142 THE ROYAL SOCIETY OF CANADA 



are present) to X, where the Ca ions are in equilibrium with calcite 

 as solid phase for that particular temperature and pressure. Similarly 

 the horizontal line represents a seawater with no Ca ions, but an in- 

 creasing amount of Mg ions from O to Y. Between these lines all 

 variations in the concentration of Mg and Ca in the seawater may be 

 represented. We are concerned here particularly with the relation- 

 ship between calcite and dolomite; and the conditions of stability of 

 these solid phases are represented at normal temperature and moderate 

 pressure diagrammatically by XP and PQ respectively. There is 

 no attempt at quantitative accuracy, but the final conditions of equi- 

 librium must be somewhat as indicated. The solubility of CaCO, 

 is probably not lowered by MgCOg when NaCl is present.* On the 

 other hand, unstable phases of CaCOg — particularly aragonite — may 

 form.t The solubility curve for aragonite lies however, above that for 

 calcite; and under conditions of maximum stability calcite is the solid 

 phase, even in presence of a certain amount of Mg". The stability 

 of calcite in normal seawater at ordinary temperatures and moderate 

 depths indicates that the equilibrium curve for calcite must under 

 these conditions extend considerablely beyond the bisector of the 

 angle XOY, as the proportion of Mg to Ca in normal seawater is roughly 

 3:1. The actual position of the intersection of the two curves is 

 undoubtedly considerably affected, not only by temperature and 

 pressure, but also by the character of the seawater. % The nature of 

 the curve PQ in its extension towards the OY axis, and its intersection 

 with equilibrium curves for Mg salts, are problematical; but with this 

 we are here not specially concerned. 



Three Dolomitization Processes. 



With the aid of the diagram, three dolomitization processes may 

 be considered. I. Dolomitization of the type suggested in the paper 

 on the Manitoba limestones, where the process is supposed to be due 

 to magnesian salts liberated from the algae. II. Dolomitization due 

 to evaporation and concentration of the waters of inland seas. III. 

 Dolomitization due to percolation of Mg-bearing waters through 

 limestones (subsequent dolomitization). It is not to be understood 

 that these three typical cases are meant to embrace all dolomitization 

 processes. Many cases of dolomitization may be due to the leaching 

 of calcium carbonate out of elevated limestones by presumably acid 

 meteoric waters. This process is specially emphasized by Clarke.* 



*Hofmeister Journ. Prakt. Geol. 2, 14, 176. 

 tLinck Jenaisch. Zeitschr. f. Naturw., 1909, 267. 

 JSee Pfaff's Experiments, 1. c. 

 *Data of Geochemistry, p. 541. 



