The two species with felspar-lathes ·2 to ·3 mm. and ·3 to ·5 mm. are represented in my collection.
4. Granular Sub-order of the Augite-Andesites
Formula.—Aug, matr, flu, gran.
13. Genus
Formula.—Aug, matr, flu, gran, phen, vitr.
Characters.—In the groundmass the felspar-lathes or prisms are in flow-arrangement and the augite is granular. The plagioclase phenocrysts are glassy.
Description.—This genus readily splits up into two sub-genera, the non-porphyritic, where the plagioclase phenocrysts are less than 3 mm. in size, and the porphyritic where they are larger.
1. Non-Porphyritic Sub-genus.—Dark-brown or blackish rocks displaying small plagioclase phenocrysts, usually only 1 or 2 mm. in size. Three of the four species defined by the length of the felspar-lathes are represented in my collection.
Species A.—Felspar-lathes .02-.1 mm. in average length. This may again be sub-divided according to the degree of basicity of the rocks:—
(a) Sub-species of greater basicity.—Sp. gr. 2·76 to 2·82.... Such rocks are represented in dykes and in the prevailing basic agglomerates. They are at times scoriaceous. The small plagioclase phenocrysts, which are fairly numerous, give lamellar extinctions of andesine labradorite (20 to 30°). Two kinds occur which may or may not be represented in the same slide. In the one the crystal is much corroded and contains abundant magma-inclusions. It belongs in such a case to an earlier period. In the other the outlines are clean and regular, and the crystal is often cross-macled to such an extent that it may be inferred from its unbroken condition to have been formed in situ. Augite phenocrysts when present are small and scanty, pale-yellow, idiomorphic, and giving extinctions of +30°. The felspar-lathes, which average ·06-·08 mm. in length, give extinctions indicating andesine labradorite. The augite granules are small (·01-·02 mm.). Interstitial glass, generally scanty, is sometimes abundant when it is smoky, showing fibrous devitrification, with irregular “lacunæ” filled with a brownish yellow opaque glass like palagonite.