GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 495 



of aplitic and pegmatitic veins which passes gradually into 

 the normal rock of the mass. It is found both in large 

 crystals of irregular outline, and playing the part of an 

 interstitial material between the crystals of pyroxene and 

 of both felspars, in optical continuity with either of the 

 latter. In the pegmatites all three kinds of felspar are 

 combined in each crystal. The core is labradorite and 

 becomes gradually more acid outwards as in the felspar of 

 the main rock until perhaps a basic andesine is reached ; 

 then there is a sudden change to alkali felspar, first 

 apparently a narrow zone of albite and then microperthite, 

 which at least in one or two cases, seems to have a rough 

 radial arrangement and would therefore be primary. 



Pyroxenes. 



In the main rock the dominant ferro-magnesian mineral 

 is a titaniferous augite of the usual violet-brown colour, 

 and pleochroic. The content of titanium-oxide shown by 

 the analysis is, however, less than might have been expected 

 from its colour. The "hour-glass" structure is well shown. 

 Occasionally (as in specimen X) it is copper colored, and 

 therefore presumably more titaniferous. The augite never 

 plays a wholly interstitial role, most of the crystals having 

 a tendency to idiomorphism. Sometimes, and apparently 

 in the neighbourhood of segregation veins, is is completely 

 idiomorphic. 



In the segregation veins, both aplitic and pegmatitic, 

 the pyroxene is mainly a pale green augite not pleochroic. 

 Its composition as calculated from the bulk analysis of the 

 coarser aplitic rock will be found in the appendix. There 

 is also aegyrine-augite occurring into it, and as separate 

 small crystals frequently idiomorphic. It has a duller and 

 rather more olive-green colour than pure aegyrine, and its 

 pleochroism is not so strong. 



