In the section they exhibit phenocrysts of plagioclase, and occasionally of augite in a groundmass of felspar-lathes, pyroxene prisms and granules (the former predominating), with a fair amount of altered interstitial glass.... The plagioclase phenocrysts owe their opacity to the great number of fine and sometimes parallel cracks filled with alteration products, that traverse them. Although much of their original material has often disappeared, they still display the lamellar twinning of medium andesine (15° to 20°).... The phenocrysts of pale yellowish augite, which give the large extinctions of that mineral, exhibit but little alteration, although lying in the same slide with those of the plagioclase.... The pyroxene prisms of the groundmass are of the same yellowish augite. They are broad with rounded extremities and are arranged in a loose plexus.... The felspars of the groundmass, which average ·1 mm. in length and give extinctions of medium andesine, are either lathe-shaped or short and broad when they display lamellæ. The last-named approach the orthophyric type, and such rocks come near the porphyrites; but I do not feel justified in placing them in a separate orthophyric order.
III Ophitic Sub-order of the Augite-Andesites with the
Felspars of the Groundmass not in Flow-arrangement.
Formula.—Aug, matr, non-flu, oph.
These rocks form generally ancient flows. They are for the most part semi-ophitic, large ophitic “plates” being uncommon.
9. Genus of the Augite-Andesites
Formula.—Aug, matr, non-flu, oph, phen, vitr.
Characters.—The felspar-lathes and prisms of the groundmass are not in flow-arrangement. The augite of the groundmass is ophitic or semi-ophitic. Glassy plagioclase phenocrysts.
This genus may be divided into two sub-genera,
- (a) Porphyritic, where the average size of the plagioclase phenocrysts
- is 3 mm. and over.
- (b) Non-porphyritic, where the size is less than 3 mm.