CHAPTER XX 



THE VOLCANIC ROCKS OF VANUA LEVU {continued) 



Hypersthene-Augite Class 



II. Sub-Class. Hypersthene-Augite-Andesites 



Formula. — Plag, hypersth-aug, matr. 



Characters. — The pyroxene phenocrysts usually are re- 

 presented by separate crystals of the monoclinic as well as the 

 rhombic type, and the two forms are often associated in the same 

 crystal. The monoclinic form prevails in the groundmass in most 

 cases. 



Remarks. — It is not possible to draw a sharp line between 

 the augite and the hypersthene-augite-andesites ; but where two or 

 three phenocrysts of the rhombic type occur in a slide the rock may 

 be placed in this division. Between this variety and that where 

 rhombic pyroxene prevails, both among the phenocrysts and in the 

 groundmass, numerous intermediate kinds exist. These rocks 

 mostly occur in agglomerates and form small and large dykes or 

 sills, but rarely are found in flows. They are distributed over most 

 of the island except in the western portion (the basaltic districts of 

 Wainunu, Seatura, and Solevu), but reappear again in the jMbua 

 peninsula in places, as at Mount Koroma. 



The pale yellow rhombic pyroxene is uniform in its optical 

 behaviour. The prisms are noticeably pleochroic, being nearly 

 colourless when lying across the long axis of the lower nicol and 

 pale yellow when parallel with it. The intergrowths with mono- 

 clinic pyroxene often take the form of lamellar bands, whilst in 

 some cases a nucleus of the one (usually rhombic) is invested by a 

 growth of the other. 



