16. Genus of the Olivine-Basalts
Formula.—Oliv, matr, cop, flu, gran, non-phen, parv.
Characters.—Abundant olivine. In the groundmass the felspar-lathes are in flow arrangement and the pyroxene is granular. There are no plagioclase phenocrysts; but there are numerous small phenocrysts, under 2 mm. in size, of olivine and occasionally a few of pyroxene.
Description.—As a highly basic genus this ranks next to the preceding one, the specific gravity of the rocks ranging from 2·91 to 3·01. All the four species indicated by the varying length of the felspar-lathes are represented in my collection. The rocks of the first two, with the felspars averaging less than ·1 mm. and between ·1 and ·2 mm., are compact aphanitic basalts, only displaying an occasional small phenocryst of augite and blackish-grey or bluish-black in colour. Those of the last two species, with the average length of the felspar-lathes ·2-·3 mm. and ·3-·5 mm. respectively, are lightish-grey coarse-textured rocks of the doleritic type. In all the rocks no pyroxene phenocrysts are displayed in the slide; and the olivine phenocrysts, which are very numerous, do not usually exceed 1 mm., though occasionally the average size is 1·3 mm., and not infrequently it is only ·5 mm. In some cases where the larger olivine phenocrysts lie athwart the current of the felspar-lathes, the smaller (·5 mm.) lie with their long axes parallel to the flow. The olivine is either fresh, or it may be beginning to serpentinise in the cracks, or it may be in part hematised. The crystals may have regular outlines, or they may be rounded and sometimes deeply eroded. The pyroxene granules of the groundmass are of pale-brown augite, and average ·01 to ·03 mm. in size. Occasionally a few prism forms occur, giving extinctions of 30° to 40°. In the case of the more compact rocks, with the felspar-lathes averaging less than ·2 mm. in length, lamellar twinning is but scantily to be noticed in the lathes, which give extinctions measured from the long axis of 20° to 25° and by the twin lamellæ of 30° to 35°, indicative of acid labradorite in both instances. With the coarser doleritic grey basalts, where the felspar-lathes are stouter and have an average length exceeding ·2 mm., lamellar twinning is more frequent; the extinctions afforded by the lamellæ range between 15° and 25° (medium andesine). Residual glass is scanty in these rocks, and in the grey dolerites it is often difficult to recognise any.