xxi ACID ANDESITES 303 



morphs. It is only at times, as in the instance of the Navuningumu 

 rock, that the brown hornblende phenocrysts are in part unchanged 

 and that complete pyroxene phenocrysts occur. In such cases the 

 last may be entirely rhombic, or may exhibit at times intergrowths 

 with the monoclinic form. 



The plagioclase phenocrysts, 2 mm. in size, are often tabular 

 and zoned and give two sets of extinctions, indicating acid and 

 basic andesine. Their abundant inclusions are arranged in zones. 

 The quartz-crystals, 1 to 2 mm. in size, present hexagonal sections 

 with rounded angles. They are sometimes traversed by cracks 

 occupied by iron oxide films. The pseudomorphs after hornblende, 

 which consist of fine pyroxene mixed with magnetite, exhibit often 

 the building up in their interior of pyroxene phenocrysts, apparently 

 rhombic, by long parallel rows of stout prisms. In other cases 

 the pseudomorphs display the different stages of dispersion. The 

 fine pyroxene of the groundmass consists mostly of rhombic 

 prisms (/02 — *o6 mm. long) with some granules. The " grain " of 

 the groundmass is usually between *oi and "02 mm. There is little 

 or no residual glass. 



The rocks of Wawa Levu and Ngaingai are closely similar, 

 but they differ in the size of the prismatic pyroxene of the ground- 

 mass, which is coarser in the first-named mountain (-055 mm. long) 

 than it is in the second (-025 mm. long). In both the " grain " of 

 the mosaic is about the same ("014 mm.). 



