AGENCIES IN DEPOSITION OF CALCIUM CARBONATE 749 



instability, for in presence of water the ju-form transforms to calcite 

 in a few days. Calcite also appears as spherulites and as "amor- 

 phous" CaC0 3 ; but there is little question that the divergent prop- 

 erties of the latter are due entirely to its fineness of grain, i.e., to its 

 extent of surface in proportion to its mass. Consequently the only 

 form other than calcite which we need consider here is aragonite. 



Apparently aragonite is formed in nature (a) through organic 

 agencies (e.g., in certain shells), (b) by deposition from hot springs, 

 (c) when an isomorphous carbonate is present to serve as nucleus, 

 and (d) by chemical precipitation in saline waters, even at ordinary 

 temperatures, under circumstances which we are unable to specify 

 except by saying that the presence of sulphate appears to be a 

 favorable factor. But pure aragonite cannot persist for any length 

 of time in presence of water and calcite, hence only in special cir- 

 cumstances will it be found persisting in the sea. There is, however, 

 the possibility that aragonite may take up in solid solution enough 

 material to bring its own solubility below that of calcite, and hence 

 in the saline solution in equilibrium with the solid solution to render 

 the latter stable with respect to calcite; on this basis it is possible 

 that such impure aragonite may persist in contact with sea-water 

 under certain circumstances, although when exposed to the action 

 of meteoric waters it would soon transform to calcite. However this 

 may be, the circumstance that CaC0 3 precipitates otherwise than as 

 calcite would not of itself affect appreciably anything stated in this 

 paper, since the whole effect would be that ensuing upon the substi- 

 tution for the solubility-product constant of calcite of the corre- 

 sponding value for the other form, the latter being certainly no more 

 than twice as great as the former; so the precipitation of the less 

 stable forms is therefore of only subsidiary importance in the present 

 connection. 



Summary. — Though organic agencies are predominantly respon- 

 sible for the deposition of calcium carbonate, yet the purely inorganic 

 factors should also be taken into account in discussions of the mode 

 of deposition. In this paper emphasis has been laid on one point 

 which has not received adequate recognition; namely, the concen- 

 tration of calcium relative to the limiting saturation concentration 

 of calcium carbonate under the particular conditions, or, in other 



