GEOLOGICAL OBSKUVATIONS L\ FIJI. 129 



2. OUrlne basalt. The government sehoolhonse at Thoumbo 

 stands on a low spur from the central hills. The spur has a veneer 

 of limestone but, beneath, lies a series of flows and agglomerates which 

 are deeply lateritized. The basalt was collected from a residual 

 boulder about one hundred yards northeast of the schoolhouse. 



The rock is dark gray in color, massive and coarsely porphyritic. 

 Feldspar phenocrysts (labradorite, AbioAueo), 10 to 15 mm. in diam- 

 eter, form 40 to 50 per cent of the rock. The rock is remarkable in 

 having no augitc phenocrysts. The olivines are small and scattering. 



The ground-mass is an ophitic aggregate of minute crystals 

 of augite and feldspar. 



Ofw Croup. 



1. Vesicular andcsiic. Xdoi consists of agglomerate and flows 

 dipping north and northeast. Its summit, at an elevation of 279 

 feet, is composed of agglomeratic rocks. This specimen was broken 

 there from a boulder in the agglomerate. The andesite has a spongy 

 vesicular structure, with the vesicles elongated in the direction of flow. 

 The rock is slate-drab in color. 



The microscope shows that a glassy rim, altered to kaolin, surrounds 

 the vesicles, between which is a mat of feldspar needles. The feldspar 

 is a medium andesine, Ab62An38. 



The ground-mass is hyalopilitic. 



2. Olivine andesite from Ndoi. Interbedded lava flows stand up as 

 hog-back ridges between ash and agglomerate deposits on the north- 

 western side of Xdoi. The specimen was collected from one of these 

 flows. 



The rock is highly vesicular and grayish brown in color. The feld- 

 spar phenocrysts are most abundant. They are of two generations. 

 The first generation are large, zoned, and show inclusions of glass 

 regularly arranged. Large augite phenocrysts are of the same 

 generation. The second generation are smaller and show less distinct 

 lines of growth. The zoned feldspars have the composition of medium 

 andesine and acid labradorite, Ab59An4] to Ab48An52. Andesine occurs 

 in the ground-mass. 



Small olivine phenocrysts are scattered through the ground-mass and 

 are associated with the second generation of feldspars. 



The texture of the ground-mass, little else but a mat of feldspar, is 

 pilotaxitic. 



3. Basalt. The basalt in question was taken from a dike cutting 

 the olivine andesite just described. 



