13. Genus
Formula.—Aug, matr, flu, gran, phen, vitr.
Characters.—In the groundmass the felspar-lathes or prisms are in flow-arrangement and the augite is granular. The plagioclase phenocrysts are glassy.
Description.—This genus readily splits up into two sub-genera, the non-porphyritic, where the plagioclase phenocrysts are less than 3 mm. in size, and the porphyritic where they are larger.
1. Non-Porphyritic Sub-genus.—Dark-brown or blackish rocks displaying small plagioclase phenocrysts, usually only 1 or 2 mm. in size. Three of the four species defined by the length of the felspar-lathes are represented in my collection.
Species A.—Felspar-lathes .02-.1 mm. in average length. This may again be sub-divided according to the degree of basicity of the rocks:—
(a) Sub-species of greater basicity.—Sp. gr. 2·76 to 2·82.... Such rocks are represented in dykes and in the prevailing basic agglomerates. They are at times scoriaceous. The small plagioclase phenocrysts, which are fairly numerous, give lamellar extinctions of andesine labradorite (20 to 30°). Two kinds occur which may or may not be represented in the same slide. In the one the crystal is much corroded and contains abundant magma-inclusions. It belongs in such a case to an earlier period. In the other the outlines are clean and regular, and the crystal is often cross-macled to such an extent that it may be inferred from its unbroken condition to have been formed in situ. Augite phenocrysts when present are small and scanty, pale-yellow, idiomorphic, and giving extinctions of +30°. The felspar-lathes, which average ·06-·08 mm. in length, give extinctions indicating andesine labradorite. The augite granules are small (·01-·02 mm.). Interstitial glass, generally scanty, is sometimes abundant when it is smoky, showing fibrous devitrification, with irregular “lacunæ” filled with a brownish yellow opaque glass like palagonite.
(b) Sub-species of lesser basicity.—Sp. gr. 2·65-2·70.... The remarks on the plagioclase phenocrysts of the more basic sub-species here apply, except that the lamellar extinctions indicate medium andesine (12°-20°). The characters of the augite phenocrysts and granules are in both groups the same; but in this case there is more frequently a suspicion of intergrowth with rhombic pyroxene. The felspar-lathes are very small, ·04 or ·05 mm, and give simple extinctions of acid andesine (5-10°). Interstitial glass exists in moderate amount.
Species B.—Felspar-lathes ·1-·2 mm. in average length.
Blackish or dark-grey rather compact rocks, sp. gr. 2·75-2·79, that cannot be readily divided into groups according to their basicity. They form dykes and volcanic “necks” and are sometimes scoriaceous. The small plagioclase phenocrysts, which are most evident in the slide, present the two kinds above described under Species A. They give lamellar extinctions varying from those of medium andesine to acid labradorite (15-30°). The augite phenocrysts, which are small and scanty, occasionally show intergrowths of rhombic pyroxene. The augite granules are generally ·02 to ·03 mm. in size, and here and there a prism form gives extinctions of +25°. The felspar-lathes which average ·11 to ·15 mm. long, are often rather stout, showing a few lamellæ that give extinctions of medium and basic andesine. Interstitial glass occurs in fair amount.
Species C.—Felspar-lathes ·2-·3 mm. in average length.
Blackish rocks with sp. gr. 2·75-2·84. The description of Species B applies here. The plagioclase phenocrysts are for the most part microporphyritic. The size of the augite granules is as above given.
2. Porphyritic Sub-genus.—This group of rocks is mostly confined to the slopes and vicinity of Mount Seatura in the western part of the island, being prevalent in the Mbua and Ndama plains, and occurring also as dyke-rocks in the Nandi Gorge leading into the Ndriti Basin, and at and near the coasts of Wainunu Bay between the Tongalevu and Wainunu rivers. They come near in appearance to the porphyritic forms of the blackish olivine-basalts belonging to genera 13, 25, and 37 of the olivine rocks; but they differ in the absence of that mineral, in their lower density, and in other characters. They are the type to which the term “porphyritic basaltic andesite” is most frequently applied in the text when the ophitic structure is not displayed.
They are blackish rocks having a specific gravity of 2·71 to 2·81 and exhibiting large porphyritic crystals of plagioclase, but they vary in their minute structure on account of the different size of the felspars of the groundmass. Those forming dykes in the Nandi Gorge are often more or less propylitic in character. The felspar-lathes, which have an average length of ·2 to ·3 mm., sometimes show a few lamellæ giving extinctions of medium andesine (12°-20°). The plagioclase phenocrysts of the same andesine are 3 to 5 mm. in size. They are eroded and contain abundant magma inclusions. There are a few small phenocrysts of pale brown augite. The augite granules are ·03 or ·04 mm. in diameter, and there is a little dark opaque residual glass.
The rocks of the Mbua and Ndama plains have a specific gravity of 2·81. The plagioclase phenocrysts, which yield extinctions of basic andesine (21-27°), are sometimes a centimetre in length. They are traversed by cracks filled with dark altered glass or occupied by brownish films. The felspar-lathes, which average ·11 mm. in length, are often stout and lamellar and give extinctions like the phenocrysts. Augite phenocrysts are either absent or scanty; whilst the granules average ·02-·03 mm. in size. There is usually a little interstitial glass.
A rock, almost holocrystalline and 2·74 in density, which was obtained from the Tongalevu district in Wainunu Bay, approaches the orthophyric type in the character of the ground mass. The felspars are short (·06 mm.) and stout, and yield lamellar extinctions of oligoclase (5°-10°). The plagioclase phenocrysts are of basic andesine. Amongst the granules (·025 mm.) of pale brown augite occur prismatic forms giving oblique extinctions of +30°.