BY W. G. WOOLNOUGH. 533 



index being about normal. Cleavages parallel to -<| 110 [- are 

 rather imperfect, while those parallel to the vertical pinacoids 

 are both rather strongly developed, though to a very unequal 

 degree. A few of the crystals are twinned in the usual manner. 

 The greatest extinction angle in the vertical zone was 39°, indi- 

 cating (Levy & Lacroix, 'Les Mineraux des Roches,' p. 262) a low 

 percentage of iron. The double refraction is, for augite, rather 

 weak. In convergent light a section rather oblique to the 

 prismatic zone shows the emergence of a positive bisectrix almost 

 perpendicular to the plane of section. The optic axial angle is 

 small, since the hyperbolic brushes do not separate widely. The 

 dispersion is not noticeable. 



The augite is fairly free from inclusions, an occasional grain of 

 magnetite being the only kind noticeable. 



Magnetite is only moderately plentiful in grains and crystals 

 scattered through the rock. 



1 i V i n e-b earing Andesite (Tama ni Ivi, Mt. Victoria). 



Plate xxxvi., fig. 8. 



MacroscopicaUy the rock is bluish-grey in colour and very fine 

 in grain. It is rendered strongly porphyritic by perfectly fresh 

 striated felspar up to 5 or 6 mm. in length. Less abundant than 

 felspar is augite, notably dark in colour when compared with 

 that of the other rocks hitherto described. An occasional grain 

 of yellow-green olivine is met with. Magnetite is plentiful in 

 brightly reflecting octahedra of small size. Specific gravity, 2*73. 



Microscopic charactei^s. — In thin section the base is almost 

 entirely crystalline. The amount of residual glass is small, so 

 that the texture may be termed pilotaxitic. Where glass is recog- 

 nisable, it is colourless and appears to be free from crystallites. 



Of the microlitic constituents of the base, by far the most 

 abundant is felspar. This occurs in lath-shaped sections which 

 do not show any trace of fluxion arrangement. The refractive 

 index is much about the same as that of the Canada balsam; if 

 anything, rather lower on an average. In polarised light the 

 laths appear either simple or twinned only according to the 

 34 



