302 A NATURALIST IN THE PACIFIC chap. 



with the rarity or absence of phenocrysts of pyroxene. They are 

 to be seen, however, in the process of being built up within the 

 substance of the hornblende pseudomorphs, which consist entirely 

 of minute prisms and granules of pyroxene and of fine magnetite. 

 The process seems to consist in the formation of parallel layers of 

 rhombic and monoclinic pyroxene. 



The Thoka-singa rocks are more basic (spec. grav. 272 to 274), 

 and in the scanty holo-crystalline groundmass approach the plutonic 

 type. They are dark grey granitoid rocks displaying abundant 

 macroscopic pyroxene crystals 2 to 3 mm. long. The original 

 hornblende phenocrysts are only represented by traces of pseudo- 

 morphs of fine pyroxene and magnetite, the process of dispersion, 

 described on page 307, being almost completed. The pyroxene 

 phenocrysts are mostly rhombic ; but intergrowths with the mono- 

 clinic form occur. The " grain " of the mosaic of the groundmass is 

 coarse ('023 mm.), and there is a fair amount of prismatic with a 

 little granular pyroxene, the prisms, '04 mm. long, giving usually 

 straight extinctions, whilst the granules are apparently monoclinic. 



Sub-Class Quartz-Hornblende-Hypersthene-Andesites 



OR Dacites 



These rocks are infrequent. They compose in mass the 

 adjacent mountains of Ngaingai and Wavva Levu in the Ndran- 

 dramea district, and appear also on the lower slopes of the neigh- 

 bouring mountain of Navuningumu. They differ chiefly from the 

 hornblende-hypersthene-andesites in the presence of porphyritic 

 quartz, which, however, is not as a rule abundant. In their general 

 origin and affinities and in their mode of occurrence they cannot 

 be separated from the two sub-classes of hypersthene-andesites 

 and hornblende-hypersthene-andesites before described. They 

 all belong to the felsitic order of the sub-class, and all are referred 

 to the sub-order with prismatic pyroxene and to the section with 

 plagioclase phenocrysts. 



They are light grey rocks, with a specific gravity of 2*57 to 

 2*6 1, showing usually dark pseudomorphs after hornblende and a 

 little porphyritic quartz. In the slide they display these pseudo- 

 morphs and quartz crystals, associated with abundant plagioclase 

 phenocrysts, in a felsitic groundmass, evidently a mixture of felspar 

 and quartz, with fine pyroxene, mostly prismatic and rhombic. 

 Pyroxene phenocr>'sts are absent or rare ; but they may be seen 

 in process of formation in the substance of the hornblende-pseudo- 



