590 PROCEEDINGS OP THE AMERICAN ACADEMY. 



OVALAU. 



This is a very important island of the <^roup, lying just east and near 

 Viti Levu, but no specimens other than some mm-h decomposed rock 

 from the vicinity of Levuka were collected. The specimens appear to be 

 altered andesites, either hornblende or augite andesites. According to 

 VVichman the prevailing rock is augite andcsite, although some hornblende 

 andesite does occur. 



Olivine Basalt fkom Wakaya. 



Wakaya is about four miles long, lying nine miles east of Levuka. 

 Its highest peak rises 595 feet above the sea. 



The basalt is a dark greenish brown vesicular rock containing amyg- 

 daloids of calcite. 



Microscopically the rock shows a brown vitro})hyric base which has a 

 slight tendency to perlitic structure developed in the consolidation. 

 Long, slim feathery forms of feldspar are scattered through the amor- 

 phous base. There are only one or two lath-shaped sections present, and 

 these show by their extinctions that they are labradorite. The pheno- 

 crysts are olivine and augite. Olivine occurs in perfect automorphic 

 crystals which are often stained yellowish by the oxide of iron which has 

 resulted from a slight alteration of the olivine. Embayments formed by 

 the groundmass are seen in some of the crystals. 



Augite is less than the olivine in amount. It occurs in pale green 

 automorphic crystals, and contains inclusions of the older minerals, olivine 

 and magnetite, besides some glass. 



The rock from its mineral composition might be considered a lim- 

 burgite, yet the large amount of glass base present would doubtless show 

 the rock to be, chemically, a more acidic type of rock than a limburgite. 



Olivine Basalt from Makongai. 



Makongai is two miles long by one and a half miles wide, lying 

 seven and a half northwest of Wakaya. It has two peaks in the centre, 

 with an average height of 875 feet. 



The specimens from this island are too much altered to determine 

 their petrographic characters, but the rock appears to have been like 

 that from Wakaya. Well developed olivine and augite crystal? are still 

 preserved in the decomposed base. 



