THE WORK OF THE OCEAN 323 



material deposited is sometimes uneven, owing to shore and tidal 

 currents. The result is that the surface of the shallow-water 

 deposits is often affected by low elevations and shallow depressions. 

 The elevations and depressions may be elongate, circular, or irreg- 

 ular in form. This topography is sometimes preserved on newly 

 emerged lands. 



Chemical and organic deposits in shallow water. There is no 

 sharp line of distinction between the deposits usually classed as 

 chemical and those classed as organic. The latter are chemical 

 in the broader sense of the term, but as they are immediately 

 associated with life 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 evaporation, and (2) those due to chemical re- 

 actions between constituents so brought together that new and 

 insoluble compounds are formed and precipitated. 



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

 or in bodies of shallow water isolated from the sea, are chiefly 

 simple precipitates resulting from evaporation. 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 leading salts are concerned, only a few are thrown down in 

 quantity sufficient to be of geological importance where evaporation 

 is incomplete. The principal deposits of this sort are calcium car- 

 bonate (limestone, CaCO 3 ) and sulphate (gypsum, CaSO 4 , 2H 2 O), 

 common salt (rock salt, NaCl), and the magnesium salts, usually 

 the chlorides or sulphates. 



While there is more than ten times as much lime sulphate as 

 lime carbonate in the ocean (p. 289), deposits of the carbonate 

 (including shells, coral, etc.,) have been very much greater than 

 those of the sulphate. This is due to the following facts: (1) The 

 sulphate is much more soluble in natural waters than the carbonate, 



(2) rivers bring much more carbonate than sulphate to the sea, and 



(3) marine plants and animals extract lime carbonate from the 

 water for their skeletons, shells, etc. The secretion of lime car- 

 bonate by organisms is not dependent on the saturation of the water, 

 but may be carried on when the amount in solution is very small 



