1. Genus of the Augite-Andesites
Formula.—Aug, matr, non-flu, gran, phen, vitr.
Characters.—In the groundmass the felspar-lathes and prisms are not in flow-arrangement and the augite is granular. Phenocrysts of glassy plagioclase.
Description.—These rocks frequently form dykes; and it is probable that most of the instances where the nature of the exposure could not be ascertained also fall into this category. They are dark-brown or blackish, and their sp. gr. ranges, except in the semi-vitreous rocks, from 2·7 to 2·83. They are sometimes vesicular, and rocks with abundant interstitial glass are common. They admit of grouping into two sub-genera according to the size of the plagioclase phenocrysts:
- (a) Porphyritic, where the average size is 3 mm. or over.
- (b) Non-porphyritic where the average size is less than 3 mm.
Nearly all the rocks in my collection belong to the second group.
In the sections they display phenocrysts of plagioclase and occasionally of pyroxene in a groundmass formed of a plexus of felspar-lathes, augite granules, magnetite, and usually a fair amount of smoky more or less opaque interstitial glass.... The felspar phenocrysts, which are sometimes abundant, give lamellar extinctions of andesine labradorite (15° to 30°). They are frequently small (1 to 2 mm.) and contain often many magma-inclusions. Whilst the corroded aspect of some indicate that they belong to an earlier period, the aggregate character and regular outlines of others suggest that they have been produced in position.... Pyroxene phenocrysts are absent in half the rocks. When present they are generally small and of a pale augite which gives extinctions of 30°. Their size does not usually exceed 2 mm.; and they may consist of single crystals (sometimes twinned) or of an aggregate of smaller crystals. At times there is a suspicion of intergrowth with rhombic pyroxene; but no phenocrysts formed alone of that mineral occur.... The augite granules of the groundmass as a rule vary from ·02 to ·04 mm. in diameter. Occasional prism-forms giving large extinctions occur.... The felspar-lathes vary much in length in different rocks. In some they average as little as ·05 mm., and in others as much as ·2 mm.; but the doleritic type with yet longer lathes is not represented in the collection except among the altered rocks. Most of the lathes show a single median twin-line, and when broader they display twin-lamellæ. The angles of extinction indicate acid and basic andesine.
Three out of the eight species distinguished by the length of the felspar-lathes occur in my collection, that with the longest lathes (·3-·5 mm.) being not represented.