284 PENTTI ESKOLA 



magnesium-bearing silicates. Quartz and calcite do not occur in 

 contact with each other. Instead either of the combinations 

 calcite-wollastonite or wollastonite-quartz occurs. 



Actual rocks have commonly been found in fair accordance 

 with these rules of association. As was pointed out above, wol- 

 lastonite is in itself stable below the transformation temperature of 

 calcite-silica, if there is no free carbon dioxide present. In the 

 same way diopside and tremolite are stable below those tempera- 

 tures where they may be formed from the carbonates and silica, 

 again provided that there is no carbon dioxide present. Now 

 there actually is no free carbon dioxide after the formation of the 

 silicates, as it is carried away either as gas or in solutions. There- 

 fore, although the reactions are reversible, they are not reversed 

 during the period of gradual cooling when the rocks are brought 

 up toward the earth's surface through the process of denudation. 

 This fact, of course, adds very much to the usefulness of meta- 

 morphic limestone as a geologic thermometer. We always read 

 the highest temperature to which it has ever been exposed. It is 

 a maximum thermometer. 



The formation of different minerals under different conditions 

 in limestones presents a special case of the more general rules of 

 adjustment of mineral composition in response to the conditions 

 existing. The writer has proposed the term mineral facies^ to 

 designate a group of rocks which have originated under conditions 

 so similar that a definite chemical composition has resulted in the 

 same set of minerals. Applying this principle to various kinds of 

 rocks, we arrive at a natural rock classification in which main 

 divisions are the groups called mineral facies. 



The four paragenetic types of limestone may be paralleled with 

 the facies system of the silicate rocks in the following way : 



Under conditions similar to those that give rise to quartz 

 limestones, rocks having the bulk composition of gabbros have 

 been metamorphosed into chlorite-epidote-albite rocks. This min- 

 eral facies has been called the greenschist facies. 



'P. Eskola, "The Mineral Facies of Rocks," Norsk geologisk tidskrift, Vol. VI 

 (1920), pp. 143-94- 



