MARINE DEPOSITS 215 



standing for some time the excess will 

 be deposited as crystalline calcium carbon- 

 ate, thus causing the filling-up of the inter- 

 stices of massive corals with crystalline car- 

 bonate. Free carbonic acid in sea-water in- 

 creases the solubility of calcium carbonate, cal- 

 cium bicarbonate being formed, but this salt is 

 much more soluble than the normal carbonate 

 and very unstable, so that it may be broken 

 up into normal carbonate and free carbonic 

 acid by evaporation and by rise of temperature. 

 Part of the calcium carbonate in sea-water 

 is withdrawn by lime-secreting organisms 

 to form shell and coral. Generally speaking, 

 organisms secrete calcium carbonate much 

 more abundantly and rapidly in warm than 

 in cold water. In the Arctic and Antarctic 

 Oceans and in the deep sea, where the tem- 

 perature approaches the freezing point of fresh 

 water, there are no great accumulations of 

 calcium carbonate due to secretion by ben- 

 thonic organisms, and the calcareous shells 

 and skeletons secreted by pelagic organisms 

 are thin and fragile. On the other hand, the 

 most abundant secretion of calcium carbonate, 

 both by benthonic and planktonic organisms, 

 occurs in tropical and sub -tropical waters. 

 Thus coral reefs are developed in greatest 

 perfection in those ocean waters where the 

 temperature is highest and the annual range 



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