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:--
- (a) with felspar-lathes ·2-·3 mm.
- (b) with felspar-lathes ·3-·5 mm.
44. Genus of the Olivine-Basalts
Formula.—Oliv, matr, pauc, flu, prism, non-phen, parv.
Characters.—Olivine scanty. In the groundmass the felspar-lathes are in flow-arrangement and the pyroxene is in great part prismatic. There are no phenocrysts of plagioclase; but there are a few small phenocrysts of olivine and sometimes of pyroxene, less than 2 mm. in size.
Description.—A dark grey compactish rock; sp. gr. 2·9; showing a little macroscopic olivine; forming a dyke in the tuffs on the summit of the hill of Vatui (p. [54]).
In the section it exhibits a few phenocrysts of olivine (more or less serpentinised) and of augite in a groundmass formed of stout augite prisms and small augite granules with felspar-lathes, magnetite, and a little devitrified yellowish interstitial glass. The augite prisms and the felspar-lathes are in flow-arrangement.... The pyroxene phenocrysts, which are pale brown and give extinctions of over 30° from the single cleavage-lines, may be aggregates of five or six smaller crystals or single crystals presenting sometimes lamellar twinning. The first are doubtless formed in situ. The second though showing regular outlines may have a nucleus giving a different extinction and possessing eroded margins. The stout augite prisms of the groundmass, which are occasionally twinned, have an average length of ·2 to ·3 mm. and give angles of extinction with the long axis of 30° to 40°. The felspar-lathes average only ·07 mm. in length and afford extinctions, when untwinned, of 18° to 24° (acid labradorite).