COLLECTED AT CAPE ROYDS 183 



The phenocrysts are chiefly purplish titaniferous augite whose size is about *56 rnm. 

 by 0*34 mm. These are slightly pleochroic, have an extinction angle on (010) of 41° 

 from the cleavage, birefringence notably weak for an augite, and a wide optic axial 

 angle. 



Phenocrysts of labradorite up to 7 "0 mm. by 2 - 5 mm. also occur. These have 

 a composition about the same as those of the base and exhibit Carlsbad and albite 

 twinning. One very noticeable feature is the great abundance in them of corrosion 

 hollows or inclusions of brownish glass, a characteristic which ajapears to be common 

 in rock types intermediate in mineral composition and texture between basalt and 

 andesite. 



Large crystals of olivine (L7 mm. by - 8 mm.) occur. The cleavage is very good for 

 olivine, and the optic axial is almost 90°. 



1278. Actinolite Gneiss (Plate II, Fig. 6) 



Macroscopic Character s. — Medium-grained, strongly foliated rock, the colour 

 bands of which are about 25 mm. wide. The lighter coloured bands consist of quartz and 

 white felspar, the darker ones of these minerals together with an equal amount of light 

 green actinolite in roughly prismatic grains. 



Microscopic Characters. — The rock is porphyroblastic with a granoblastic 

 ground fabric. Quartz is the most abundant constituent. In the large, complex grains 

 and in the ground fabric also it shows considerable evidence of strain. The large grains 

 are almost certainly secondary growths. There is a good deal of another colourless 

 mineral whose refractive index is greater than that of the quartz ; this is most likely 

 un twinned plagioclase. An occasional grain shows exceedingly fine lamellar twinning 

 through part of it. Still more colourless material has a refractive index less than that 

 of quartz, and is probably orthoclase. There are occasional phenocrysts of andesine 

 up to 2 mm. by 1 mm. Actinolite is in irregular patches up to 1 mm. by *5 mm. 

 showing sieve- structure. On rotation it changes from faint green to colourless. 



There is a little sagenite. 



A good deal of colourless epidote is present in prisms and aggregates, generally 

 associated with the actinolite. 



440. Tremolite Gneiss (Plate II, Fig. 5) 



Macroscopic Character s. — Grey schistose rock, medium grained, somewhat 

 foliated. On fresh fractures the lustre is almost silky owing to the flashing cleavage 

 faces of the very fine tremolite prisms in parallel orientation. 



Microscopic Character s. — Granoblastic, with a decided tendency to lepido- 

 blastic texture in places. Average grainsize 15 mm. 



Untwinned and multiple-twinned felspar and tremolite are the most abundant 

 constituents, though the amount of quartz is notable. 



Felspars showing albite twinning give symmetrical extinctions up to 23°, indicating 

 labradorite of the composition Ab n An 9 . These felspars are very slightly sericitised. 



There is also a very little untwinned orthoclase. Tremolite is in well-defined rods 

 which are almost colourless, and show, at most, a very faint brownish-grey tint when 

 the vibrations are parallel to the axis of greatest absorption, namely, C. 



Quartz is in irregular grains with well-marked undulose extinction due to strain. 

 Small irregular grains of sphene and crystals of apatite and pyrites are rare. 



