126 FOYE. 



described was taken from a flow in the east central part of the island 

 at an elevation of 200 feet. 



The basalt is slate-gray in color and fine-grained. An occasional 

 feldspar phenocryst, 4 to 5 mm. in length, may be seen, but usually 

 the feldspar is in the form of minute needles associated with small 

 augite crystals, 1 to 2 mm. in diameter. The rock is very fresh and 

 breaks with conchoidal fracture. 



Phenocrysts of augite, olivine and plagioclase feldspar are abundant 

 in the thin section. The ground-mass, with occasional traces of a 

 diabasic texture, is composed of augite, olivine, feldspar, and glass 

 with abundant magnetite dust. 



The feldspar phenocrysts show zonal inclusions and frequently 

 are zoned, having the composition, Ab42An58, labradorite, and Ab^sAnw, 

 bytownite. 



Thikombia-i-lau. 



1. Andesiiic Basalt. A little to the east of the extreme south- 

 western point of Thikombia-i-lau is a point of land, built up of a coarse 

 agglomerate. Cutting the agglomerate is a dike of basalt, 5 to 6 

 feet in width. This intrusive is fine-textured and porphyritic. It is 

 fresh, dark gray, almost black, in color. Augite (diallage) and 

 labradorite (Ab4oAn6o) phenocrysts, 2 to 3 mm. in diameter, form 30 

 per cent of the rock. The structure of the ground-mass is hyalopilitic. 



The chemical composition is as follows: — 



Norm 



SiOo 58.30. 



TiOs 0.80 Quartz 12.36 



AI2O3 17 10 Orthoclase 5.00 



FesOs 2 . 10 Albite 31 .44 



Feb 5.58 Anorthite 27 52 



MnO 0.08 Diopside 6.93 



]VIgO 2 . 70 Hypersthene 10 . 74 



CaO 7.40 Magnetite 3.02 



NaoO 3.72 Hmenite 1.52 



K26 0.83 Apatite 31 



H2O 1.15 98.84 



P2O5 0.22 Water 1 . 1 5 



99.98 99.99 



Specific gravity at 20°C = 2.700 



