120 FOYE. 



V. Rocks from Tavium. 



Thirteen rocks were studied from the collections made in Taviuni. 

 Eleven are olivine basalts; the other two contain basic andesine and 

 are closely related types. Most of the specimens came from the western 

 side of the island, but a few were collected from the eastern shore at 

 its northern and southern extremities. The small island of Vunim- 

 bani, off the northeastern shore, is capped with a water-laid ash deposit 

 of fine texture, 20 to 30 feet in thickness. Beneath the ash lies a 

 vesicular olivine basalt closely related to the basalts of the adjacent 

 shore of the main island. The two types have acid labradorite merg- 

 ing into andesine as their feldspar. 



The textures of the basalts of TaA'iuni are generally diabasic; in 

 two cases the ground-mass has pilotaxitic texture while a third rock 

 shows a trachytoid arrangement of its feldspar needles. 



The rocks were collected from widely separated localities and are 

 considered to be typical. Two varieties only will be described. 



(1) Andesiiic Basalt. The northeastern slopes of Taviuni are cov- 

 ered at intervals by a series of recent flows emanating from two well 

 preserved, accessory cones. The flows rest upon a lateritized sur- 

 face of vesicular basalt ha\ing a composition very similar to the 

 more recent la\'as. The specimen described is from a recent flow at 

 Nangasau. 



The rock is fresh, highly vesicular, slate-gray in color and has a 

 cryptocrystalline ground-mass in which are many minute phenocrysts 

 of feldspar and olivine. The phenocrysts are hardly noticeal)le in 

 the hand specimen. 



The olivine phenocrysts are euhedral and larger, as well as more 

 abundant, than the plagioclase phenocrysts. The plagioclase pheno- 

 crysts are zoned and have a composition Aarying from Ab55An45 to 

 AbooAnso or from basic andesine to acid labradorite. 



The ophitic ground-mass consists of olivine and plagioclase with 

 abundant minute bits of iron ore. 



(2) Olivinc-cjahhro Porphyritc. Near the center of Taviuni, back of 

 the village of Somo Somo, there is an extinct crater whose walls ha\e 

 not yet been breached by erosion. From the outer slope of the western 

 walls, at an elevation of approximately 4000 feet, the rock to be 

 described was collected. 



It is a fresh, massive, slate-gray rock containing abundant pheno- 

 crysts of feldspar (S to 10 mm. in diameter) with augite ami olivine 

 phenocrysts of the same size but less abundant. 



