4. Genus of the Olivine-Basalts
Formula.—Oliv, matr, cop, non-flu, gran, non-phen, parv.
Characters.—Abundant olivine. In the groundmass the felspar-lathes are not in flow-arrangement and the pyroxene is granular. No plagioclase phenocrysts. Small phenocrysts of olivine and occasionally pyroxene under 2 mm.
Description.—A grey coarse-grained rock. Sp. gr. 2·9. It displays abundant small phenocrysts of olivine, all less than a millimetre in size (·2-·8 mm.), which are hematised in the fissures and at the borders. The felspar-lathes, which display a few twin-lamellæ giving extinctions of 16°-18° (medium andesine), vary greatly in size. The smaller are ·1 to ·3 mm. and the larger ·3 to ·5 mm. long; but the two are connected by felspars of intermediate length. The abundant augite granules average ·037 mm. in breadth. Pyroxene phenocrysts are not represented in the slide. From its coarsely crystalline texture this rock merits the field-name of a grey doleritic basalt; and except in the arrangement of the felspar-lathes it does not differ from the grey doleritic basalts of genus 16.