38. Genus of the Olivine-Basalts
Formula.—Oliv, matr, pauc, flu, gran, phen, opac.
Characters.—Olivine scanty. In the groundmass the felspar-lathes are in flow-arrangement and the pyroxene is granular. Opaque plagioclase phenocrysts.
Description.—Grey rocks, sp. gr. 2·78 to 2·83, showing small opaque porphyritic crystals of plagioclase with a few phenocrysts of olivine and pyroxene in a groundmass of parallel felspar-lathes, augite granules, and magnetite, with very scanty, if any, interstitial glass.... The plagioclase phenocrysts, 2 to 3 mm. in size, are often aggregates of smaller crystals. They contain colourless granular inclusions and are sometimes zoned, giving extinctions of medium andesite (15°-18°), and of andesine labradorite (25°-29°).... The pyroxene phenocrysts are pale-brown, scanty, 2 to 3 mm. in size, often twinned and give the large extinctions of augite.... The olivine phenocrysts, which do not exceed 2 or 3 mm., are deeply eroded by the magma and are hematised and schillerised. Small grains also occur in the groundmass.... The felspar-lathes, which in the species here represented average ·15 mm. in length, are stout and give lamellar extinctions of andesine (18°-22°).... The augite granules are pale-brown and usually ·02-·03 mm. in diameter.
The only species represented in my collection is that with the felspar-lathes ·1-·2 mm. long.