323 



as moderately dark-greyish, fine-grained aggregates, with 

 the occasional development as phenocrysts of the characteristic 

 blue opalescent quartz seen in the main granite. Microscopic- 

 ally the segregations are seen to consist of the minerals of 

 the main granite. Biotite is present in ragged flakes and, 

 in parts, is altered to chlorite; epidote is also present as a 

 secondary product. In some cases the remains of biotite are 

 now only represented by chlorite and epidote together. 



Ilmenite is sparingly present with its leucoxenic 

 decomposition product. 



Phenocrysts of quartz and oligoclase-andesine or andesine 

 are present, and the remaining mass consists of a fine-grained 

 assemblage of allotrimorphic quartz and microcline, mostly 

 untwinned.< 2 ) 



The microcline is heavily dusted with kaolin. With the 

 plagioclase the secondary development of scaly mica appears 

 more usual. There is a minor amount of graphic intergrowth 

 of quartz and felspar (microcline). 



The specific gravity of one of these ovoid segregations 

 was determined as 2'655 (16° C). The specific gravity of 

 the main granite is 2"668 (16° C). D 1 ^ = 2-668. 



The segregations show a variable amount of biotite, the 

 one in question being, if anything, freer from this mineral 

 than the average. In some cases their slightly darker colour, 

 as an indication necessarily of a greater concentration of 

 biotite than in the main granite, is probably illusory, in 

 that they are finer grained, and the biotite is more evenly 

 distributed than in the main granite. 



(b) THE MINOR INTRUSIONS. 



For the purpose of later discussion these minor intrusions 

 can be separated into three distinct groups: — (a) The grey 

 aplite; (b) the pink aplite; (c) the white pegmatite. These 

 will be treated seriatim. 



(a) The Grey Aplite. — This occurs as an intrusive mass, 

 elliptical in plan, behind Barn Bluff (vide map). In hand 

 specimens it is a fine-grained light-grey rock with development 



(2) The absence of microcline twinning in some sections of 

 the potassic felspar is not considered sufficient evidence to 

 interpret the rock as possessing orthoclase in addition. The 

 presence of microcline is definitely fixed by its characteristic 

 "grating" structure, but it is quite possible for microcline to 

 occur with albite twinning alone or no twinning at all. The very 

 general presence of microcline in the older plutonic rocks is 

 suggestive that this mineral is really the stable phase of potassic 

 felspar. On this question cf. C. H. Warren, "A Quantitative 

 Study of certain Perthitic Felspars" (Proc. Amer. Acad. Arts 

 and Sciences, vol. 51, No. 3, 1915, pp. 127-154). 

 l2 



