18 



shows the rock to have the following percentage compo- 

 sition (mode) : — 



Orthoclase 25-58 



Albite 29-34 



Anorthite 5 • 00 



Quartz 37-68 



Biotite 0-90 



Magnetite 1-39 



Water 0-26 



Total 100-15 



It will thus be seen that the amount of anorthite 

 present is sufficient, when united with the albite, to give 

 34-34 per cent of an acid oligoclase having the formula 

 Ab 6 An, thus bearing out results obtained by the Thoulet 

 separation. This combination as compared with the 

 orthoclase, is present in about the proportion off 3 to 2. 

 This rock, in the quantitative classification, is a tehamose. 



Dark inclusions are present throughout the granite- 

 gneiss almost everywhere in the area. These are often 

 very abundant, and consist of amphibolite or closely 

 allied rocks. In some places, on account of their abundance 

 and angular character, the granite presents the appear- 

 ance of a breccia. These fragments, while usually more 

 or less angular, have sometimes been softened and drawn 

 out in the direction of the movement of the gneiss so as 

 to impart to the rock a streaked or banded appearance. 

 In other places, the inclusions have been so completely 

 permeated by the granite-magma that they are utterly 

 disintegrated. Every stage of passage from the sharply 

 angular inclusions to the final products of disintegration 

 can be traced in many places, although in most cases the 

 inclusions are well marked and sharply defined against 

 the enclosing gneiss. At many points throughout the 

 red granite-gneiss of the batholiths, moreover, streaks 

 of grey gneiss are found. It is estimated that, taking 

 the granite-gneiss of the whole area examined, the amphib- 

 olite inclusions represent about 10 per cent of its volume 

 and this grey gneiss another 10 per cent. 



The origin of these amphibolite inclusions and of the 

 masses of grey gneiss is not only a question of much 

 interest, but one of considerable importance to the true 

 understanding of the geological processes which have been 



