OF MOUNT EREBUS, ANTARCTICA 105 



The anorthoclase phenocrysts are often zoned both optically and mechanically, 

 the former being due to varying composition, the latter a corrosion effect. They show 

 good Carlsbad twinning usually combined with cross-hatching due to fine lamellar twin- 

 ning in directions which cannot be assigned definitely to any of the common twinning 

 laws (see Plate II, fig. 3). 



The ground-mass felspar seldom shows this fine twinning, though occasionally it has 

 distinct albite and pericline twinning, as well as Carlsbad. The plagioclases are likewise 

 frequently zoned, and have well-marked Carlsbad, albite, and pericline twinning. 

 They belong chiefly to the oligoclase species, though they may range to the basicity of acid 

 labradorite. 



The other minerals have already been described to a sufficient extent. 



2. GROUP OF ACID KENYTES 



General Characteristics. — Hand-specimen, light grey to reddish with large felspars, 

 forming about half the mass. The texture and general composition have already 

 been indicated. The large anorthoclase and microcline microperthite phenocrysts, 

 forming from 30 per cent, to 50 per cent, of the mass, have in addition to the properties 

 already mentioned a refractive index very near that of Canada Balsam ; their peculiar 

 microperthitic and polysynthetic twinning only become visible near the position of 

 extinction : included in them we have rounded quartz and orthoclase grains, apatite, 

 olivine, magnetite, pyroxene shreds of glass and iron ores. 



The other phenocrystalline minerals are, a?girine and segirine-augite, which are 

 usually uncorroded, but sometimes slightly resorbed ; magnetite in idiomorphic or 

 more or less corroded crystals ; apatite in stout diomorphic rods ; olivine in corroded 

 grains. The base varies from holocrystalline, either pilotaxitic (strahlenkornig) or 

 trachytic, to kypocrystalline (hyalopilitic), hemicrystalline (vitrophyric), and 

 holohyaline. The minerals of the base are often porphyri tic- serial, and consist of 

 felspar microlites, a?girine, soda amphiboles, apatite, magnetite, and glass. In a few 

 cases colourless felspathoid or other isotropic material which gelatinises with dilute 

 acid is also present ; this may be sodalite, nosean, or analcite ; the analyses indicate 

 a frequent abundance of nosean. 



The most typical acid kenytes were got from the Skuary. 



J. 47 (1956). The Skuary. — Hand-specimen, grey, rough with coarse crystals. 



Texture. — Porphyritic hiatal in phaneric rhomben- felspars, sempatic, magnophyric. 

 The base is holocrystalline, pilotaxitic, approaching panidiomorphic granular or 

 diabase structure. 



Composition. — Anorthoclase of the habits described above forms about 35 per cent, 

 in phenocrysts, and 35 per cent, of the rock in the base. The phenocrysts contain 

 numerous included zircon needles. They have each a dark zone near the rim, consisting 

 of included granules of the materials of the base, and into this zone the crystals of 

 the base often project. The phenocrysts are highly resorbed and rounded by corrosion, 

 yet of very large size, reaching a length of half an inch. 



Magnetite, olivine, apatite, and pyroxene occur in the manner described under 

 general characteristics. The pyroxene phenocrysts consist of a?girine-augite of 

 greenish colour, faint pleochroism and high extinction angle (38°). 



The base is a pilotaxitic mass consisting of felspar laths, with Carlsbad and some- 

 times very faint lamellar twinning, probably anorthoclase; acicular prismatic 

 microlites of aegirine-augite similar to that of the phenocrysts; laths of a brown 

 hornblendic mineral which is pleochroic from deep brown along the length to opaque 



