GNEISS AND LIMESTONE CONTACT PHENOMENA 283 



that magnesium-bearing carbonates will react with the silica at 

 lower temperatures than calcium carbonate. Each silicate to be 

 formed may have its own equilibrium curve. 



This assumption has been very strongly supported by petro- 

 logical investigations which I have carried out in the pre-Cambrian 

 Umestone of Finland.^ In the southeastern part of the pre- 

 Cambrian area of Fennoscandia there are extensive formations all 

 of whose characters point toward metamorphism at low tempera- 

 tures. In the limestones of this area, quartz occurs together 

 with dolomite, and fine scaly micaceous minerals (biotites rich in 

 magnesia) are usually the only silicates. Sometimes epidote 

 occurs, a fact indicating that this hydrated calcium aluminium 

 silicate also belongs to the low temperature minerals. 



Northwest of this low temperature area there is a broad zone 

 within which the limestones contain, besides mica, amphiboles of 

 the tremolite-actinolite series, but no other silicates. Going 

 farther northwest diopside-hedenbergite is added to the list of 

 minerals of the metamorphic limestones, together with many others, 

 such as scapolite, vesuvianite, and grossularite-andradite. Finally 

 we find, as local developments near the contacts of igneous masses, 

 but never in regional distribution, wollastonite limestone, and 

 in this most of the other silicates may occur also. This mode of 

 occurrence clearly indicates that wollastonite, among the lime- 

 bearing silicates, requires the highest temperature to form. 



We may thus discriminate the following four paragenetic types 

 of limestones : 



1. Quartz limestone, in which quartz is coexistent with the 

 dolomite. 



2. Tremolite limestone. Tremolite is usually present; quartz 

 occurs in calcite-rock, but is not coexistent with dolomite. 



3. Diopside limestone. Diopside is usually present, quartz 

 occurs together with calcite but not with dolomite. 



4. Wollastonite limestone. Wollastonite is present, provided 

 the rock contains silica in excess of the amount needed to form the 



' Pentti Eskola, Victor Hackman, Aame Laitakari ja W. W. Wilkman, Suomen 

 kalkkikivi. With an English summary by P. E., "Limestones in Finland," Suomen 

 Geologinen Toimisio, Geoteknisid Tiedonanioja, No. 21 (1919). 



