eaklp::. — hocks from the fi.ji islands. 591 



Olivine Basalt fuom Ngau. 



The island of Ngau lies twenty-seven miles southeast of Ovalau, and is 

 eleven miles long by four miles wide. 



The rock is porous, of a gray color, and under the microscope shows 

 a base composed mainly of stout plagioclase laths, whose angular inter- 

 spaces are filled with a speckled mixture of bluish green augite microlites, 

 minute feldspathic rods, and magnetite grains, the whole forming an 

 intersertal structure. No glass is however apparent. The feldspathic 

 constituent is limited to the plagioclase laths and rods in the groundmass, 

 and is subordinate to the pyroxene in amount. Albitic twinning com- 

 bined with the Carlsbad is common, and the extinction angles on sections 

 normal to the twinning planes are those of labradorite, averaging 28°. 

 Most of the phenocrysts are large automorphic augites, the olivine being 

 subordinate in amount and size. 



Augite occurs in light green perfect crystals, some of which show a 

 very weak pleochroism to yellowish tones. Zonal structure and twin- 

 ning is seen in several of the sections. Inclusions of olivine and magne- 

 tite are present and some of the sections have been penetrated along the 

 cleavage by the groundmass. Olivine occurs here and there in the slide 

 in rounded grains without any original crystal boundaries, and most of it 

 has altered slightly so as to be coated yellowish brown. 



A few auhedrons of magnetite occur, but most of the magnetite is in 

 small grains, 



Augite Axdesite from Nairai. 



Nairai Island is four miles long by one and a half to three miles wide, 

 lying about ten miles east of Ngau. 



The rock has a hyalopilitic base, consisting of minute short rods of 

 feldspar with augite grains and magnetite particles in a glass cement. 

 The feldspar phenocrysts which were present have all been altered to a 

 brown carbonate, leaving only the rims of the original mineral. The 

 brown carbonate is apparently calcite stained with the iron oxides, this 

 staining solution saturating a good portion of the groundmass and filling 

 the cavities wiih banded walls of brownish opalitic material. 



Augite occurs in automorphic crystals, which occasionally show twin- 

 ning parallel to tlie orthopinacoid. A few grains of olivine can be 

 detected stained reddish brown. 



Na Kobii. — This is a peak in a small island of the Nairai group, just 

 off the south coast of the main island. 



