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.