PSEUDOBRECCIATION IN ORDOVICIAN LIMESTONES 413 
phenomenon when the calcareous mud was as yet only partially 
solidified. We have then to account for the selective dolomitiza- 
tion of the calcareous ooze. Dolomite is formed according to the 
following reversible reaction: 
2CaCO,+ MgCl.= CaCO; - MgCO;+ CaCl. 
and the action is found to proceed from left to right when a tempera- 
ture of 100° C. is reached. Increase of temperature accelerates 
the action. It is well known, however, that dolomitization has 
taken place, and is taking place today, where the proportion of 
Mg salt to Ca salt is much below that represented by the above 
equation. The process that goes on in nature is perhaps more 
accurately represented, as Klement has suggested,’ by a continu- 
ous readjustment of equilibrium between the solution pressure 
of the solid CaCO, and the pressure of the Mg ions out of solution. 
Dolomitization would then ensue after the CaCO, had been precipi- 
tated; or, more correctly, a transformation takes place by which 
crystals of the optical characters of dolomite are formed, though 
the percentage of MgCO, may be smaller than that required for a 
true dolomite. In other words, dolomite is seemingly capable of 
forming mixed crystals—up to a certain limit—with MgCO,, a 
substance not strictly isomorphous with itself. If then the reaction 
be stated crudely as Mg’’sCa, as an abbreviation for the statement 
that the two reactions Mg’ Mg and CasCa” are not independ- 
ent, but regulate each other, three factors would affect the equilib- 
rium: (1) percentage of Mg” in the sea water, (2) temperature of 
the water, (3) character, though not of course the quantity, of the 
CaCO;. An increase of Mg ions, and increase of temperature 
move the equilibrium point from left to right; while CaCO, in the 
form of aragonite is more readily affected, especially at moderately 
high temperatures, than is calcite, presumably because of the greater 
solution pressure of the CaCO, in the aragonite modification. In 
seeking for an explanation of selective dolomitization in the Mani- 
toba limestones, one may practically discard the second and third 
factors. Local temperature changes may be neglected, except 
in so far as taken into account in a hypothesis outlined below, 
and there is no indication that aragonite shells have been attacked. 
1 Tscherm. Min. Petr. Mitteil., XIV (1895), 530. 
