1 Sub-Order, Granular, of the Hypersthene-Augite-Andesites

(Felspar-lathes not in flow-arrangement.)

Formula.—Hypersth-aug, matr, non-flu, gran.

1genus(Vitr.) See Synopsis, p. [247].
2"(Opac.)
3"(Magn.)
4"(Parv.)

Nearly all of the rocks of this sub-order that are represented in my collection belong to the genus (1) with phenocrysts of glassy plagioclase. They vary considerably in appearance and in colour (black to grey), and occur under very different conditions, as in “necks,” old flows, large intrusive masses, dykes, agglomerates, &c. Their specific gravity has rather a wide range according to the degree of basicity. In the heavier rocks where the rhombic pyroxene is scanty, it is usually 2·7 to 2·8. In the others, where rhombic pyroxene is more predominant and where the felspar is less basic, it is 2·6 to 2·7.

In the slide small phenocrysts of plagioclase and pyroxene occur in a groundmass of felspar-lathes and pyroxene granules, whilst there is as a rule a fair amount of residual glass. The plagioclase phenocrysts, which give extinctions in different rocks of acid and basic andesine and contain abundant magma-inclusions, are generally one to two mm. in size. The pyroxene phenocrysts are small, and may be represented by separate crystals of the monoclinic and rhombic kinds, or by crystals displaying intergrowths of the two sorts. The pyroxene granules vary much in size and are evidently in great part of augite. In most of the rocks the felspar-lathes are less than ·1 mm. in length. In those where the length is ·1 to ·2 mm. they are sometimes stout and display a few lamellæ, yielding extinctions corresponding to those of the phenocrysts.

A singular dark grey almost holocrystalline doleritic rock (sp. gr. 2·85) is exposed in the Thulanga Ridge (p. [211]). It shows no plagioclase phenocrysts, but those of pyroxene are numerous, which, however, do not exceed 2 mm. in size, so that the rock would be referred to genus 4 of this sub-order. It appears to be a doleritic form of the plutonic rock found at Nawi in this neighbourhood (p. [211]). The pyroxene phenocrysts are mostly of brownish-yellow augite, but rhombic pyroxene, either as separate crystals or as intergrowths, is not uncommon. The plagioclase lathes are long and fairly stout, giving at times lamellar extinctions of 20°. Their average length is ·3 mm., and it is to their large size that the doleritic texture is due. The pyroxenes of the groundmass are similarly coarse (·2 mm.), and include both monoclinic and rhombic forms, the latter infrequent. There is a slight tendency to semi-ophitic behaviour in places; but generally these pyroxenes are irregular in shape or rudely prismatic.