13. Genus
Formula.—Hypersth-aug, matr, flu, gran, phen, vitr.
Characters.—Glassy plagioclase phenocrysts.
Description.—This is a group of rocks that comes near the basaltic andesites represented in genera 1 and 13 of the augite sub-class; and to the more basic kinds the terms of basaltic andesite is equally applicable. These rocks, however, differ in the prevalence of rhombic pyroxene, which occurs as phenocrysts, but always accompanied by monoclinic pyroxene, whether as separate crystals or as inter-growths. Such rocks are intermediate between those of the augite and rhombic pyroxene classes. They are particularly characteristic of the Savu-savu peninsula, but they are also found in other scattered localities. Sometimes they appear to form ancient flows, and at other times intrusive masses and dykes; but they are rarely scoriaceous.
Almost all the rocks in my collection referred to this genus belong to the species where the felspar-lathes of the groundmass are ·02-·1 mm. long. They are generally blackish or dark-brown, and the specific gravity ranges usually from 2·72 to 2·83. They display in the slide a fair number of plagioclase and pyroxene phenocrysts in a groundmass of felspar-lathes, pyroxene granules, and magnetite, the interstitial glass being scanty or moderate in amount.... The plagioclase phenocrysts are rarely as large as 3 mm., so that most of the rocks belong to the non-porphyritic group of the genus. These phenocrysts, which are often zoned, give extinctions of basic andesine (15°-25°). They contain magma-inclusions, sometimes in abundance, which are arranged in zone-lines.... The pyroxene phenocrysts are small, the two kinds being always represented in the same slide. In some cases separate crystals occur, and in others the two are associated as intergrowths, but in most cases separate and compound crystals occur in the same section. Not infrequently the phenocryst is an aggregate of lesser crystals of the two pyroxenes. The monoclinic form is a brownish yellow augite with large extinctions and often twinned. The felspar-lathes of the groundmass, which usually average ·05 or ·06 mm. in length, are either narrow and untwinned, or they may be stouter and display simple and at times lamellar twinning, giving extinctions of medium andesine.... The granules of pyroxene are generally ·01-·02 mm. in size; but occasional prism-forms occur which give sometimes the straight extinction of rhombic pyroxene and at other times the large oblique extinctions of the augite type.