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.
Fourth Order, Felsitic, of the Hypersthene-Augite Andesites
Formula.—Hypersth-aug, matr, fels.
Characters.—The groundmass presents a rudely granular appearance or a blurred mosaic.
This order is capable of subdivision, as in the other orders of the hypersthene-augite-andesites (see Synopsis, page [247]); but since it is only represented by six of my rock-sections, I will confine the description to the general characters.
These rocks are dark-grey, sometimes granitoid in appearance, with specific gravity 2·65 to 2·75. They usually show some alteration, arising from secondary changes within the rock-mass; and probably the felsitic or semi-mosaic appearance of the scanty groundmass is the result of such a secondary change. Such rocks in some respects approach the type of the gabbros. They are frequent on the north coast of Natewa Bay in the vicinity of Waimotu and also occur in the Valanga Ridge. They generally present themselves as deeper-seated massive rocks exposed by the stripping off of the superficial deposits.
There are as a rule more or less conspicuous phenocrysts, up to 3 mm. in size, of plagioclase and pyroxene, in a relatively scanty micro-felsitic groundmass, displaying a blurred mosaic, in which a few stout felspar-lathes can still be recognised, and composed apparently of felspar and crystalline silica. The “grain” of the mosaic may range in different rocks from ·005 mm. to ·02 mm. The pyroxene of the groundmass is largely decomposed, and the scanty residual glass is represented by viriditic materials.
The plagioclase phenocrysts, which give extinctions of medium and basic andesine (15°-25°), are often semi-opaque and corroded. They are traversed by numerous cracks and often contain many whitish alteration products, though the lamellar structure is usually well preserved. The pyroxene phenocrysts are composed of brownish-yellow augite (ext. + 30°) and pale rhombic pyroxene of the type described before, either as separate crystals or associated as intergrowths. The rhombic pyroxene crystals are often sub-rounded with dark borders; and as a rule the pyroxene phenocrysts are much fresher than the plagioclase phenocrysts. As far as can be ascertained, most of the original pyroxene of the groundmass was monoclinic with a little rhombic.