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.
16. Genus of the Augite-Andesites
Formula.—Aug, matr, flu, gran, non-phen, parv.
Characters.—In the groundmass the felspar-lathes are in flow-arrangement and the augite is granular. Plagioclase phenocrysts are absent, or if present very scanty and not usually over 1 mm. in size. When present the augite phenocrysts are under 2 mm.
Description.—This is a very extensive genus, admitting considerable variation and including most of the aphanitic augite-andesites, where the felspar-lathes are as a rule very small (under ·1 mm. in length), as well as some of the doleritic types where they are very large (·2 to ·4 mm. long). In assigning a rock a place in this genus some regard must be paid to its macroscopic aspect as well as to the presence or absence of plagioclase phenocrysts. In many cases two or three small phenocrysts may be observed in a slide, under a millimetre in size; but they do not give a character to the naked-eye appearance of the rock, and such rocks cannot be distinguished from others that do not display them.
These rocks range in specific gravity from 2·55 to 2·85. This large range is in the main concerned with different degrees of basicity depending on the character of the plagioclase, the relative abundance of the augite granules, &c.; but it is also connected with the amount of interstitial glass. The variety of plagioclase ranges between oligoclase and andesine labradorite. The fluidal structure is nearly always well-marked, and the closely packed felspar-lathes have often the peculiar “felted” appearance of many andesites. A little interstitial glass is present in most rocks.
Many, perhaps nearly all, of the rocks belong either to dykes or to larger intrusive masses. All the four species indicated by the length of the felspar-lathes are represented in my collection, especially the two with smallest felspars. They may again be split up into two sub-species according to the degree of basicity of the rocks.