20. Genus of the Augite-Andesites
Formula.—Aug, matr, flu, prism, non-phen, parv.
Characters.—In the groundmass the felspar-lathes are in flow-arrangement, and the augite is prismatic. Plagioclase phenocrysts are absent or are very small and scanty, and pyroxene phenocrysts when present do not exceed 2 mm. in size.
Description.—These dark semi-vitreous rocks occur in agglomerates and as “necks” and dykes, and are at times scoriaceous. They are usually compact and aphanitic, showing few if any plagioclase phenocrysts and having a semi-conchoidal fracture. The specific gravity varies, being generally 2·6-2·65, but according to the degree of basicity and amount of glass it may be as low as 2·5 or as high as 2·77. In the less glassy condition, as in the case of a rock from the ridge east of Na Raro, the felspar-lathes are relatively scanty and the groundmass is mainly formed of augite prisms in flow-arrangement. The lathes are generally small, less than ·1 mm., and rarely over ·2 mm. Their extinctions are those of oligoclase and acid andesine. The pyroxene prisms, which give the large extinctions of augite, have a pale muddy-brown hue and are as a rule ·03-·07 mm. long. Granular pyroxene, if present, is subordinate in amount. The glass, which is always in good quantity and is sometimes abundant, displays fibrous devitrification. In a rock from the vicinity of Narengali a variolitic structure is exhibited in the form of sheaf-like aggregates of fibre-like felspars and skeleton prisms of pyroxene.