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.