OF MOUNT EREBUS, ANTARCTICA 97 



by their characteristic properties, such as extinction angles, refractive indices, cleavage 

 angles, and modes of twinning. 



The microlites of the base, being of minute size, were not so readily identified, yet 

 the following properties point to their being anorthoclase : the refractive index was 

 but slightly below that of Canada Balsam, twinning was mainly on the Carlsbad plan, 

 but shadowy extinction of the rnoiree type indicated the presence of ultramicroscopic 

 polysynthetic twinning in some crystals, the sanidine cross cracking or parting was 

 fairly common, cleavage almost rectangular, and extinction angles varied from 0° to 8° 

 with the length of the laths. 



(b) The pyroxenes consisted chiefly of a3girine-augite, though in some specimens 

 true segirine, and in others colourless diopside accompany the segirine-augite. The 

 segirine-augite is of a greenish-yellow colour, and occurs in prismatic laths, needles, 

 and grains. Often a group of crystals of this mineral is optically continuous, so that the 

 pyroxene may be regarded as embracing the penetrating felspar needles ophitically. 

 The segirine-augite seems usually to have continued to crystallise out longer than the 

 felspar. It commonly extinguishes at angles lying between 20° and 35°, and occurs 

 in two generations, the earlier of which is usually less alkaline than the later. 



Cossyrite-like amphiboles occur only sparingly in most specimens or not at all. 

 They may be scattered about in grams or form ophitic aggregates of a deep coffee-brown 

 colour, pleochroic from olivine-brown to deep brown opaque in sections across the length. 

 In some slides these amphiboles show a deep purplish-blue absorption colour on rotating 

 the stage, indicating affinities with riebeckite or some species of kataphorite. These 

 hornblendes are seldom seen with any tendency to idiomorphism, though in one slide 

 a few small idiomorphs were noticed. Often they form the nucleus of an segirite growth 

 or a fringe round magnetite. The cleavage when noticeable was at about 65°. 



Riebeckite occurs only very rarely in deep blue ragged grams, sometimes fringing 

 cossyrite-like amphibole, or forming the nucleus of segirine-augite. Arfvedsonite is 

 occasionally secondary after segirine-augite (a uralitic decomposition product), 

 occurring around the pyroxene (or replacing it) together with ferrite decomposition 

 products. 



Olivine occurs only sparingly, and only as phenocrysts, which belong to the 

 intratelluric period of consolidation. The crystals are usually rounded grains and 

 have been strongly resorbed at the edges by the magma. It can only be considered a rare 

 or accessory constituent hi these rocks. 



Magnetite in idiomorphic grains is a common accessory in some of the slides, 

 in others soda hornblendes take its place. It is not improbable that in some of the 

 rocks the cubic and octahedral black isotropic grains provisionally identified as 

 magnetite may be a black variety of perofskite, knopite, or geikielite. 



Felspathoids are not uncommon, but only present in small amounts. Usually 

 they are interstitial and were the last minerals to crystallise. In addition to filling 

 smaller interstices these minerals may form larger patches into which felspar needles 

 penetrate. The commonest felspathoids are sodalite and analcite. 



Order of Consolidation. — There are two generations of minerals, each of which has 

 its own order of crystallisation. In the first generation magnetite is formed first, for 

 it occurs included even in the cores of olivine and pyroxene phenocrysts, and it finished 

 crystallising before the others commenced to separate. The olivine came next, and is 

 very corroded by the magma. Felspar followed, and was likewise strongly resorbed at the 

 rim on reaching surface conditions, and the segirine-augite probably formed at the 

 same time. 



As regards the second generation, magnetite and other accessories formed first, 

 then felspar commenced and at the same time cossyrite. Riebeckite followed, and after 



