282 PENTTI ESKOLA 



Certain facts known at present would seem to indicate that the 

 curve may lie at somewhat lower temperatures. Its general 

 character, however, agrees well with petrological experience and 

 is not likely to be subject to any great changes. 



From the standpoint of the phase-rule, a mixture of calcium 

 carbonate and silica is a three-component system and therefore 

 can have a maximum of three phases in coexistence under variable 

 pressure and temperature conditions, that is, in a divariant system. 

 Below the transformation curve the possible three-phase combina- 

 tions are either silica, calcite, and carbon dioxide or silica, calcite, 

 and wollastonite. Above the curve we may have either wollasto- 

 nite, carbon dioxide, and calcite or wollastonite, carbon dioxide, 

 and quartz (or any other form of silica). 



Thus wollastonite may be stable and even be formed at tempera- 

 tures below the transformation curve, but only if silica meets 

 lime in other form than carbonate and no free carbon dioxide is 

 present. The occurrence of wollastonite, therefore, is not in itself 

 a sufficient evidence that the rock had been heated above the 

 transformation temperature. On the other hand the occurrence of 

 both quartz and calcite proves conclusively that it never has. 



The temperature of transformation may be considerably 

 depressed if the partial pressure of carbon dioxide is kept low as a 

 result of its continual removal, e.g., by circulating solutions. This 

 case may occur in the formation of metasomatic ore deposits, and 

 in fissure veins. The conditions are here complicated also because 

 the addition of substances and formation of minerals may have 

 continued at different temperatures. As a matter of fact, observa- 

 tion usually indicates a definite sequence of formation of minerals 

 in such deposits. 



While, then, many complications are to be expected where 

 circulating solutions have been prominent agents, we may still use 

 limestone as a geologic thermometer with considerable confidence 

 in those cases in which quartz or the silicates occur evenly dis- 

 tributed as true rock constituents. 



As we are concerned, in natural rocks, with calcium and magne- 

 sium carbonates, the resulting silicates will be those of calcium and 

 magnesium, or double compounds of both. It is to be expected 



