GEOLOGICAL OUSEUVATIONS IN FIJI. 123 



Tn thin-section an (Krasional augito pluMiocryst is seen to be associ- 

 ated with the feldspar. The ophitie ground-mass is largely composed of 

 augite with disseminated feldspar crystals. Both generations of feld- 

 spar are labradorite, AhioAnen, though some of the crystals are zoned. 



6) Andf.sitic Bdsalt from Tata. Tota is situated at the juncture of 

 the volcanic rocks with the limestones on the northeastern shore of 

 Vanua Mbalavu. Just south of the village low spurs from the vol- 

 canic hills jut out into the sea. To the northwest abrupt limestone 

 dills rise from a narrow beach. Inland there is evidence that the 

 limestone overlies the volcanic rocks. The andesite here described 

 was collected from the volcanic spurs south of the village, but its 

 relations to the limestone is uncertain. It is l:)elieved, however, to 

 represent a flow from the basaltic cone of Koro Mbasanga rather than 

 to be allied to the andesite underlying the limestone. 



The rock is reddish brow'n in color, holocrystalline and fine grained. 

 Phenocrysts of feldspar and diallage are evenly disseminated through 

 the rock. Larger olivine phenocrysts are not so common. The 

 ground-mass is composed of augite and feldspar in intersertal relation, 

 associated with magnetite particles and some olivine. 



The feldspars are zoned and show a composition varying from 

 Ab56An44 to xAb48An52 or from basic andesine to acid labradorite. The 

 texture of the ground-mass is coarser than that of the average rock 

 studied. Minute inclusions which appear to be olivine are some- 

 times found in the feldspars. 



The analysis of the basalt is as follows : — 



Si02 55.75 Norm 



AbOa 16 53 



Fe^Oa 0.53 Q^a^tz 1.50 



FeO 7.14 



MgO 3.19 



MnO 0.55 



Orthoclase 6.12 



Albite 35.63 



Anorthite 23.27 



(^^Q 9 j2 Diopside 18.62 



js-j^^Q 4 24 Hypersthene 12.13 



KoO.. 0.95 Magnetite 0.70 



HoO (below 100°C) 1 . 04 97 . 97 



H9O (above 100°C) 0.51 Water 1 .55 



CO. 0.32 CO2 0.22 



Total 99.87 Total 99.74 



Specific Gravity at 20°C = 2.845 

 Analyst, L. F. Hamilton. 



