COLLECTED AT CAPE ROYDS 181 



larger twinned microlites shows symmetrical estimations of 8° and the smaller ones 

 give extinctions from 0° up to 8°, so that they consist of soda-sanidine. Many of them 

 are forked and bent. The larger crystals are crowded with apatite and magnetite 

 inclusions, the latter being zonally arranged. 



There is a good deal of interstitial augite of a green colour often ophitically enclosing 

 the felspar laths. In some cases the augite grains are covered with a network of black 

 needles strongly suggesting skeleton crystals of magnetite. 



The infrequent phenocrysts consist of tabular felspar with undulose extinction, augite 

 with an extinction angle of 41 °, and large corroded grains of brown sphene. 



One small grain of light blue sapphire occurs in the slide. 



387 (E. 11). Coeundum-beabing Teachyte (Plate II, Figs. 1 and 2) 



Microscopic Characters. — Porphyritic, with trachytic base. 



The base is composed of sanidine needles, augite needles, magnetite, and a little 

 glass. There is a very strong fluidal arrangement of the constituents. Sanidine in 

 crystals 5 mm. long and very narrow shows Carlsbad twinning only. The extinction 

 nowhere measures more than 3° from the length of the needles. 



Augite is light green, much corroded, and of the same order of size as the sanidine. 



Magnetite. Idiomorphic sections very minute but pretty numerous. 



Phenocrysts are numerous. 



Plagioclase in sections up to 4 - 5 mm. by 0*9 mm. with symmetrical extinction of 

 the albite lamelke of 8°, and a refractive index decidedly higher than that of Canada 

 balsam. The plagioclase contains very remarkable inclusions. There are small apatite 

 crystals and grains of greenish augite and of almost opaque dark hornblende ; these 

 are not abundant. There are, however, large irregular patches and inclusions in the 

 form of negative crystals of colourless glass. From the edges of some of the larger 

 cavities partial spherulites composed of slightly greenish fibres extend inwards. 



There are a few large phenocrysts of anorthoclase up to 2 mm. by - 8 mm. of sanidine- 

 like habit. These show Carlsbad twinning, and very occasional small patches of gridiron 

 twinning. The extinction angles are almost straight and the refractive index is about 

 equal to that of Canada balsam. The optical sign is negative and the optic axial angle 

 small. The mineral is perfectly fresh and is probably anorthoclase. The most con- 

 spicuous constituents of the rock are numerous, almost completely altered crystals 

 of dark hornblende up to 1 mm. by 0*5 mm. The resorption has completely darkened 

 the sections in most instances, but here and there small elliptical patches of the original 

 mineral are preserved. The colour is brown and there is strong pleochroism, brownish- 

 yellow when the vibrations are transverse, brownish-orange when they are longitudinal 

 in prismatic sections. In every grain examined the extinction angle was very small, 

 not more than 2°. Some of the sections are distinctly twinned, the extinction being 

 just sufficiently oblique to make the phenomenon apparent. The perfection of cleavage, 

 the colour, the double refraction, and the very small extinction angle all suggest biotite ; 

 but the optic axial angle is very considerable, hence I have called the mineral horn- 

 blende. It is possible that both minerals may be represented. Each, very much altered, 

 crystal is surrounded by a halo of small augite needles and grains in the base. 



There is a little bright red micaceous haematite. 



Augite in purplish phenocrysts, stout prisms - 25 mm. long, is easily confused with 

 the corundum on casual inspection. 



a = brownish-grey; (] = grey- violet ; 



C = purple ; 2V is large. 



