40. Genus of the Olivine-Basalts

Formula.—Oliv, matr, pauc, flu, gran, non-phen, parv.

Characters.—Olivine scanty. In the groundmass the felspar-lathes are in flow-arrangement and the pyroxene is granular. No plagioclase phenocrysts; but there are a few small phenocrysts of olivine and sometimes of pyroxene under 2 mm. in size.

Description.—Compact-looking non-porphyritic blackish-brown rocks, sp. gr. about 2·9. Occasionally a little vesicular. For the most part dyke-rocks.

In the slide are displayed a few small phenocrysts of olivine and pyroxene in a groundmass formed of more or less parallel felspar-lathes, augite granules, magnetite, sometimes in rods, and a little greenish devitrified residual glass.... The pyroxene phenocrysts are of pale-brown augite and are generally less than a millimetre in size. They may be single crystals or they may be formed of an aggregate of a few smaller crystals.... The olivine phenocrysts rarely exceed 2 mm. in size and are in part serpentinised.... The augite granules vary usually from ·01 to ·03 mm. in diameter.... The felspar-lathes of the rocks in this collection are large, often exceeding ·2 mm. in length, giving the rock a doleritic texture in the slide. In a single slide they may range from ·1 to ·6 mm. When lamellar they give extinctions of 15° to 25° (basic andesine).

Two species are represented in this collection:--