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).
6. Genus of the Augite-Andesites
Formula.—Aug, matr, non-flu, prism, phen, opac.
Characters.—In the groundmass the felspar-lathes are not in flow-arrangement and the augite is for the most part prismatic. The plagioclase phenocrysts are opaque.
Description.—Light and dark grey rocks displaying abundant opaque plagioclase phenocrysts not exceeding 2·5 mm. They are somewhat altered, one of the specimens having a sp. gr. of 2·68.