14. Genus of the Augite-Andesites
Formula.—Aug, matr, flu, gran, phen, opac.
Characters.—In the groundmass the felspar-lathes are in flow-arrangement and the augite is granular. The plagioclase phenocrysts are opaque.
Description.—Only two rocks are referred to this genus. One which is dark grey with a specific gravity of 2·72 is exposed in the gorge of the Mbutu-mbutu River below the falls of Na Savu. Flow-arrangement is displayed both by the felspar phenocrysts and lathes. The phenocrysts, 2 to 3 mm. in size, owe their opacity to the abundance of inclusions of brown glass. They are corroded and give extinctions of acid labradorite (26-32°). Pyroxene phenocrysts are scanty and small. The groundmass has a characteristic “pilotaxitic” appearance, the densely packed felspar-lathes averaging only ·05 mm. in length, whilst the pyroxene granules are ·01 mm. in size. Residual glass scanty.
The other rock is from the range behind Sueni. It shows large porphyritic crystals (5 or 6 mm.) of medium andesine which contain magma-inclusions in abundance. The average length of the felspar-lathe is ·08 mm. and the size of the augite granules is ·02 mm. There is but little glass. The rock is somewhat altered.