BY W. G. WOOLNOUGH. 531 



the next zone inwards. Twinning after the albite and Carlsbad 

 laws is practically universal, and lamellne after the iDericline law 

 are by no means infrequent. Measurements of extinction angles 

 indicate that the outer zone of felspar is totally different in com- 

 position from the inner portion, and it is, therefore, in all pro- 

 bability of secondary origin. In the zone perpendicular to 

 (010) the central portions give a maximum symmetrical 

 extinction of 28^, while sections parallel to (010) give -15° 

 to- 19°, thus indicating labradorite. The secondary rim gives 

 extinctions not sensibly varying from 0° in all sections, while its 

 refractive index, is almost exactly that of the Canada balsam. It 

 is therefore oligoclase, that is, is identical in composition with the 

 microlites of the base. 



The inclusions in the felspar do not differ essentially from 

 those described for the other andesites, except in the fact that the 

 zone of dusty inclusions, which I take to be really solution-cavities, 

 is noticeably narrower than in the other rocks. This, combined 

 with the fact that the zone of secondary felspar is comparatively 

 wide, indicates that the phenocrysts have suffered less corrosion 

 before secondary addition of felspar began to take place than the 

 corresponding crystals in the Buki Levu rock for instance. 



As we should expect, in addition to augite and magnetite, we 

 have, included in the felspar, grains of pleochroic hornblende. 



Next in abundance to felspar is hornblende, occurring in per- 

 fectly idiomorphic crystals. In the prism zone these are bounded 

 by I 110 ]> and ■{ 010 j- , the latter less developed than is usually 

 the case, so that cross sections appear almost rhombic. Alon<:>' 

 the vertical axis the crystals are terminated by what appear to 

 be pyramid planes. The colours in ordinar}- light var}^ between 

 dark clove-brown and greenish-brown. In all cases there is a 

 very strongly marked resorption rim rendered opaque by the 

 abundance of secondary magnetite. In some cases the resorption 

 rim is narrow and sharply defined, but a complete series of 

 structures from this to a pseudomorph of magnetite is met with. 



The ordinary prismatic cleavages are strongly developed, and, 

 in addition, there are indications of another slightly irregular 



