cob PROCEEDINGS OF THE AMERICAN ACADEMY. 



allotriomorphic feldspar and quartz grains, while both plagioclase and 

 quartz appear as plienocrysts in this microcrystalline base. 



An extinction angle normal to the albitic twinning was 20° on the 

 best section. Some of the sections show no polysyntlietic twinning, and 

 resemble glassy sanidine with Carlsbad twinning common. 



Quartz occurs as an original constituent in large crystals with rounded 

 edges. They contain inclusions of glass and licjuid. Some of the sec- 

 tions have a wide resorption rim. 



Some of the feldspar has altered to calcite. Magnetite dust is plenti- 

 ful. There are some areas of yellowish green material which seem to be 

 mixtures of chloritic substance with carbonates, which may have resulted 

 from the alteration of some ferromagnesian mineral, but further than this 

 there is no indication of a dark silicate present. 



The rock is {)erhaps more a dacite than an andesite, but from its pres- 

 ent condition little can be learned regarding its former nature. 



Hornblende Andesite fuom Waia. 



Waia is a small island three and a half miles long by about three 

 miles wide, lying tliirty miles west of Viti Levu. It is one of the most 

 southern of a train of volcanic islands belonging to the Yasawa group, 

 and has several high peaks, one rising 1,870 feet above the sea level. 



The specimen from this island is a hornblende andesite having a lead 

 gray color and compact texture. Small glassy feldspars can be detected 

 in the groundmass. 



The base lacks glass, and appears microfelsitic with some microlites, 

 and many allotriomorphic grains, of feldspar. Plauioclase phenocrysls 

 occur, having an average extinction on sections normal to 010 of 21°. 

 Polysyntlietic and Carlsbad twinning are common. Many of the sections 

 show the zonal structure. Some calcite has been developed from the 

 alteration of the feldspar. Well defined basal and prismatic outlines of 

 hornblende crystals are shown by magnetite, but the original hornblende 

 has completely altered, yielding a mixture of chlorite with calcite. 



Pyroxene was also a constituent of the rock, although not so abundant 

 as the hornblende. One crystal, presumably diopside, is seen which has 

 completely altered to calcite. 



Magnetite occurs abundantly in fine dust and occasionally in good- 

 sized anhedrons. 



This rock is probably the same as the hornblende andesite from Kan- 

 davu, but is in a more advanced stage of alteration. 



