530 THE CONTINENTAL ORIGIN OF FIJI, II., 



hornblende prisms up to 2 or 3 mm. in length. Both minerals 

 are strikingly fresh and idiomorphic. The felspars are strongly 

 striated on the cleavage faces and are more numerous than the 

 hornblende crystals. On closer examination fairly abundant 

 augite is detected in the form of very small crystals of light 

 honey-yellow colour, looking, in fact, almost like olivine. A 

 little magnetite can also be observed. 



The specific gravity of the rock is 2-61. 



Microscopic characters. — The base is hyalopilitic in texture, 

 consisting of microlites of felspar and augite, with some magnetite, 

 and colourless interstitial glass in relatively rather small propor- 

 tion. All the mineral constituents of the base are very minute 

 in size. The felspars are the most abundant. In most cases 

 their size is so small that twinning cannot be detected, but 

 whenever the size is slightly gi-eater than the average twin stria- 

 tion is observable. Measurements of extinction angles are 

 unsatisfactory. The best of them never exceed 2^ to 3^ from 

 the direction of elongation of the microlite. The refractive index 

 is much the same as that of the Canada balsam. The species is 

 therefore oligoclase. 



The augite of the second generation does not appear to differ 

 noticeably from that of the other andesites already described. 

 If hornblende microlites are developed, they are undistinguishable 

 from the augite. The magnetite is niostl}^ in more or less sharpty 

 defined crystals of small size. In addition to these constituents 

 whose nature can be satisfactorily determined, there are rather 

 scarce and very minute prisms with moderate refractive index 

 whose double refraction is so weak as to be scarcely noticeable. 

 These are probably apatite. 



Felspar is the most abundant mineral of the first generation. 

 It is rather tabular in habit, parallel to (010) and is perfectly 

 fresh, and, like the phenocrysts in the previously described 

 andesites, has exceedingly abundant inclusions. Zoning is 

 strongly marked by differences of refractive index, and in all 

 cases there is a narrow peripheral zone wliose refractive index 

 exhibits a very considerable and very sudden drop from that of 



