44. Genus of the Olivine-Basalts
Formula.—Oliv, matr, pauc, flu, prism, non-phen, parv.
Characters.—Olivine scanty. In the groundmass the felspar-lathes are in flow-arrangement and the pyroxene is in great part prismatic. There are no phenocrysts of plagioclase; but there are a few small phenocrysts of olivine and sometimes of pyroxene, less than 2 mm. in size.
Description.—A dark grey compactish rock; sp. gr. 2·9; showing a little macroscopic olivine; forming a dyke in the tuffs on the summit of the hill of Vatui (p. [54]).
In the section it exhibits a few phenocrysts of olivine (more or less serpentinised) and of augite in a groundmass formed of stout augite prisms and small augite granules with felspar-lathes, magnetite, and a little devitrified yellowish interstitial glass. The augite prisms and the felspar-lathes are in flow-arrangement.... The pyroxene phenocrysts, which are pale brown and give extinctions of over 30° from the single cleavage-lines, may be aggregates of five or six smaller crystals or single crystals presenting sometimes lamellar twinning. The first are doubtless formed in situ. The second though showing regular outlines may have a nucleus giving a different extinction and possessing eroded margins. The stout augite prisms of the groundmass, which are occasionally twinned, have an average length of ·2 to ·3 mm. and give angles of extinction with the long axis of 30° to 40°. The felspar-lathes average only ·07 mm. in length and afford extinctions, when untwinned, of 18° to 24° (acid labradorite).
The only species represented is that where the average length of the felspar-lathes is between ·02 and ·1 mm.
CHAPTER XIX
THE VOLCANIC ROCKS OF VANUA LEVU (continued)
Augite Class
Sub-class II
Augite-Andesites including the Basaltic Andesites
(Plag, aug, matr.)
This sub-class, which comprises 40 per cent. of the volcanic rocks, is characterised by the absence of olivine on the one hand, and by the rarity or absence of rhombic pyroxene on the other. On the basic side it shades into the olivine-basalts through the basaltic andesites, and on the acid side by intermediate stages into the hypersthene augite andesites; and for these reasons it is not always possible to draw a sharp line of distinction. In cases where a hand-specimen displays no macroscopic olivine and where a solitary small phenocryst of olivine is alone observed in the slide, it should be referred to this sub-class; and here also all doubtful specimens as regards the occurrence of olivine should be placed. When the question of the occurrence of rhombic pyroxene arises, it should be remembered that the great prevalence of monoclinic pyroxene amongst the phenocrysts and the practical absence of rhombic pyroxene from the groundmass are essential characteristics of this sub-class. Rhombic pyroxene is only indicated at times by intergrowths in the phenocrysts.
The basaltic andesites enter into the formation of old “flows,” as in the Mbua and Ndama plains. The other rocks enter principally into the composition of dykes, necks, and agglomerates.