IV DEPTHS AND DEPOSITS OF THE OCEAN 177 



whilst the carbonic acid owes its origin more or less indirectly 

 to the atmosphere and to infra-oceanic respiration. 



In considering by what agencies calcium carbonate may be 

 precipitated from the sea, we can at once set aside two which 

 are of importance in terrestrial geology, viz. removal of solvent 

 by evaporation and change of temperature ; neither are operative 

 in adequate degree in the hydrosphere. Turning to chemical 

 processes we note, in the first place, that the solubility of calcium 

 carbonate in water is nearly proportional to the cube root of the 

 COg-tension,^ i.e. the amount of free CO^ present in solution. 

 Calcium carbonate as such is scarcely soluble at all, but in 

 presence of CO., the bicarbonate Ca(HC03).3 is formed, and 

 this is soluble to a considerable extent. Hence, if CO^ be 

 abstracted, calcium carbonate will tend to come out of solution. 

 Here we have what seems to be the niodzis operandi of cal- 

 careous algae. The plant absorbs CO., by way of nutrition, 

 precipitates calcium carbonate, and thus builds its skeleton. 

 That this process takes place in fresh water, where the bicar- 

 bonate is the chief salt of calcium present, may be considered 

 as established. The mosses Hypiium, Eucladimn, Trichostovia 

 are cases in point, as also Chara. These plants deposit coral- 

 like growths, known to mineralogists as tufa and travertine. 

 Many occurrences have been noted in the Yellowstone Park 

 and other American localities. In some instances the calcium 

 carbonate is aragonitic, as at Carlsbad. The calcareous algae, 

 which are well represented at the surface and at the bottom of 

 the warmer oceans (coccolithophoridae), no doubt secrete their 

 skeletons in the same way as the fresh-water algae enumerated. 



But there is another far more important agency at work. 

 Calcium carbonate must separate out if the product of the con- 

 centrations of its ions Ca"* and CO3" happens to exceed a certain 

 definite limit. Small increases in the concentration of Ca" ions 

 may be disregarded, since their concentration is already consider- 

 able ; but small local accessions of CO3" ions, which, in the shape 

 of alkaline carbonate, may and do occur, are more effective. 

 Marine animals generate, as ultimate products of the metabolism 

 of their proteid food, ammonia and carbon dioxide. These 

 combine to form ammonium carbonate, which in aqueous solution 

 is largely dissociated into NH^' and CO3" ions ; thus calcium 

 carbonate is precipitated with liberation of ammonia, and a shell 

 or coral growth may be formed. The reaction here described, 



1 Schloesing, Coinptes Retidiis Acad. Sci. Paris, vol. Ixxv. p. 70, 1872 ; Bodlander, Zeiischr. 

 Phys. Cheni., vol. xxxv. p. 23, 1900. 



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