12. Genus of the Augite-Andesites
Formula.—Aug, matr, non-flu, oph, non-phen, parv.
Characters.—In the groundmass the felspar lathes and prisms are not in flow-arrangement and the augite is ophitic or semi-ophitic. No plagioclase phenocrysts. Augite phenocrysts when present less than 2 mm. in size.
Description.—These rocks come near to the non-porphyritic group of genus 9; but differ in the absence or rarity of plagioclase phenocrysts, in their more frequently vesicular and scoriaceous character, and in the fresher condition of the rock. Their sp. gr. is about 2·77. They present themselves usually as blackish-brown doleritic rocks and form ancient flows in the coast-plains, sometimes exhibiting a columnar structure as in the crossing of the Ndreketi above Mbatiri. They are, however, of limited occurrence and are mostly represented in the Ndreketi plains and in the district between the Lekutu River and Wailea Bay.
Typically they display in thin sections no phenocrysts either of plagioclase or of pyroxene, and exhibit a plexus of usually long stout felspar-lathes partly invested by the lesser augites in a copious smoky glass.... The felspar-lathes, ·25 to ·4 mm. in length, give lamellar extinctions of 10° to 20° (andesine) and contain a few magma-inclusions.... The semi-ophitic augites, ·1 to ·2 mm. in size, are sometimes twinned.... The smoky glass polarises feebly and displays dark feathery aggregates of microliths. Within it are brownish-yellow semi-opaque patches of residual glass, which polarise faintly and behave like palagonite.
The two species with felspar-lathes ·2 to ·3 mm. and ·3 to ·5 mm. are represented in my collection.