236 PETROGRAPHICAL NOTES ON ROCKS 



The plagioclase is mostly contained in and makes up about half the ground -mass. 

 The crystals form an interlacing network between the meshes of which are contained 

 the diopside and allanite ( ?) grains. lhe average size of the felspar laths is about -2 mm. 

 by -02 mm. 



Diopside is very abundant in the ground-mass and most of the phenocrysts are of 

 this mineral. In the base the crystals take either a granular or tabular habit. 



Allanite ( ?). This mineral is very abundant, and is quite comparable in quantity 

 with diopside. It is strongly pleochroic from light to dark brown. Prismatic sections 

 show a good cleavage parallel to the length. The refractive index and double refraction 

 are both high. The crystal grains are very small and uniform in size and average only 

 • 02 mm. in diameter. On this account the sign could not be determined. The mineral 

 resembles, and probably is, allanite, but the small size of the grains makes the deter- 

 mination difficult. 



Quartz is present only as a few inclusions. One large piece 2 mm. in diameter was 

 found quite rounded. A well-marked zone about -4 mm. in width surrounded the grain 

 and was evidently the result of the interaction of the magma and quartz. The zone 

 consists of small diopside crystals arranged with their lengths perpendicular to the 

 outlines of the grain. 



Magnetite occurs as a few scattered grains in the ground-mass. 



Chlorite occurs in feather-like aggregates but does not appear to be due to the 

 alteration of any of the primary minerals. 



No flow structure is present. This rock may be termed an augite porphyrite. 



Granite, Slightly Foliated, from Depot Island 



Crystallinity : holocrystalline. 



Grain size : relative — even to coarse ; absolute — 1 to 5 mm. 



Fabric : granitoid, with very small patches showing micrographic intergrowth. 



Minerals present in order of their abundance : orthoclase, quartz, albite, microcline, 



biotite, labradorite, apatite. 

 Secondary minerals : chlorite and kaolin. 



The orthoclase is fairly fresh, rather more than 50 per cent, being clear and un- 

 decomposed. It has a cloudy extinction and possesses a great number of small wavy 

 fracture lines. These indicate strain after consolidation. 



The quartz for the most part has crystallised after the felspar, but small rounded 

 or oval patches occur as inclusions in the orthoclase. This with the small amount 

 of micrographic structure shows that some of the quartz crystallised before some of 

 the felspar. In this respect it resembles the Skiddaw granite.* On the whole the 

 bulk of the orthoclase crystallised before the bulk of the quartz. 



These two minerals, quartz and orthoclase, constitute more than 85 per cent, of the 

 rock. The remaining minerals occur rather sparingly. Only one crystal of labradorite 

 was observed. This was about 2 mm. in diameter and gave a symmetrical extinction 

 of 28° on the albite twin lamellae. 



Very little apatite was found. 



The rock is a granite having affinities with the pegmatite group. 



* Q.J.G.S. (1895), vol. li. p. 145. 



