SALINITY, GASES 51 



latter which impart to sea-water its alkaline 

 reaction. 



Salts imthdrawn from Sea-Water. — Carbonate- 

 of-lime-secreting organisms withdraw large 

 quantities of calcium carbonate from ocean- 

 water to form shell and coral. Part of the 

 material so withdrawn accumulates in coral 

 reefs and in the pelagic deposits of moderate 

 depths in the ocean, while part is redissolved 

 after the death of the organisms, for the 

 calcareous shells of many pelagic organisms 

 never reach the greatest depths of the ocean, 

 and only relatively few of the heavier pelagic 

 shells reach the depths where red clay covers 

 the bottom. In like manner silica is with- 

 drawn from the ocean by organisms such as 

 sponges, diatoms, and radiolarians, and nxay 

 be again redissolved. 



Potassium salts are withdrawn from ocean- 

 water to combine with the silicates of alumina 

 in argillaceous materials to form glauconite 

 in the presence of organic matter. Potassium 

 and iodine are selectively absorbed by certain 

 species of seaweed ; indeed, until recent times 

 seaweed was the chief industrial source of 

 iodine. 



When sea-water freezes many of the salts 

 in solution are left behind in the brine, so 

 that the salinity of the water from which 

 the ice has been formed is increased, the ice 



