Characters.—Olivine scanty. In the groundmass the felspar-lathes and prisms are not in flow-arrangement and the pyroxene is granular. The plagioclase phenocrysts are opaque.
Description.—Dark grey porphyritic rocks, which, from the opacity of the felspar phenocrysts, look like porphyrites. They are not very frequent and occur mostly on the northern slopes of Mount Seatura. Two different types occur in my collection which may be regarded as sub-genera. In the most basic kind, where the sp. gr. is 2·86 to 2·89, the plagioclase phenocrysts, 2 to 3 mm. in size, owe their opacity chiefly to their aggregate structure. They give lamellar extinctions (15°-30°) of andesine labradorite. Porphyritic olivine is scanty and more or less hematised; but a fair amount of olivine grains, less than ·1 mm., occur in the groundmass. Pyroxene phenocrysts are scanty, but microporphyritic pale brown augite (·1 mm.) is frequent. In the groundmass are found stoutish felspar-lathes, averaging ·2 mm. long, together with an abundance of fine augite granules (·01-·02 mm.) and fine magnetite, the residual glass being scanty.... In the other type the sp. gr. is 2·75; and the plagioclase phenocrysts 4 to 6 mm. in size give extinctions of andesine and acid labradorite (10°-30°). There is an approach to the orthophyric structure in the groundmass, as is indicated by the number of short broad felspars, averaging ·2 mm. in length and giving lamellar extinctions of acid and basic andesine. The granular augite of the groundmass is coarse (·04 mm.), and occasional prism-forms give extinctions of 40°.
33. Genus of the Olivine-Basalts
Formula.—Oliv, matr, pauc, non-flu, oph, phen, vitr.
Characters.—Olivine scanty. In the groundmass the felspar-lathes are not in flow-arrangement and the pyroxene is ophitic or semi-ophitic. Plagioclase phenocrysts glassy.
Description.—These brownish-black rocks are all of the semi-ophitic type. Although no ophitic “plates” occur in the slide, the augites of the groundmass have no longer the granular form, but are large, ·08-·1 mm. in size, and tend to invest the felspar-lathes. The specific gravity ranges from 2·78 to 2·86. As in other of the ophitic and semi-ophitic rocks of this collection (genera 9, 10, 12, of the augite-andesites), the large size of the felspar-lathes (·2-·3 mm. long) of the groundmass gives a doleritic texture in the slide. In most of the other characters these rocks approach those of genus 25 which possess felspar-lathes more than ·2 mm. in length. But they are more often semi-vitreous, and display a considerable amount of dark smoky glass showing numerous magnetite rods and skeletal crystals with fibrous devitrification. The plagioclase phenocrysts, which vary much in size in different rocks (in some 2 or 3 mm., in others 4 or 5 mm.), give extinctions of andesine labradorite (20°-35°). They are often eroded and contain numerous large black inclusions of the magma. The pyroxene phenocrysts, which are of pale-brown augite, often have an aggregate-structure, having been formed in situ. Others again consist of single crystals and have been much affected by the magma. The olivine phenocrysts, which are at times deeply eroded, are generally small and in part serpentinised.
37. Genus of the Olivine-Basalts
Formula.—Oliv, matr, pauc, flu, gran, phen, vitr.
Characters.—Olivine scanty. In the groundmass the felspar-lathes are in flow-arrangement and the pyroxene is granular. Glassy plagioclase phenocrysts.
Description.—Brownish-black rocks which cannot be distinguished, except in the flow-arrangement of the felspars of the groundmass, from those described under genus 25. Like them they enter into the formation of the basaltic plains of Sarawanga and Mbua and elsewhere. Most of the rocks of this genus group themselves into one type where the felspar-lathes average in length ·15-·21 mm. The sp. gr. is usually between 2·87 and 2·91. Though rarely porphyritic, such rocks display to the eye a few small scattered glassy phenocrysts of plagioclase and an occasional grain of olivine. It is to this type of the genus that the following description applies.