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:
- (a) felspar lathes ·1-·2 mm.
- (b) felspar lathes ·2-·3 mm.
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.