RESEARCHES IN THERMAL METAMORPHISM. 291 



during thermal metamorphism, the carbonates are not de- 

 composed except in presence of silica in some available 

 form to replace the carbonic acid. Consequently the most 

 striking effects are to be observed in limestones or dolomite 

 rocks containing a large percentage of impurities and in 

 calcareous shales, tuffs, etc. Here the carbonic acid is 

 entirely expelled, and rocks are formed consisting in great 

 part or wholly of lime-bearing silicates. Such a rock — the 

 Kalksilicathornfels of the Germans — is often of very com- 

 plex constitution, and may be of so fine a texture as to 

 require very minute study. Of this type are the compact 

 porcellanous-looking rocks consisting essentially of a fine 

 mosaic of augite, wollastonite, felspar, etc., sometimes en- 

 closing rather larger crystal plates of some pyroxene, nests 

 or patches of tremolite needles, etc. On the other hand 

 there are lime-silicate rocks of which the constitution is 

 more simple and manifest, such as the type which consists 

 of dodecahedra of lime-garnet set in a matrix of crystalline 

 idocrase. These and other types are found in various beds 

 of the Coniston Limestone group, where they are metamor- 

 phosed by the Shap granite (3), and in these rocks English 

 geologists may study almost all the phenomena that have 

 been described in the classic districts of the Harz and the 

 basin of Christiania. 



A survey of the recorded facts regarding the lime-silicate 

 rocks leads us to two or three general conclusions. In the 

 first place we note that quite a moderate proportion of cal- 

 careous matter in a rock, which would not cause a field 

 geologist to style it a limestone, is sufficient to make the 

 metamorphism follow this line, and that such a rock, in 

 contrast with a purer limestone, has its carbonic acid com- 

 pletely eliminated in the process of metamorphism. Further, 

 we see that such changes in impure calcareous rocks are 

 quite readily induced, the lime-bearing silicates apparently 

 demanding no very elevated temperature for their produc- 

 tion in this way : this fact seems to connect itself with the 

 rather easy fusibility of the minerals in question, though 

 the conditions of metamorphism are, of course, different 

 from those of dry fusion. Lastly, the great variety in 



