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 princi- 

 pally into the composition of dykes, necks, and agglomerates. 



I. Granular Sub-order (Augite-Andesites) 

 FORMULA. — Aug, matr, non-flu, gran. 



