In the thin sections they display small plagioclase phenocrysts, with a few of olivine and occasionally of pyroxene, in a groundmass where the flow-arrangement of the felspar-lathes is well marked, the rest of the groundmass being made up of granular augite with magnetite and generally a little residual glass.... The plagioclase phenocrysts do not usually exceed 2 mm. in size and contain magma inclusions. Two kinds are often indicated in the same slide by the extinctions, namely, one of medium andesine (17°-22°), and the other of acid labradorite (28°-33°).... The pyroxene phenocrysts are of pale brown augite; but they are small (less than 2 mm.), scanty, and often absent when their place is taken by microporphyritic augite, ·2 mm. in size.... The olivine phenocrysts are generally small. Though sometimes showing the long hexagonal sections, they are often rounded and more or less serpentinised.... The felspars of the groundmass present more typical lathes than are to be observed in the non-parallel felspars of the rocks of genus 25. The twin-lamellæ, when present, are fewer; but give similar extinctions (15°-21°) of medium andesine.... The augite granules are, as a rule, ·02 or ·03 mm. in diameter; but occasional more prismatic forms occur, two or three times the length, which give extinctions of over 30°. The magnetite is abundant, and the scanty interstitial glass is green or brown and displays fibrous devitrification.
The following three species, as indicated by the length of the felspar-lathes, are represented in my collection:
- (a) ·02-·1 mm.
- (b) ·1-·2 mm.
- (c) ·2-·3 mm.
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.
40. Genus of the Olivine-Basalts
Formula.—Oliv, matr, pauc, flu, gran, non-phen, parv.