37. Genus of the Olivine-Basalts

Formula.—Oliv, matr, pauc, flu, gran, phen, vitr.

Characters.—Olivine scanty. In the groundmass the felspar-lathes are in flow-arrangement and the pyroxene is granular. Glassy plagioclase phenocrysts.

Description.—Brownish-black rocks which cannot be distinguished, except in the flow-arrangement of the felspars of the groundmass, from those described under genus 25. Like them they enter into the formation of the basaltic plains of Sarawanga and Mbua and elsewhere. Most of the rocks of this genus group themselves into one type where the felspar-lathes average in length ·15-·21 mm. The sp. gr. is usually between 2·87 and 2·91. Though rarely porphyritic, such rocks display to the eye a few small scattered glassy phenocrysts of plagioclase and an occasional grain of olivine. It is to this type of the genus that the following description applies.

In the thin sections they display small plagioclase phenocrysts, with a few of olivine and occasionally of pyroxene, in a groundmass where the flow-arrangement of the felspar-lathes is well marked, the rest of the groundmass being made up of granular augite with magnetite and generally a little residual glass.... The plagioclase phenocrysts do not usually exceed 2 mm. in size and contain magma inclusions. Two kinds are often indicated in the same slide by the extinctions, namely, one of medium andesine (17°-22°), and the other of acid labradorite (28°-33°).... The pyroxene phenocrysts are of pale brown augite; but they are small (less than 2 mm.), scanty, and often absent when their place is taken by microporphyritic augite, ·2 mm. in size.... The olivine phenocrysts are generally small. Though sometimes showing the long hexagonal sections, they are often rounded and more or less serpentinised.... The felspars of the groundmass present more typical lathes than are to be observed in the non-parallel felspars of the rocks of genus 25. The twin-lamellæ, when present, are fewer; but give similar extinctions (15°-21°) of medium andesine.... The augite granules are, as a rule, ·02 or ·03 mm. in diameter; but occasional more prismatic forms occur, two or three times the length, which give extinctions of over 30°. The magnetite is abundant, and the scanty interstitial glass is green or brown and displays fibrous devitrification.

The following three species, as indicated by the length of the felspar-lathes, are represented in my collection: