This genus may be divided into two sub-genera, porphyritic and non-porphyritic, according to the average size of the plagioclase phenocrysts, whether above or below 3 mm.
A. Porphyritic Sub-genus.—This is represented by a light grey porphyritic rock, with sp. gr. 2·75, from the lower part of Mount Freeland. It comes near to the porphyrites, and displays large opaque white phenocrysts of plagioclase 5 or 6 mm. long. It is a somewhat altered rock.
In the thin section it displays the plagioclase phenocrysts in a ground mass of doleritic and semi-ophitic texture showing a plexus of long felspar-lathes partly invested by small augites with a fair amount of altered greenish opaque interstitial glass.... The plagioclase phenocrysts, which give extinctions of 11° to 15° (acid andesine), are traversed by a network of fine cracks and contain a quantity of colourless dust-like inclusions and alteration-products. They are long and rectilinear in outline and are not much affected by the magma.... The felspar-lathes, which average ·26 mm. in length, are occasionally lamellar when the angle of extinction indicates acid andesine. Like the phenocrysts they contain many dust-like inclusions.... The augite may at times form an aggregate phenocryst of small size; but usually it occurs as semi-ophitic masses ·1 mm. in diameter.
B. Non-Porphyritic Sub-genus.—The specimen representing this group is a coarse-grained greyish altered rock, sp. gr. 2·81, found in blocks near Waikatakata (p. [204]), showing small somewhat opaque plagioclase phenocrysts. In the section these phenocrysts are displayed in numbers together with a few of augite. The groundmass, doleritic in texture in this species, displays a plexus of long stout felspar-lathes with numerous semi-ophitic lesser augites, chloritic pseudomorphs after pyroxene, and scanty interstitial altered glass.... The plagioclase phenocrysts, 2 to 3 mm. in size, give extinctions (22° to 28°) of basic andesine. They are traversed by many fine cracks and contain an abundance of colourless dust-like inclusions, apparently altered magma products.... The small pyroxene phenocrysts consist of aggregates of smaller crystals of pale augite. The lesser augites, ·1 mm. in size, partly invest the felspars.... The broad felspar-lathes, which average ·25 mm. in length and give lamellar extinctions of medium andesine (15°), contain inclusions similar to those of the phenocrysts.... The scanty interstitial glass is converted into viriditic and chloritic materials. Secondary calcite also occurs here and in the chloritic pseudomorphs.
The only species represented is that with felspar-lathes ·2 to ·3 mm. in length.
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.