426 THE GEOLOGY OF THE NEW HEBRIDES, 



The island is entirely volcanic. Rocks collected on the south- 

 east side of Dives Bay proved to be porphyritic basalts(l) and 

 an interesting type of basic lava(2) possessing an andesitic facies. 



Merlav, 2900 ft., Merig, 200 ft., Vatganai, 250 ft., and 

 several minor islets not mentioned, we were not able to visit, but, 

 like the rest, they are no doubt of volcanic origin. 



1. Porphyritic Basalt from the south shore of Dives Bay. 

 Macroscopic Characters. — A compact grey rock 



homogeneous in appearance. 



Microscopic Character s. — A fine-grained basaltic base 

 through which are porphyritically distributed abundant corroded 

 labradorite felspars up to 1 mm. diameter, constituting about 

 18 % of the rock; occasional porphyritic individuals of a light 

 gveQH i^yroxene frequently twinned on the (100) face and measured 

 up to 1 mm. diameter; relatively large grains of magnetite, up to 

 0-75 mm. diameter. The base consists mainly of tiny lath-shaped 

 felspars with augite microlites, magnetite dust, and an indistinct 

 glassy residue. 



2. Olivine Pyroxene Porphyry from the south shore of Dives 

 Bay, near the Mission Station. 



Macroscopic Character s. — A dense dark-coloured 

 rock with gleaming porphyritic pyroxenes and abundant rect- 

 angular felspars. 



Microscopic Character s. — Distributed through a 

 more or less glassy base are porphyritic individuals present to 

 the extent of about one-half the rock-mass. The felspars are 

 rectangular, pointing in the direction of flow, and regular in 

 size, averaging 1 mm. x 3 mm.; they are well zoned, dusty bands 

 being especially characteristic; judging by the extinction angle 

 on the albite twin, its composition must be in the vicinity of 

 Abg An 4. From the description these felspars will be seen to 

 have the facies of those developed in andesite lavas; appearing 

 in so basic a rock as this, they are therefore of special interest. 

 In addition to felspar individuals, which compose about 25 % of 

 the rock, are large apple-green pyroxenes to the extent of 12%; 



