33. Genus of the Olivine-Basalts
Formula.—Oliv, matr, pauc, non-flu, oph, phen, vitr.
Characters.—Olivine scanty. In the groundmass the felspar-lathes are not in flow-arrangement and the pyroxene is ophitic or semi-ophitic. Plagioclase phenocrysts glassy.
Description.—These brownish-black rocks are all of the semi-ophitic type. Although no ophitic “plates” occur in the slide, the augites of the groundmass have no longer the granular form, but are large, ·08-·1 mm. in size, and tend to invest the felspar-lathes. The specific gravity ranges from 2·78 to 2·86. As in other of the ophitic and semi-ophitic rocks of this collection (genera 9, 10, 12, of the augite-andesites), the large size of the felspar-lathes (·2-·3 mm. long) of the groundmass gives a doleritic texture in the slide. In most of the other characters these rocks approach those of genus 25 which possess felspar-lathes more than ·2 mm. in length. But they are more often semi-vitreous, and display a considerable amount of dark smoky glass showing numerous magnetite rods and skeletal crystals with fibrous devitrification. The plagioclase phenocrysts, which vary much in size in different rocks (in some 2 or 3 mm., in others 4 or 5 mm.), give extinctions of andesine labradorite (20°-35°). They are often eroded and contain numerous large black inclusions of the magma. The pyroxene phenocrysts, which are of pale-brown augite, often have an aggregate-structure, having been formed in situ. Others again consist of single crystals and have been much affected by the magma. The olivine phenocrysts, which are at times deeply eroded, are generally small and in part serpentinised.