740 JOHN JOHNSTON AND E. D. WILLIAMSON 



and is the only procedure which will yield accurate results and lead 

 to correct conclusions. Moreover, a comprehension of this question 

 is desirable because this apparently complex equilibrium is typical 

 of what takes place in many other systems, aqueo-igneous and igne- 

 ous as well as aqueous; it is in but very few cases, however, that we 

 know even what molecular species are important factors in the 

 equilibrium, and in still fewer is any information available as to the 

 quantitative relations at equilibrium. 



There has, moreover, been considerable misapprehension as to 

 the role of hydrostatic pressure in increasing the solubility of CaC0 3 ; 

 thus in a recent paper Daly 1 writes: "On account of the higher 

 temperatures and lower bottom pressures (pressure increasing the 

 solubility of the carbonate) of the shallower water we should expect 

 the rate of chemical precipitation of calcium carbonate at the bottom 

 to be concentrated in the neritic (epicontinental) and shallower 

 bathyal regions." And many smiliar statements relative to the 

 effect of hydrostatic pressure might be quoted. As a matter of fact, 

 the hydrostatic pressure 2 acting on the water is of itself an abso- 

 lutely negligible factor; thus water a mile below the surface of the 

 sea will hold in solution an amount of CaC0 3 which does not differ 

 by an appreciable quantity from the amount the same water at the 

 surface will hold, provided that the concentration of free C0 2 and 

 the temperature be the same in both cases. The increased solubility 

 with depth in the ocean is due entirely to the lower temperature of 

 the water and to the increased proportion of free C0 2 , but not at 

 all to the increased hydrostatic pressure there prevailing. 



The only pressure which does affect the solubility is the partial 

 pressure, i.e., the proportion, of C0 2 in the layer of atmosphere 3 in 



1 R. A. Daly, Bull. Geol. Soc. Am., XX (1909), 156; also in "Geology of North 

 American Cordillera," Memoir No. 38, Geol. Survey of Canada, Part II (1912), p. 651. 

 Italics are ours. 



2 Increase of hydrostatic pressure decreases the solubility of some substances. In 

 any case the effect is very small indeed; its magnitude and direction can be calculated 

 if the appropriate data on volume changes are known. Cf. Johnston and Niggli, Jour. 

 Geol., XXI (1913), 504, where references are given. 



3 The proportion of C0 2 in the air increases, ceteris paribus, as we pass from higher 

 to lower levels; but this is a factor of no moment to the present discussion because 

 the diffusion downward through the water is in all probability very slow in comparison 

 with the natural circulation of the water. 



