194 WORK OF THE OCEAN 



The principal deposits of this sort are calcium carbonate (limestone, 

 CaCO 3 ) calcium sulphate (gypsum, CaSO 4 ,2H 2 O), common salt 

 (rock salt, NaCl), and magnesium salts, chiefly the chloride and 

 sulphate. 



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

 lime carbonate in the ocean (p. 167), 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: (i) The 



Fig. 200. Sun-cracks; flat of the Missouri a few miles above Kansas City. The 

 sun-cracks on shore deposits are not essentially different. (Calvin.) 



sulphate is much more soluble than the carbonate, (2) rivers bring 

 much more carbonate than sulphate to the sea, and (3) marine 

 plants and animals extract the carbonate from the water for their 

 skeletons, shells, etc. The secretion of lime carbonate by organisms 

 is not dependent on the saturation of the water, but is carried on 

 when the amount in solution is very small. 



The chief deposits of lime carbonate have been made through 



i the agency of plants and animals, in the form of shells, coral, bones, 



I and other devices for supporting, housing, protecting, and arming 



themselves; but while it is agreed that the larger part of the lime 



carbonate deposited in the open sea is of organic origin, it is equally 



clear that in closed seas subject to concentration from evaporation, 



direct precipitation may take place. There is difference of opinion 



as to the quantitative importance of this last class of deposits. 



