257 



The refractive indices were obtained by the Shroeder van der Kolk method 

 of oil immersion, while the specific gravities were determined with a Pycnometer. 



Petrological Features of the Minerals. 



Magnetite. — The grains are rather variable in appearance, some being 

 lustrous black, others dull and rusty. They are always opaque and generally 

 well rounded, but occasionally abraded octahedrons are present. The specific 

 gravity varies considerably in different samples owing to partial hydration, etc., 

 but ranges between 41 and 51. 



Ilmenite. — Opaque grains, black and lustrous, chiefly well rounded, but some- 

 times platy or showing abraded crystal forms. Many of the grains show patches 

 of yellowish-white leucoxene. The magnetic permeability varies considerably 

 in different grains, probably in accord with a fluctuation in the titanium content. 

 Specific gravity is 502, a value corresponding to titaniferous magnetite rather 

 than ilmenite proper. 



Almandine Garnet. — Occurs essentially as irregular grains with rounded 

 outlines, though well-formed crystals are occasionally to be seen, especially in 

 the Petrel Cove sand. The colour varies from light pink to deep red. The 

 grains are perfectly isotropic and show no sign of cleavage. The refractive 

 index is high and greater than that of methylene iodine. vSome of the grains 

 show an abundance of solid black inclusions. The specific gravity is 4' 16. 



Tourmaline. — Rather common, as well-rounded grains, in the Middleton 

 sand, rarer in Rosetta Head deposits, but more frequently euhedral. Colour 

 very variable, from light smoky-grey through brown to bluish-black or opaque. 

 Pleochroism very strong, light-coloured varieties giving complete absorption of 

 the ordinary ray. No cleavage shown, prismatic forms give straight extinction. 

 Refractive index high, about 163. Some grains show acicular inclusions. The 

 specific gravity varies from 31 to 33. 



Green Spinel. — Typically as rounded grains of olive-green to light-green 

 colour. Occasionally an abraded octahedral form occurs. No cleavage observ- 

 able. With crossed nicols the grains are quite isotropic. Inclusions are rare 

 though acicular rods sometimes occur. Refractive index high and near to that 

 of methylene iodide, about 172. Specific gravity, 40. 



Staurolite. — This mineral is the chief component of the weakly magnetic 

 group. Although very variable in appearance most of the grains are anhedral 

 with a pronounced rough surface, due to an abundance of solid inclusions. 

 Well-formed crystals are occasionally present in the Petrel Cove sand. The 

 colour varies from brown to yellow in the opaque grains, and in the rarer trans- 

 parent varieties it is a deep reddish-brown. The clear grains show strong 

 pleochroism in yellow and brown, and also give straight extinction. The refrac- 

 tive index is high and very near to methylene iodide ; the birefringence is moder- 

 ately low. 



An interrupted cleavage, parallel to (010), is occasionally shown; twinning 

 not observed. 



The nature of the "inclusions varies considerably; in some instances they 

 are gaseous, but more commonly appear to consist of quartz ; occasionally they 

 are carbonaceous, but in no instance is there a symmetrical distribution of the 

 included material. 



The specific gravity varies considerably ; in the purer varieties it is generally 

 3'57, but often falls as low as 3'45, when inclusions are abundant. 



Kyanite. — Rather common in the non-magnetic group. Almost invariably 

 as flat grains and lath-shaped forms. Colour generally bluish-green but often 

 colourless or altering to brown among the cleavage cracks. Many grains show 

 colourless borders and blue centres. Cleavage cracks, parallel to (001), are 

 generally shown across the laths. 



Pleochroism often rather weak, in coloured grains only. Refractive index 

 high, a little less than that of methylene iodide, about 171. 

 I 



