2. Sub-order, Prismatic, of the Hypersthene-Augite-Andesites
(Felspar-lathes not in flow-arrangement.)
Formula.—Hypersth-aug, matr, non-flu, prism.
| 5 | genus | (Vitr.) | See Synopsis. | |
| 6 | " | (Opac.) | ||
| 7 | " | (Magn.) | ||
| 8 | " | (Parv.) |
This sub-order includes rocks varying much in appearance, but all alike in the presence of prismatic pyroxene in the groundmass and in the absence of flow-arrangement of the felspar-lathes. They belong to the two genera, with glassy and opaque plagioclase phenocrysts. These crystals are not usually over 2 mm. in size and are of medium andesine (15°-20°). The pyroxene phenocrysts are small and may be entirely of monoclinic or of rhombic pyroxene, or the two may be associated either as lamellar intergrowths, or by displaying an eroded nucleus (generally rhombic) around which a regular crystal of monoclinic pyroxene has grown. The felspar-lathes are in some cases less than ·1 mm. long, and in others ·1-·2 mm. The pyroxene prisms of the groundmass average ·01-·04 mm. in length, and give both straight and oblique extinctions, the last prevailing. The specific gravity ranges from 2·55 to 2·75 according to the degree of basicity and amount of interstitial glass, which is usually in fair quantity.