The Several Forms of Calcium Carbonate. 509 



Bahamas,* and found that they contain about 07 per cent of sul- 

 phate reckoned as CaSO, — a quantity of precisely the same order 

 as we found in samples of aragonite precipitated from solutions 

 containing sulphate at ordinary temperature. The fact of the 

 solid solution is significant for two reasons : that it probably 

 determined (1) the precipitation as aragonite; (2) the preserva- 

 tion of aragonite as such. 



(1) There is reason for believing that aragonite does not form in 

 nature except {a) through organic agencies (e. g. in shells); (b) by 

 deposition from hot springs ; (c) when an isomorphous carbonate 

 is present to serve as nucleus; (d) and, apparently, by chem- 

 ical precipitation in saline waters, even at ordinary tempera- 

 ture, under certain circumstances which we are unable to specify 

 except that the presence of sulphate appears to be requisite. 

 It is, however, not possible to state in what way these factors 

 favor the formation of aragonite, nor can we be sure that the 

 above comprise all the important factors. 



(2) Pure aragonite in presence of water transforms slowly 

 into calcite,f so that the preservation of aragonite in any but 

 recent geological formations^: may depend upon its having been 

 kept dry. In the older rocks fossil shells of types such that 

 they were presumably aragonite originally, now consist of cal- 

 cite. Now the solubility of pure aragonite is not much greater 

 than that of calcite, and this difference would be diminished 

 by the presence of other material in solid solution in the aragon- 

 ite; it is conceivable, therefore, that such impure aragonite 

 should be really stable in contact with the natural waters§ in 

 which it formed. On this basis, then, it is plausible (though 

 not yet established) that natural aragonite containing other 

 substances in solid solution should be practically stable under 

 certain conditions. Incidentally, it may be remarked that the 



* The sample (No. 177) was furnished to us by Dr. T. W. Vaughan, who 

 has recorded a mechanical analysis of it in the 1914 Year Book of the Car- 

 negie Institution of Washington. 



fAn interesting example of this is given in a very recent paper by W. 

 Watanabe (Beitrage Min. Japan No. 5, 237, 1915). Cones have formed (and 

 are still forming, depositing at the rate of about 44 grams daily) at the mouth 

 of a series of springs or geysers, the temperature of the water being about 

 94°; these cones have formed in less than a year, yet, though the outer and 

 more recent portions are aragonite, the inner and older part has changed to 

 calcite. 



X For example, the aragonite associated with zeolites in the basalt of Table 

 Mountain. Colorado, described by Cross and Hillebrand (U. S. G. S., Bull. 

 20, 39, 1885), may have been formed in early Tertiary time. 



§ This implies that these waters contained sufficient of the admixed sub- 

 stance that it was in equilibrium as between the liquid and solid solution. 

 Such impure aragonite would cease to be permanently stable in contact with 

 pure water ; thus it may be that chemically precipitated aragonite, which 

 apparently persists under some circumstances in saline waters, is soon trans- 

 formed to calcite when exposed to the action of meteoric waters. 



