12. Genus of the Olivine-Basalts
Formula.—Oliv, matr, cop, non-flu, oph, non-phen, parv.
Characters.—Olivine abundant. Felspars of the groundmass not in flow-arrangement. Pyroxene of the groundmass ophitic or semi-ophitic. No plagioclase phenocrysts. Small phenocrysts (under 2 mm.) of olivine and occasionally a few of pyroxene.
Description.—A dark greenish-brown rock, with sp. gr. 2·91, showing abundant micro-porphyritic olivine in a groundmass consisting of ophitic pale-brown augite inclosing the felspar-lathes, together with small olivines, patches of magnetite, and a little altered interstitial glass. The olivine-phenocrysts are about ·5 mm. in average size and are more or less hematised. The felspar-lathes, which average ·15 mm. in length, often show twin-lamellæ that give extinctions of 23°-30° (andesine-labradorite).
The species with felspar-lathes ·1-·2 mm. long is alone represented.