584 PROCEEDINGS OF THE AMERICAN ACADEMY. 



in the island, which agrees quite closely with this rock, and may possibly 

 be the same rock. 



Mt. Victoria. — A specimen of augite andesite is from this mountain, 

 which rises in the nortlieru part of the island. The rock shows mega- 

 scopic prisms of augite, magnetite grains, and a few feldspars in a dense 

 greenish black base. Under the microscope the groun<lmass appears as 

 a web of feldspar and augite microlites with many magnetite grains, ce- 

 mented by a colorless to yellowish glass, the whole having a hyalopilitic 

 structure. 



Plagioclase, augite, and anhedrous of magnetite occur as phenocrysts. 

 The feldspar phenocrysts predominate. The sections are large twinned 

 plates and give an average extinction of 28° normal to 010, showing 

 them to be labradorite. Zonal structure is seen well in some of the 

 brachypinacoidal sections, and the extinction angles of the zones range 

 from +7° on the outer shell to —39° in the centre. Colorless and 

 yellowish glass inclusions are abundant in the feldspar phenocrysts in 

 zonal arrangement. Augite or diopside occurs in pale green, large basal 

 and prismatic sections occasionally containing inclusions of glass and 

 magnetite with a few apatite needles. 



The feldspar and augite phenocrysts have been penetrated along the 

 cleavages and fissures by a yellowish brown oxide of iron which also 

 lines some of the cavities with brown banded layers. This oxide has 

 apparently been derived from an iron-rich olivine, whose former presence 

 is indicated by a few irregular sections of fibrous serpentine mixed with 

 brown iron oxide and carbonates. 



The base of the rock w^eathers to a light brown, leaving the augite 

 crystals standing out prominently unaltered. 



Na dari Vatu. — An olivine-bearing augite andesite is labelled from 

 this locality, which id said to be in the northern part of the island. 



The rock has a dark gray holocrystalline base in which megascopic 

 crystals of black augite are prominently disseminated. The base is largely 

 feldspathic, but includes some small augites, although most of the py- 

 roxenic constituent of the rock is in large phenocrysts. 



Plagioclase, augite, and olivine are the chief constituents. 



Sections normal to 010 of the feldspars give an average extinction of 

 22°, indicating an andesine or perhaps the soda-lime end of labradorite, 

 as the kind of plagioclase phenocrysts. Zonal structure is common, and 

 inclusions of the older formed minerals augite, olivine, and apatite, besides 

 much glass, zonally arranged, are seen. 



Augite occurs in automorphic sections, containing apatite and olivine 

 inclusions. It is suboi'dinate to the feldspars in amount. 



