25. Genus of the Olivine-Basalts
Formula.—Oliv, matr, pauc, non-flu, gran, phen, vitr.
Characters.—Olivine scanty. Felspars (lathes and prisms) of the groundmass not in flow-arrangement. Pyroxene of the groundmass granular. Glassy plagioclase phenocrysts.
Description.—About two-thirds of these rocks have a common facies, being closely similar in appearance, brownish-black in colour, and with spec. grav. usually between 2·84 and 2·92. They belong to the species with the felspar-lathes ·1 to ·2 mm. in length. They are essentially the rocks of the old submarine basaltic flows; and they are often columnar, the columns being 2 to 4 feet across. My remarks will mainly apply to this predominant group.
To the eye they are somewhat compact and show scattered porphyritic crystals of plagioclase. In the slide they display numerous phenocrysts of plagioclase, with a few of olivine and pyroxene, in a groundmass formed of stout lamellar felspar-lathes and small prisms forming a plexus with granular augite in the meshes. There is a good deal of magnetite and generally scanty residual glass. The plagioclase phenocrysts are usually 2 to 3 mm. in size, but they may be smaller (1 to 2 mm.) or larger (3 to 5 mm. or more) when the rock has a porphyritic appearance.[[111]] They are often cross-macled and at times show zoning. In many slides two kinds are distinguished by the extinctions which indicate in one case medium andesine (15° to 22°) and in the other acid labradorite (27° to 32°). They contain inclusions of the magma and are often eroded. The pyroxene phenocrysts are of pale brown augite, scanty and small, and give extinctions of 30° to 40°. They are sometimes twinned and may be eroded and contain inclusions of the magma. The olivine phenocrysts, which do not usually exceed 2 or 3 mm., are mostly rounded, but sometimes have the regular outlines, and are in various stages of serpentinisation. The felspars of the groundmass, which average ·17 mm. in length, are mostly stout and lamellar; but they exhibit all transitions from the lathe-shape with one or two lamellæ to broad multi-lamellar prisms where the breadth is half the length. They give lamellar extinctions averaging 15° to 18° (andesine). The augite granules of the groundmass are pale brown and average ·02 to ·03 mm. in size. In a few cases they are larger (·05 mm.) which is an indication of an approach to the ophitic type. In most slides occur a few small augites of prism-form, two or three times the size of the granules, which give extinctions of over 30°. Where the phenocrysts of augite are very scanty or absent, there exist large grains (·1 mm.) of an intermediate size. The magnetite is often abundant, occurring in crystals, rods, and irregular masses, the last associated often with the interstitial glass which is present in small quantities in most rocks, being greenish or brownish and showing fibrous devitrification.
In some localities semi-vitreous rocks referable to this genus are frequent. This is especially the case in the Naivaka peninsula, where the rocks show a fair amount of glass in the groundmass, the porphyritic augite being well developed, whilst the pyroxene of the groundmass is only in part differentiated. Three of the four species are here represented. Those with large felspar-lathes (·2-·3 mm. long) and coarse augite granules (·05) approach the semi-ophitic rocks included in genus 33.