THE WORK OF THE OCEAN. 375 



Chemical and organic deposits. — There is no sharp Une of distinction 

 between the deposits usually classed as chemical and those regarded as 

 organic. The latter are chemical in the broader sense of the term, but 

 as they are immediately associated with Hfe and are dependent upon 

 it, it is a matter of practical convenience to separate them. Aside from 

 the organic deposits, the chemical deposits made in shallow sea-water 

 embrace (1) those due to reactions between constituents so brought 

 together that new and insoluble compounds are formed and precipitated, 

 and (2) those due to evaporation. The points of saturation for the 

 various substances dissolved in sea- water are reached at different stages, 

 and hence they are deposited more or less in succession. 



The chemical deposits made in the shallow water of the sea, or in 

 shallow bodies of water isolated from the sea, are chiefly simple precipi- 

 tates resulting from evaporation; but new combinations are sometimes 

 made in the process of concentration and precipitation. All substances 

 in solution are necessarily precipitated on complete evaporation, but 

 since the sea-water is in general far from saturation, so far as all its lead- 

 ing salts are concerned, only a few are thrown down in quantity sufficient 

 to have geological importance where evaporation is incomplete. The 

 leading deposits are lime carbonate (CaCOg), lime sulphate (gypsum, 

 CaS04,2H20), common salt (rock-salt, NaCl), and the magnesium salts, 

 usually the chlorides and sulphates, which are later changed to carbonates. 

 In investigations on Mediterranean water ^ which had an initial density 

 of 1.02, no deposit took place until concentration by evaporation had 

 brought the water to a specific gravity of 1.05. Between this density 

 and that of 1.13, hme carbonate and some iron oxide were deposited. 

 Between 1.13 and 1.22, lime sulphate was the most abundant precipitate, 

 while between 1.22 and 1.31, 95% of the deposit was common salt. 

 With still further concentration, the remaining substances in solution, 

 especially the magnesiimi salts, were thrown down. 



While there is somewhat more than ten times as much Hme sul- 

 phate as hme carbonate in the ocean (p. 324), the deposits of the car- 

 bonate (including the organic) have been very much greater than those 

 of the sulphate. This is due partly to the fact that the sulphate is 

 much more soluble in natural waters than the carbonate. Rivers 

 bring much more carbonate than sulphate to the sea, so that the point 

 of saturation for the sulphate would normally be reached much later 

 * Usiglio. Encyclopaedia Britannica. Article on Salt. 



