4. Genus of the Augite-Andesites
Formula.—Aug, matr, non-flu, gran, non-phen, parv.
Characters.—In the groundmass the felspar-lathes and prisms are not in flow-arrangement and the augite is granular. There are no plagioclase phenocrysts, and those of augite when present are small (under 2 mm.).
Description.—Two groups of these rocks occur in my collection. In the one there are vesicular and scoriaceous rocks forming dykes near Nukunase and near the village of Ndriti. They display a plexus of felspar-lathes with abundant smoky more or less devitrified glass, the augite granules not being always differentiated. The felspar-lathes vary from ·1 to ·2 mm. in average length, and when lamellar give extinctions of basic andesine (25°). There are no pyroxene phenocrysts, and the augite granules when present average ·02 mm. in size.
In the other group are included some propylites from the dykes of the Ndriti Basin. They are greyish or greenish rocks, have a sp. gr. of 2·72 to 2·76, sparkle often with pyrites, and contain so much secondary calcite that they effervesce freely with an acid. Except in the rarity or absence of plagioclase phenocrysts, they come near to the propylitic rocks described under genus 2. They usually display a doleritic groundmass exhibiting long felspar-lathes, ·2 to ·33 mm. in length, which present the same pseudo-flow arrangement from their being gathered into bundles. The alteration corresponds precisely to that previously described, chlorite, epidote, pyrites, &c., occurring in quantity as secondary products.
II Prismatic Sub-Order of the Augite-Andesites where
the Felspar-lathes are not in Flow-arrangement
Formula.—Aug, matr, non-flu, prism.
The augite-andesites, which display in the groundmass a plexus of felspar-lathes and much prismatic pyroxene, are not frequent in my collection. About half of the specimens belong to agglomerates, whilst the rest are of the massive type, none apparently being obtained from dykes. They admit of the same classification as that generally adopted for the “granular” sub-orders; and it must be not forgotten that granular pyroxene also occurs but is not predominant.
5. Genus of the Augite-Andesites
Formula.—Aug, matr, non-flu, prism, phen, vitr.
Characters.—In the groundmass the felspar-lathes and prisms are not in flow-arrangement and the augite is for the most part prismatic. Plagioclase phenocrysts glassy.
Description.—Except as regards the prismatic pyroxene these rocks do not differ much from the “granular” augite-andesites. Those before me show phenocrysts of plagioclase and sometimes of augite in a groundmass displaying a mesh-work of felspar-lathes, prismatic pyroxene, and much interstitial glass.... The plagioclase phenocrysts, 1 to 3 mm. in size, show abundant magma-inclusions arranged either zone-wise or parallel to the twinning-planes. They are often eroded.... The phenocrysts of augite, which give extinctions of over 30°, are often rounded and display glass and other inclusions.... The prismatic pyroxenes of the groundmass vary in average length from ·03 to ·08 mm. They have the peculiar pale muddy brown hue characteristic of the prismatic augite in these rocks, and give oblique extinctions up to 30° and over. They may be short and broad or long and slender, and when there is much glass in the rock they resemble the felspar-lathes in their forked and imperfect extremities. Granular pyroxene occurs, but is subordinate.... The felspar-lathes, ·1 mm. long, are rarely lamellar, and give extinctions measured from their long axis of 20° (basic andesine).