OF MOUNT EREBUS, ANTARCTICA 101 



refractive index is greater than that of Canada Balsam, and the extinction angles are 

 those typical of oligoclase-andesine. 



Anorthoclase. — Both as large and small microscopic phenocrysts with the 

 characteristic shape of sanidine. This felspar is less corroded and not so commonly 

 zoned. Its refractive index is less than Balsam, and the extinction angles agree with those 

 of anorthoclase. The twinning is Carlsbad, and sometimes faint lamellar twinning 

 accompanies it. 



Augite. — This mineral is of a peculiar lilac or bistre colour, and has idiomorphic 

 outlines. It usually extinguishes at from 38° to 45°. Corresponds closely with the 

 bistre-coloured pyroxene described by J. Allan Thomson in an eclogite from Kakanui, 

 New Zealand. (See Geol. Mag., May 1907 ; Dec. 5, vol. iv.) 



Nepheline. — Nepheline occurs as small idiomorphic crystals in the base. Secondary 

 cancrinite also occurs sporadically scattered in patches throughout the rock. Other 

 felspathoids also occur interstitially. Olivine exists as corroded phenocrysts only. 

 Apatite and magnetite as small crystals in the base. Secondary iron ores constitute an 

 important accessory. Brownish glass forms the balance of the rock. 



J. 3 (1913). Kaersuetite (?), JEgirine-augite Trachyte, Observation Hill. — Megascopic 

 characters. — Very fine-grained aphanitic rock of dark grey colour and containing 

 reddish inclusions. 



Microscopic characters. — Holocrystalline, even- grained pilo taxi tic; consisting of 

 lath-shaped and acicular felspars, brown hornblendes, green soda pyroxene, and a little 

 opacite. Of these constituents the first two are partly microporphyritic serial, mini- 

 phyric. 



The felspar laths have straight extinction and appear to be anorthoclase. Green 

 pyroxene was last to crystallise; it occurs in grains and acicular prisms, which have 

 an extinction angle of 34°. A brown amphibole, pleochroic in colours from dark reddish- 

 brown to yellowish-brown, occurs as numerous scattered idiomorphic grains and also 

 as clusters of grains (Plate I, figs. 3 and 4). These clusters represent the remains of 

 almost completely assimilated fragments of a camptonitic rock, inclusions of which 

 remain in the specimen. The brown camptonitic amphibole has the properties of 

 kaersuetite, and this mineral has not been resorbed by the magma, but numerous 

 grains and microphenocrysts of it lie scattered through the base together with opacite 

 grains. (For Analysis see Table I, E, p. 98.) 



J. 50 (1959). Baked Alkali-trachyte, Ross Island, Tuff Cone. — This specimen is 

 a very weathered or baked rock of reddish colour and even grain-size. It has a 

 microholocrystalline, pilotaxitic fabric and consists of anorthoclase, oligoclase- 

 andesine, iron-stained eegirine-augites with an extinction of 35° in phenocrysts, (see 

 Plate II, fig. 6), laths, and grains, magnetite and iron ores largely secondary, and 

 interstitial felspathoids (nosean or sodalite). 



J. 51 (997). Anorthoclase Trachyte. A compact grey specimen marked old lake 

 deposit. — This is typical trachytic trachyte consisting mainly of anorthoclase which 

 shows Carlsbad and Baveno twinning. The other minerals are interstitial rods 

 of aegirine-augite (and its decomposition products, epidote, iron ores, etc.), and 

 idiomorphic magnetite grains. The texture is best described as holomicrocrystalline, 

 uneven, seriate, trachytic. 



J. 62 (1961). Anorthoclase Trachyte. An erratic from Cape Royds. — This was 

 very similar to J. 51. Megascopically, porphyritic-hiatal in felspars of the first 



