Third Order, Orthophyric, of the Hypersthene-Augite-Andesites
Formula.—Hypersth-aug, matr, orth.
Characters.—Felspars of the groundmass short and broad.
Since the material is insufficient for the separate description of each genus, a general account of the order is alone given. These rocks are often represented in agglomerates or they occur as large blocks, either lying on the surface or imbedded in tuffs. Many of them are somewhat altered.
They are for the most part dark grey dull-looking rocks, with a specific gravity of 2·7 to 2·8, showing macroscopic plagioclase together with conspicuous pyroxene phenocrysts. The plagioclase phenocrysts are usually small (1 to 3 mm.), and give extinctions of medium andesine (15°-20°) and in some rocks of acid labradorite (30°). They are as a rule corroded and are penetrated by numerous fissures, whilst they contain a considerable amount of altered magma-inclusions with sometimes other alteration products. The pyroxene phenocrysts are from 2 to 4 mm. in size. Brownish-yellow augite, giving extinctions of over 30°, and pale-yellow rhombic pyroxene of the type before described occur generally in the same slide, and are frequently associated as intergrowths in the same crystal. They may have regular outlines or dark eroded borders, and at times they exhibit abundant dark opaque inclusions. The broad felspars of the groundmass are sometimes rectangular and give lamellar extinctions of medium and acid andesine (12°-17°). They vary in length in different rocks from ·05 to ·2 mm. and more. The pyroxene of the groundmass is generally granular and coarse (·02-·05 mm.). As indicated by the extinctions of occasional prism-forms it is composed of both augite and rhombic pyroxene, the former prevailing. The prismatic sub-order is also represented, and here the pyroxene of the groundmass is in great part prismatic, the length of the prisms not often exceeding ·05 mm., whilst both the monoclinic and rhombic kinds occur. Interstitial glass varies in amount, sometimes absent, sometimes scanty and viriditic, at other times abundant and opaque. Magnetite abounds in the groundmass much of it often being of secondary origin.