122 FOYE. 



2) Andcsitc from the western side. The andesite in question came 

 from the coast near the roadstead indicated on the charts. It occurs 

 in cliffs cut about the bases of mature hills. 



It is a slate-black, porphyritic rock. Abundant lar^e phenocrysts 

 of feldspar, 10 to 13 mm. in diameter, and less conspicuous augites 

 are set in a ground-mass consisting largely of glass with disseminated 

 augite, feldspar, and magnetite. 



The feldspar phenocrysts are very irregularly intergrown and 

 twinned. Most of them are zoned and show a composition varying 

 from Ab6f,An35 to Ah)47An53. The feldspar of the ground-mass is ande- 

 sine, AbesAnss. 



3) Basalt from Koro Mbasanc/n. Koro Mbasanga is the highest 

 peak in Vanua Mbalavu. It rises in the north central part of the 

 island. Two specimens of basalt were collected; one from the floor of 

 the breached crater and the other from a spur ridge south of the sum- 

 mit. Their textures vary but their composition is similar. The speci- 

 men from near the summit is holocrystalline and fine-grained with 

 very small phenocrysts. The other has larger phenocrysts, the olivine, 

 especially, being distinct. The latter specimen will be described. 



The rock is gray-black in color, holocrystalline, and porphyritic. 

 The feldspar phenocrysts are 2 to 3 mm. in diameter while those of 

 olivine are 5 to 8 mm. in diameter. Abundant large phenocrysts of 

 olivine and smaller ones of augite and feldspar appear in the slide. 

 Augite, feldspar, pyrite, niiignetite, and a little olivine compose the 

 ophitic ground-mass. The feldspars are zoned labradorites, varying 

 in composition from Ab43An57 to AbsgAnea- 



4) J'esicular basalt from Mva Mva. On the western side of Vanua 

 Mbalavu, somewhat south of the center of the island, there is a man- 

 grove swamp. At its northern edge the swamp abuts directly against 

 rather steep slopes of sub-Recent, porphyritic lavas. These lavas 

 are fresher than most of the other rocks of the island, in spite of the 

 fact that they are very vesicular. 



The olivine phenocrysts are large and euhedral. The feldspar, a 

 medium labradorite, (Ab4oAn6o) occurs in short, stubby laths. The 

 ground-mass is largely feldspar with a few minute olivine crystals in 

 the interstices. Some augite is also present. 



5) Basalt from the north side. This basalt occurs within a valley 

 in the limestone, 4 to 5 feet above sea-level, just north of the point 

 forming the northern side of the bay at Dalithoui. 



It is a fresh dark purple, vesicular rock, with a very fine holocrystal- 

 line structure. Phenocrysts of feldspar, 5 to 7 mm. in diameter, occur 

 in an aphanitic ground-mass. 



