13. Genus of the Olivine-Basalts
Formula.—Oliv, matr, cop, flu, gran, phen, vitr.
Characters.—Olivine abundant. Felspars of the groundmass in flow-arrangement. Pyroxene of the groundmass granular. Phenocrysts of glassy plagioclase.
Description.—Dark grey or dark brown rocks with sp. gr. 2·88 to 2·99. Phenocrysts of olivine, pyroxene, and plagioclase occur in a groundmass showing partially parallel felspar-lathes, abundant pyroxene grains, and fine magnetite, residual glass being scanty or absent. The olivine-phenocrysts do not exceed 3 or 4 mm. in size and in some rocks are less than 1 mm. They are usually more or less serpentinised and hematised at the borders and in the cracks; but sometimes they are almost fresh and present regular outlines. In some rocks the olivine also occurs as grains (·3 mm.) in the groundmass. When the phenocrysts of olivine have blackish borders they are surrounded by a halo, as though the crystal had attracted the magnetite from the groundmass immediately around. The plagioclase phenocrysts vary from 1 to 3 mm. in size. They often contain abundant magma-inclusions and give lamellar extinctions of 15° to 25° (basic andesine). In some rocks they are rudely parallel. The pyroxene phenocrysts, which are of pale brownish-yellow augite giving extinctions of over 30°, do not usually exceed 3 mm. They present regular octagonal cross-sections and sometimes display lamellar twinning. Occasionally there is a suspicion of intergrowth with rhombic pyroxene. The felspar-lathes, which according to the species vary much in length, at times show a few lamellæ. The augite grains of the groundmass are abundant and are as a rule about ·02 mm. in size; but in some rocks they are larger and in others smaller.
This genus may be divided into two sub-genera, the porphyritic sub-genus where the felspar phenocrysts are larger than 3 mm., and the non-porphyritic where they are smaller. All four species, as indicated by the length of the felspar-lathes, are represented.