14. Genus of the Olivine-Basalts
Formula.—Oliv, matr, cop, flu, gran, phen, opac.
Characters.—Olivine abundant. Felspars of the groundmass in flow-arrangement. Pyroxene of the groundmass granular. Opaque plagioclase phenocrysts.
Description.—Dark grey rocks, with sp. gr. 2·9 to 2·93, showing phenocrysts of olivine and pyroxene with opaque whitish phenocrysts of plagioclase in a groundmass of felspar lathes, pyroxene grains, and magnetite, with occasional fine olivine. The olivine phenocrysts, which are sometimes 5 or 6 mm. in size, are often deeply eroded. They are at times so extensively hematised along the schiller-planes that they appear like brown mica. The plagioclase phenocrysts owe their opacity in part to their consisting of an aggregate of lesser crystals which are clear and glassy and give lamellar extinctions of 20° to 30° (andesine labradorite). They do not usually exceed 3 mm. and are sometimes scanty. The pyroxene phenocrysts, which are at times infrequent, may be 5 mm. in size. They are of pale yellowish-brown augite, giving extinctions of 40°. The felspar-lathes are rarely lamellar; but in one such case the angle of extinction was 17° (medium andesine). The grains of augite in the groundmass average ·02 mm. in diameter.
The only species represented is that with the felspar-lathes ·1 to ·2 mm. long.