HYDROGRAPHY 



155 



fourths of the whole, whereas in the water of rivers bearing 

 material from the land to the sea it only amounts on the 

 average to about 2 per cent, of the dissolved salts. On the 

 other hand, carbonates, of which only minute quantities are 

 present in the sea, make up over half the total in the case 

 of river-water. It is these and other differences that have 

 given rise to the view that the saltness of the sea is not due 

 merely to the dissolved salts now being conveyed from the 

 land to the sea, and accumulated there throughout the ages 

 as the result of the constant evaporation of pure fresh water 

 from the surface, but may be due also in part to salts present 

 in the primeval ocean when condensation first took place 

 on the globe. We know little or nothing, however, of the 

 proportions in which such salts may have been present in 

 the earliest oceans, and as little of the chemical changes 

 which may have taken place in the dissolved salts accumu- 

 lating in the sea during geological ages. 



The volume of the total salts in the sea has been calculated 

 to be 4,800,000 cubic miles ; and one of the most recent 

 estimates of the age of oceans on the earth (not necessarily 

 the present ones) is nearly a hundred million years. 



The principal salts present in average sea-water are usually 

 stated (from Dittmar's " Challenger " results) to be as 

 follows : — • 



Sodium chloride . 

 Magnesium chloride 

 Magnesium sulphate 

 Calcium sulphate 

 Potassium sulphate 

 Calcium carbonate 

 Magnesium bromide 



35 000 



It is, however, probable that by far the greater part of 

 these materials are not present in the above form combined 

 as salts, but are dissociated as " ions," and therefore a more 

 correct statement of the constitution of the thirty-five parts 



