26. Genus of the Olivine-Basalts
Formula.—Oliv, matr, pauc, non-flu, gran, phen, opac.
Characters.—Olivine scanty. In the groundmass the felspar-lathes and prisms are not in flow-arrangement and the pyroxene is granular. The plagioclase phenocrysts are opaque.
Description.—Dark grey porphyritic rocks, which, from the opacity of the felspar phenocrysts, look like porphyrites. They are not very frequent and occur mostly on the northern slopes of Mount Seatura. Two different types occur in my collection which may be regarded as sub-genera. In the most basic kind, where the sp. gr. is 2·86 to 2·89, the plagioclase phenocrysts, 2 to 3 mm. in size, owe their opacity chiefly to their aggregate structure. They give lamellar extinctions (15°-30°) of andesine labradorite. Porphyritic olivine is scanty and more or less hematised; but a fair amount of olivine grains, less than ·1 mm., occur in the groundmass. Pyroxene phenocrysts are scanty, but microporphyritic pale brown augite (·1 mm.) is frequent. In the groundmass are found stoutish felspar-lathes, averaging ·2 mm. long, together with an abundance of fine augite granules (·01-·02 mm.) and fine magnetite, the residual glass being scanty.... In the other type the sp. gr. is 2·75; and the plagioclase phenocrysts 4 to 6 mm. in size give extinctions of andesine and acid labradorite (10°-30°). There is an approach to the orthophyric structure in the groundmass, as is indicated by the number of short broad felspars, averaging ·2 mm. in length and giving lamellar extinctions of acid and basic andesine. The granular augite of the groundmass is coarse (·04 mm.), and occasional prism-forms give extinctions of 40°.