2. Genus of the Augite-Andesites
Formula.—Aug, matr, non-flu, gran, phen, opac.
Characters.—In the groundmass the felspar-lathes and prisms are not in flow-arrangement and the augite is granular. Opaque plagioclase phenocrysts.
Description.—This genus may be divided into two groups according to the size of the plagioclase phenocrysts, the first “porphyritic,” where they average 3 mm. and over, the second “non-porphyritic,” where they are smaller than 3 mm., usually not over 2 mm. The former would include some of the “porphyrites,” and to this only one of the rocks sliced is to be referred. All the rest belong to the non-porphyritic type; and several of them are rocks that have undergone the propylitic change, as indicated by the formation of pyrites, chlorite, calcite, and other alteration-products.
(a) Porphyritic Sub-genus.—A greyish rock, with sp. gr. 2·78, showing abundant porphyritic opaque plagioclase (4 to 7 mm.), from the vicinity of Satulaki. These phenocrysts are often aggregates of lesser crystals, or they may be extensively cross-macled. They are traversed by numerous fine cracks and show much dust-like included material. They are in part corroded by the magma and give evidence of fracture in their present position, the re-union being sometimes effected by the growth of new substance. Their lamellar extinctions (10° to 20°) are those of andesine. The groundmass displays a plexus of stout felspar-lathes, averaging ·1 mm. long, with the meshes occupied by coarse augite granules, ·03 to ·05 mm., with little, if any, interstitial glass. The felspars are often lamellar and give extinctions like those of the phenocrysts.
(b) Non-porphyritic Sub-genus.—Reference will first be made to some of the propylitic rocks of the dykes of the Ndriti Basin which belong to this group (see p. [70]). They are greenish or greyish, with sp. gr. 2·76 to 2·8, and often sparkle with pyrites and contain secondary calcite, sometimes to such an extent that they might be taken at first sight for impure limestones.
The small opaque plagioclase phenocrysts (under 2 mm.), that they contain, are more evident in the slide than in the hand-specimen, and scarcely give a macroscopic character to the rock. They give extinctions (10° to 30°) ranging from those of acid andesine to acid labradorite, and are traversed by numerous cracks occupied by calcitic and other alteration products. The few pyroxene phenocrysts that once existed are now entirely represented by chloritic pseudomorphs. The groundmass displays a doleritic texture, exhibiting a plexus of long felspar-lathes, ·2 to ·4 mm. in average length, which often present a false resemblance to a flow-arrangement from their aggregation into bundles. They are often clouded by secondary products, but occasionally give lamellar extinctions (20° to 30°) indicating andesine labradorite. The rest of the groundmass is greatly altered, the granular augite and the interstitial glass, which originally existed in fair amount, being replaced by calcite, chlorite, pyrites, and occasionally epidote, so that the rock mass appears largely impregnated with alteration products. In addition there is much secondary magnetite, and in some cases there are a few minute cavities filled with chalcedonic silica and zeolites.
Reference may here be made to a singular rock from Ruku-ruku Bay, which resembles the Ndriti rocks in its propylitic alteration, but the felspar-lathes of the groundmass, ·21 mm. in length, give the small extinctions of oligoclase. Spec. grav. 2·61.
Most of the prevailing rocks of Mount Freeland belong to this sub-genus. They are dark grey and show small opaque plagioclase phenocrysts 1 or 2 mm. in size. They usually, however, are more or less altered, the change being often of the propylitic type, calcite, chloritic material, viridite, and occasionally pyrites occurring as alteration products. The specific gravity of the altered rocks is 2·61-2·69; that of the least affected is about 2·76. They all, however, belong to the same genus, displaying small phenocrysts of plagioclase and augite in a groundmass composed of minute stoutish felspar-lathes (·03-·06 mm.), augite granules, magnetite, and a little residual glass. The plagioclase phenocrysts owe their opacity partly to the numerous fine cracks traversing them and partly to the alteration products. The pyroxene phenocrysts, which are mostly of pale yellow augite, display at times intergrowths of rhombic pyroxene.