412 S. ALLPOKT OS THE METAMOBPHIC ROCKS 



In this instance, the lateral pressure has reduced the dimensions 

 of the rock in one direction to about two thirds of its original 

 volume. 



As previously stated, the extreme metamorphism of the Cornish 

 slates extends to a very short distance only from the granite ; indurated 

 slates traversed by quartz -veins then occupy a much wider zone ; and 

 the rocks at a still greater distance are ordinary " killas " or clay- 

 slate, having a more or less distinct slaty cleavage, but differing 

 greatly in texture and mineral constitution from those just de- 

 scribed. 



As is well known, nearly all the Cornish granite is schorlaceous ; 

 and it also very frequently contains two varieties of mica, the prin- 

 cipal mass consisting as usual of quartz and felspar. Now, of these 

 constituents, felspar is the only one absent from the altered slates ; 

 it is stated, however, by De la JBeche that in other Cornish localities 

 rocks of gneissic character are found in contact with granite. 



The facts observed appear therefore to warrant the conclusion 

 that the intrusion of the granite has not only altered the structure 

 of the slates, but has also developed in them some of its own con- 

 stituents. De la Beche was evidently inclined to adopt this view of 

 the introduction of mineral matter from the adjoining granite ; and 

 he concludes his observations on the subject with the following 

 remark : — " It is difficult to conceive that nothing should have 

 escaped from the hot fluid granite in the shape of vapour into the 

 pores and cracks of the adjoining disrupted beds, or that, when the 

 mass was sufficiently cool to allow water to permeate among both 

 rocks at a high temperature, some modifying combinations were not 

 then produced"*. 



Summary. 



It appears from the preceding investigations that the alteration of 

 the slates in contact with the granite has been of two kinds, minera- 

 logical and structural. 1. The minute particles of an ordinary clay- 

 slate have been replaced by crystalline quartz, tourmaline, and three 

 distinct varieties of mica, to which tremolite, magnetite (and anda- 

 lusite ?) have occasionally been added, and in other localities felspar 

 also occurs. 



2. The texture produced by the grouping of these minerals is 

 various, the two most remarkable being : — A, foliation more or less 

 perfect, with every gradation from nearly straight parallel lines to 

 the most complicated contortions ; and B, concretionary, showing a 

 decided tendency to segregation of both quartz and mica, the result 

 being a spotted schist. 



Minerals produced by Metamorjphic action. 



The minerals by which the foliated schists are characterized are 

 quartz, tourmaline, and mica ; each of them presents special points 



* Eeport on the Geology of Cornwall, p. 270. 



