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.
I. Granular Sub-order (Augite-Andesites)
Formula.—Aug, matr, non-flu, gran.
1. Genus of the Augite-Andesites
Formula.—Aug, matr, non-flu, gran, phen, vitr.
Characters.—In the groundmass the felspar-lathes and prisms are not in flow-arrangement and the augite is granular. Phenocrysts of glassy plagioclase.