2. Genus of the Olivine-Basalts

Formula.—Oliv, matr, cop, non-flu, gran, phen, opac.

Characters.—Abundant olivine. Felspar-lathes of the groundmass not in flow-arrangement. Pyroxene of the groundmass granular. Phenocrysts of plagioclase opaque white.

Description.—Grey compact-looking rocks; sp. gr. 2·83 to 2·9. Interstitial glass scanty. The olivine phenocrysts, which range up to 5 mm. in size, are more or less hematised; and in extreme cases of this alteration, where schiller-planes are formed, the hand-specimen appears to carry brown mica. There are sometimes small grains of olivine (·1 mm.) in the groundmass. The plagioclase-phenocrysts, varying in size from 2 to 4 mm., owe their opacity partly to their composite character, when they present an aggregate of smaller clear crystals, and partly to multiple macling. They give extinctions of acid labradorite (25°-32°). Pyroxene-phenocrysts, when present, are scanty, pale brown, not over 2 mm. in size, and give the large extinctions (+30°) of augite. In their absence small augites (·2 mm.) are frequent. Fine granules (·01-·03 mm.) of similar augite, together with magnetite, abound in the groundmass. The felspars of the groundmass are fairly stout and lathe-like and show at times a few twin-lamellæ which give extinctions of 13° to 20° (medium andesine).

Species represented: