144 



THE ROYAL SOCIETY OF CANADA 



Case II. — In the case of an inland basin exposed to solar concen- 

 tration, the initial stage may be represented by a solution of composition 

 A (Fig. 3), which has not yet reached equilibrium with the calcite 

 which has been deposited by organic agency: or, it may be, by a so- 

 lution of composition C. with no solid phase in contact with it. On 

 concentrating, the solution changes in composition along the line AB 

 (or CD) ; continuous precipitation of calcite now ensues, till the water 

 is represented in composition by P. when dolomite a' so forms. With 

 continued operation, dolomite continues to form, calcite becomes un- 



Figure 3 



stable and passes into solution, and the composition of the solution 

 moves from P towards Q. 



Case III. — In this case Mg-bearing waters descend (or ascend) 

 into limestones which are in contact with relatively small quantities 

 of underground water, and where the stable solid phase is calcite, for 

 which the water is saturated. The composition of the underground water 

 may be represented by A. With the introduction of the meteoric (or 

 abysmal) water the composition of the solution will suddenly change 

 to some point B, where dolomite is the stable phase. Calcite will go 

 into solution, and dolomite will form. If calcite continues to dissolve, 

 the concentration of the solution will gradually change to P, where 

 calcite and dolomite are both stable. 



In none of the cases considered, does the dolomitization take place 

 by the dissolving of excess of CaCOj, and consequent increase in relative 



