Description.—Coarse-looking brownish-black porphyritic rocks displaying large plagioclase crystals that often show a play of colours. Their sp. gr. is about 2·8. None of the rocks in my collection are vesicular. On account of the considerable porphyritic development of the plagioclase, the groundmass is relatively diminished, the large phenocrysts occupying about a third of the mass. They form ancient basaltic flows more especially in the vicinity of the isolated hills and mountains of acid andesite, as around Vatu Kaisia; whilst they may enter into the formation of the low basaltic plains as in the region west and south of the Ndreketi River. They are, however, limited in their extent and occurrence. From the large amount of glass in the groundmass, they may be inferred to belong to flows formed under different conditions from those under which the great basaltic plateaux were formed, where the rock contains but scanty interstitial glass.
In the slide they show the large plagioclase phenocrysts together with a few small plates of ophitic augite in a groundmass displaying in an abundant smoky glass a loose plexus of long stout lathe-like plagioclase prisms partly wrapped around by lesser augites.... The plagioclase phenocrysts, which attain a size of 4 to 6 mm., give lamellar extinctions of basic andesine (20°-27°) and show concentric zone-lines with transmitted light. They often polarise in brilliant colours and are extensively cross-macled. They contain usually abundant inclusions of the magma sometimes arranged zone-wise, and are frequently eroded.... Non-ophitic pyroxene phenocrysts are uncommon. In the slide occur one or two small “plates,” 1 to 2 mm. in size, of ophitic pale-brown augite, and a number of lesser augites, ·2 to ·3 mm. in size, which in part wrap around the felspar-lathes and by their aggregation form imperfect ophitic “plates.”... The long stout felspar-lathes, which are on the average ·3 to ·45 mm. in length, give lamellar extinctions of 15° to 20° (medium andesine).... The copious smoky glass is rendered partially opaque by the abundant development of rods and skeletal crystals of magnetite, and shows the fibrous devitrification arising from the formation of incipient microliths. In some rocks there appear in the smoky glass brownish-yellow patches of the residual magma which under the microscope cannot be distinguished from palagonite.
All but one of the specimens belong to the species where the felspar-lathes average over ·3 mm. in length.
B. Non-porphyritic Sub-genus
Description.—Blackish-brown semi-ophitic rocks, sp. gr. 2·74-2·77, frequently of doleritic texture and showing a few small macroscopic plagioclase phenocrysts. They are sometimes vesicular, and form old flows in a few localities, as in the vicinity of Natua in the eastern part of the Ndreketi plains and in the coast district between Lekutu and Wailea Bay.
They display in the slide small plagioclase phenocrysts, often abundant, in a groundmass exhibiting a loose plexus of large lathe-shaped felspar prisms, together with occasional small ophitic “plates” of augite and numerous smaller semi-ophitic augites, whilst there is much interstitial smoky glass.... The plagioclase phenocrysts are as a rule 1 to 2 mm. in size and do not exceed 3 mm. They often contain abundant inclusions of the magma sometimes arranged schiller-fashion, and are frequently eroded. Their lamellar extinctions (15°-30°)indicate andesine labradorite.... The stout plagioclase lathes, which in most of my specimens range between ·2 and ·3 mm., and contain at times magma-inclusions, give the rocks their doleritic texture.... The occasional small ophitic “plates” of pale augite are not over 1 mm. in size and give extinctions of +30° from the single cleavage-lines. The lesser augites, ·2 mm. in size, are several times larger than typical granular pyroxenes (·02-·03 mm.), and adapt their form to the interspaces of the felspar-lathes which they in part invest.... The copious interstitial glass is generally smoky and sometimes quite opaque through the deposition of magnetite. It is never clear and isotropic, but displays fibrous devitrification and is usually a little altered.
These rocks come near to those of the previous sub-genus in several respects, but they differ conspicuously in their non-porphyritic character, in being sometimes vesicular, and in their general appearance. All the four species indicated by the length of the felspar-lathes are here represented; but the doleritic types with the lathes exceeding ·2 mm. are the most frequent.
10. Genus of the Augite-Andesites
Formula.—Aug, matr, non-flu, oph, phen, opac.
Characters.—In the groundmass the felspar-lathes and prisms are not in flow-arrangement and the augite is ophitic or semi-ophitic. The plagioclase phenocrysts are opaque.