51 G THE COXTINEXTAL ORIGIN OF FIJI, II., 



The peculiarity in these felspars, which immediately attracts 

 the attention, is the enormous abundance of inclusions, and their 

 marked zonal arrangement. The actual nature of these inter- 

 positions will be considered later. Some sections contain none 

 of them. In practically eveiy instance the "inclusion zone " is 

 bounded peripherall}^ by a perfectly limpid ring. The shape of the 

 " inclusion zone " indicates that the original crystal lost its sharp 

 edges owing to corrosion, and the secondary felspathic material 

 of rather more acid composition was added peripherally in optical 

 continuity with the nucleus, reproducing the original crystal 

 edges. These crystals have a refractive index higher than that 

 of Canada balsam. The zones are remarkable, in that a more 

 basic zone is usually interposed between the nucleus and the 

 peripheral zone, sharply marked off from both by its higher 

 refractive index and different extinction angle. In some cases 

 the recurrence of several basic zones is to be noticed. 



Sections in the zone perpendicular to (010) are apparently 

 free from inclusions, and do not as a rule show much zoning, 

 especially those with the maximum extinction angle. The maximum 

 extinction angle obtained in a ver}^ large number of readings was 

 37°. The greater number of readings lay between 33'-' and 37*^. 



In sections of this zone twinning after the albite and Carlsbad 

 laws is almost universal. One section, w^hich does not exhibit 

 Carlsbad twinning, shov/s that after another law. The plane of 

 compo-sition cuts diagonall}^ across that of the albite lamellae. I 

 take this to be the Baveno law. 



Sections parallel to (010) are very strongly zoned. The 

 cleavages parallel to (001) are sharpl}^ defined, while those 

 parallel to the prism face are in some cases marked b}' rough 

 cracks. The zoning, indicating the presence of (001), (lOl), 

 (201), allows the orientation of the section. It is thus found 

 that the extinction angle varies from -17° to - 28° for the 

 different zones. The kernel is in all cases the most basic part, and 

 usually occupies from one-third to one-half the area of the section. 

 The most basic variety of felspar is thus Bytownite. This deter- 

 mination is in accord with the fact that the refracti\e index is fairly 



