3. Genus of the Olivine-Basalts
Formula.—Oliv, matr, cop, non-flu, gran, non-phen, magn.
Characters.—Abundant olivine. Felspar-lathes of the groundmass not in flow-arrangement. Pyroxene of the groundmass granular. No plagioclase phenocrysts. Large phenocrysts of olivine and pyroxene over 2 mm. in size.
Description.—A remarkable blackish-grey rock. Although somewhat scoriaceous, it has a sp. gr. of 2·91. It displays large phenocrysts of olivine and pyroxene, 4 to 8 mm. in size, in a coarse-textured groundmass of stout felspars, augite granules, magnetite and a little glass. The olivine is extensively hematised. The pyroxene phenocrysts are of brownish-yellow augite with regular outlines and giving angles of extinction up to 40°. The broad lamellar felspars of the groundmass, which are on the average ·3 to ·4 mm. long, give extinctions indicating both acid labradorite (23° to 28°) and medium andesine (16° or 17°). The abundant augite-grains average ·05 mm. in size; but the prism form occurs at times.
The species with felspar-lathes ·3 to ·5 mm. long is alone represented.