15. Genus of the Olivine-Basalts
Formula.—Oliv, matr, cop, flu, gran, non-phen, magn.
Characters.—Abundant olivine. In the groundmass the felspars are in flow-arrangement and the pyroxene is granular. No plagioclase phenocrysts, but large phenocrysts, over 2 mm., of olivine and pyroxene.
Description.—This genus includes the most basic rocks represented in my collection. They are compact heavy blackish rocks with sp. gr. 3 to 3·1, and display large porphyritic crystals of olivine and pyroxene often 3 or 4 mm. in size. The olivine phenocrysts may be fairly fresh with clean outlines, or they may be deeply eroded and stained by iron oxide, or they may be passing into serpentine. The pyroxene phenocrysts may be either eroded or possess regular outlines. They are of pale brown augite and give extinctions over 30°. The pyroxene granules, which average ·02 mm. and are very abundant, are of the same augite. The felspar-lathes are relatively scanty. In the two rocks here included they average in length ·06 mm. and ·08 mm. When lamellæ can be recognised they give extinctions of 30°-40° (labradorite). The untwinned lathes give extinctions of 20°-28° (labradorite).
The only species represented is that with felspar-lathes less than ·1 mm. in length.