AGENCIES IN DEPOSITION OF CALCIUM CARBONATE 737 



this discussion, then, it follows that three of these arguments are not 

 conclusive as to the point at issue. 



Let us now consider the modes in which CaC0 3 may be precipi- 

 tated. We shall for convenience arrange them under three heads, 

 which, however, cannot be sharply differentiated: (1) direct evapo- 

 ration of the water; (2) through organic agencies; (3) change of 

 conditions, especially of temperature and concentration of free C0 2 , 

 these being the predominant inorganic factors. 



1. By direct evaporation. — When natural waters evaporate, CaC0 3 

 is commonly (though not necessarily) the first substance to be 

 deposited, and may be very largely precipitated before any of the 

 other salts separate; 1 the more soluble salts, moreover, will tend to 

 be leached out of such deposits. But since all such deposits are of 

 obvious origin and of minor importance, they need not detain us 

 further. 



2. Deposition through organic agencies. — The agencies which come 

 under this category are of the greatest importance and are predomi- 

 nantly responsible for the deep-sea deposits, yet little as to their 

 mode of action can be definitely stated until more is known about 

 the biologic processes involved. This question is altogether beyond 

 the scope of this paper; we shall mention merely two established 

 effects of organic agencies, reverting to them later: viz., the abstrac- 

 tion of free C0 2 from fresh water by growing plants, 2 and the 

 production of ammonia in sea-water by decaying organisms or by 

 bacteria. 3 Both of these effects disturb the equilibrium in a solu- 

 tion originally saturated with CaC0 3 , the former by diminishing the 

 concentration of H 2 C0 3 , the latter by increasing the concentration 



1 Cf. Van't Hoff's Ozeanische Salzablagerungen. 



2 Various references to these effects are given in Clarke's Data of Geochemistry 

 under "Limestone." See especially C. A. Davis, Jour. Geol., VIII (1900), 485, 494; 

 IX (1901), 491. According to Murray, the calcareous algae common in the warmer 

 oceans no doubt secrete their skeletons in the same way. See also S. T. Powell, 

 "Effect of Algae on Bicarbonates in Shallow Reservoirs," Jour. Am. Waterworks 

 Assoc, II (1915), 7°3- 



3 This question has been discussed recently by G. H. Drew (Camegi f Institution 

 Pull. No. 182 [1914], p. 7), and by Kellerman and Smith {Jour. Wash. Acad. Sci., IV 

 [19 14], 400), and so need not be treated here. Many decaying organisms and bac- 

 teria (as weU as the respiration of animals) produce C0 2 , and to this extent they would 

 act as an adverse influence on the deposition of CaC0 3 . 



