EAKLE. ROCKS FROM THE FIJI ISLANDS. 589 



HyPERSTHENE AnDESITE from VomO LAI LAI. 



Vomo lai I:ii is a rock about two huiulred feet high on the island of 

 Vomo, which lies near Waia. 



The specimen is a gray porous rock liaving visible small pheiiocrysts 

 of pyroxene and feldspar. 



The groundmass has a hyalopilitic structure consisting of a dense mat 

 of feldspar rods and augite microlites with magnetite grains cemented by 

 a greenish brown glass. The amount of base is small compared with the 

 phenocryst constituent of the rock. 



The pyroxene microlites appear to be wholly augite, as they are non- 

 pleochroic and do not extinguish parallel. The phenocrysts are plagio- 

 clase, hypersthene, and augite. 



The plagioclase plienocrysts predominate, and most are short rectangu- 

 lar sections and square plates, abundantly filled with zonally arranged 

 brownish glass inclusions which impart a cloudy aspect to the sections. 

 Undulating extinction due to pressure or strain is noticeable in several 

 of the larger plates. The extinction angles on sections normal to the 

 twinning lamellsB reach 33°, showing the presence of a quite basic labra- 

 dorite. The rods, however, and some of the phenocrysts, show much 

 smaller extinction angles, under 10°, and are probably oligoclase. 



The pyroxene phenocrysts are both hypersthene and augite, which are 

 at times intimately associated. The hypersthene occurs in prismatic and 

 basal sections which give the common optical characteristics. The pleo- 

 chroism is quite marked, although not especially strong ; c = bluish 

 green, a = reddish brown, and b = brownish yellow. The plane of the 

 optic axes is 010, and sections parallel to 100 show they are normal to 

 the acute bisectrix, although the optic angle is a little wider than the field 

 of the microscope. The augite is bluish green, exactly matching the 

 color of the hypersthene in the vertical direction. It is non-pleochroic 

 and shows the large extinction angle. One section encloses an irregular 

 core of the pleochroic hypersthene. 



This is the only instance where the orthorhombic pyroxene was noted 

 as a constituent of the rocks from this region, yet its presence might be 

 more generally shown from a study of a larger amount of material. 

 There is a noticeable absence of olivine in the rocks of these islands, 

 while on the other hand it is a common constituent of the rocks from 

 all of the volcanic islands lyiiig at a distance to the east of Viti Levu. 

 These eastern islands are composed of rocks decidedly more basic in 

 composition than those from the islands immediately adjacent to the large 

 island of Viti Levu, as will be seen in the descriptions which follow. 



