CHAPTER XII. 

 CONTACT METAMORPHISM. 



THE intrusion of a large mass of igneous rock is frequently 

 accompanied by important changes in the structure and 

 mineralogical composition of the surrounding rocks. This is especially 

 the case when the surrounding rocks belong to the sedimentary 

 group. 



The extent of the metamorphism, other things being the same, 

 is dependent upon the mass of the intruded rock. The character of 

 the metamorphism is mainly dependent upon the nature of the rock 

 affected. Much work will have to be done before we shall be in a 

 position to give a general account of the phenomena of contact 

 metamorphism. There are, it is true, certain general resemblances 

 between the phenomena observed in different localities ; at the same 

 time each district exhibits certain peculiarities. The same rock, or 

 what appears to be the same rock, is not always similarly affected. 

 Limestone may, for example, in certain cases be merely rendered 

 crystalline, whereas in others it may be entirely replaced by 

 crystalline silicates such as garnet, vesuvianite, wollastonite, &c. 



Having regard, then, to the local peculiarities and to the imperfect 

 state of our knowledge it seems better to consider the facts as they 

 have been observed in a few typical localities than to attempt a 

 description of the phenomena in general terms. We will commence 

 by referring to certain foreign localities in which detailed observations 

 have been made. 



The effect of the biotite-granites (granitites) of Barr-Andlau and 

 Hochwald, in the Vosges, on a tolerably homogeneous mass of clay- 

 slate (Steiger Schiefer) lias been described in considerable detail by 

 Professor ROSENBUSCH.W The normal rock of the Steiger Schiefer 

 is clay-slate of a purplish colour. Blue, bluish-black, black and brown 

 varieties may, however, be observed. The schistosity is as a rule 

 parallel with the bedding. The rock is dull on a cross-fracture, but 

 presents a glistening appearance on the cleavage surfaces. It contains 

 grains and concretionary patches of quartz. Limestones and grau- 

 wackes occur in the series, but they play only a very subordinate 



(1) Die Steiger Schiefer. Abh zur geol. Specialkarte von Elaass-Lothringen. Band I. 

 Heft II. Strassburg, 1877. 



