538 THE CONTINENTAL ORIGIN OF FIJI, II., 



are met with. The extinction on the clinopinacoid is 52° for- 

 ward from the trace of the vertical axis. Sections nearly at right 

 angles to an optic axis give an hyperbolic brush with very little 

 curvature, showing that the optic axial angle is large. These 

 facts all indicate augite rich in iron. 



The augite is not decomposed to any notable extent, but 

 has suffered somewhat from corrosion by the magma. The 

 resulting rim is rendered dark by development of magnetite, 

 Man}' of the crystals are fractured by movement due to the flow 

 of the rock. In one or two cases solution has gone on along 

 planes parallel to the pyramid faces, producing relatively large 

 cavities filled w4th, and often visibly connected with, the ground- 

 mass. 



In addition to these pseudo-inclusions, there are numerous true 

 interpositions. The chief of these is olivine in relatively large 

 grains, and magnetite in more or less idiomorphic forms. 



Olivine is perfectly idiomorphic, the shape of the sections indi- 

 cating that none of the pinacoid faces are very strongly developed. 

 The mineral is perfectly colourless, its refractive index and double 

 refraction calling for no special remark. The cleavages are, 

 however, very much more strongly developed than usual; in fact 

 they are in certain sections quite perfect. All three pinacoidal 

 cleavages seem to be present. This may indicate that the olivine 

 is a ferriferous variety. This appears to be borne out by the fact 

 that relatively little corrosion has taken place. The optic axial 

 angle is large, judging by the slight curvature of the hyperbolic 

 brush in convergent polarised light. The crystals are slightly 

 corroded in parts, "bays" of groundmass being produced. The 

 only inclusion of note is magnetite; as noted above, the olivine 

 is older in crystallisation than augite. The cracks which traverse 

 the crystals fairly abundantly are not so irregular as usual, but 

 show a decided tendency to spheroidal arrangement strikingly 

 similar to perlitic structure in glassy rocks. The mineral is 

 often grouped in the form of nests of crystals, or occurs in 

 isolated sections. As a rule the positions of the intergrown 

 individuals do not seem to obey any fixed law, but in one case 



