analyses of six geysers in different parts of 

 the world gives 400 parts of sodium per million, existing 

 partly as chloride, but also as sulphate and carbonate. 



It should not be overlooked that the present is a calm 

 and quiet epoch in the earth's history, following after a 

 time of fiery activity. More than once, indeed, has the 

 past been distinguished by unusual manifestations of 

 volcanic energy, and these must have had some effect 

 upon the supply of sodium to the ocean. Finally, 

 although the existing ocean water has apparently but 

 slight effect in corroding the rocks which form its bed, yet 

 it certainly was not inert when its temperature was not 

 far removed from the critical point. Water begins to 

 exert a powerful destructive action on silicates at a tem- 

 perature of 180 C., and during the interval occupied in 

 cooling, from 370 to 180 C., a considerable quantity of 

 sodium may have entered into solution. 



A review of the facts before us seems to render some 

 reduction in Dr. Joly's estimate imperative. A precise 

 assessment is impossible, but I should be inclined myself 

 to take off some thirty or forty millions of years. 



