Ill 



Plagioclase. — This type of felspar is developed to a 

 greater extent than any other mineral in this series of rocks. 

 In different rocks there is some variety in the composition 

 of the plagioclase developed, the range being from albite to 

 basic oligoclase. 



In the type of pegmatite peculiar to this district, and 

 for which the name ijataliie has been suggested, the felspar 

 is a true alhitt . It forms subidiomorphic crystals up 

 to an inch in diameter and of a pale-pink colour. Cleavage 

 flakes parallel to (001) show that albite twinning is developed 

 with very fine lamella?, of which one set is always narrower 

 than the other ; the extinction angle from the twinning plane 

 is 3^ for one set, 5° for the other. Scattered about the sec- 

 tion are small patches of microcline of irregular outline, but 

 frequently elongated parallel to (010), and with one set of 

 its twin lamellae parallel to those of the host, the other set 

 being perpendicular to the first. The refractive index and 

 birefringence of the albite is distinctly higher than that of 

 the microcline. Of higher refracture index than the albite 

 and (in this position) of lower double refraction are small, 

 irregular, or round grains of quartz with many liquid inclu- 

 sions. 



Cleavage flakes parallel to (010) show an extinction angle 

 referred to the trace of (001) of -I- 17°. This measurement, 

 together with that on (001), indicates that the composition 

 of the felspar is near to Ab^^ An-. The refractive index is 

 less than that of Canada balsam. The position of emergence 

 of the acute bisectrix on the (010) is slightly oblique ; the 

 optical sign is positive. Carlsbad and pericline twinning was 

 not observed. The felspar is somewhat decomposed, being 

 dusted with kaolin or paragonite, and a great deal of epidote 

 has been introduced. Liquid inclusion of a very small size 

 is also developed, and apparently is elongated parallel to 

 the vertical axis of the crystal. 



OJifjoclaae is the dominant species in the main mass of 

 the intrusion. It forms anhedrons of a grey or greenish 

 white colour, rarely more than 3 mm. in diameter, and varies 

 down to a tenth of that length ; while in the fine inter- 

 granular material, felspar of a much smaller size is recog- 

 nizable. The twinning is almost entirely on the albite law. 

 Pericline twins sometimes occur, but twinning after other 

 laws has not yet been observed. In most cases intergrowths 

 with microcline are present, as described above, one set of 

 the microcline laminae being always parallel to those of the 

 plagioclase. These intergrowths are generally recognizable 

 in ordinary light by the more advanced kaolinization of the 

 plagioclase and its higher refractive index. In polarized 



