534 THE CONTINENTAL ORIGIN OF FIJI, II., 



Carlsbad law. The extinction angle, in a very large number of 

 measurements, never exceeds 7*^ from the length. This indicates 

 that the felspar is oligoclase, just on the boundary between albite 

 and oligoclase. 



Augite of the second generation is also abundant. It is 

 jrreenish in colour and is darker than that which occurs in the 

 less basic rocks hitherto described, and possesses apparently a 

 rather higher refractive index, and stronger double refraction. 

 These augites are distinctly prismatic. In addition, there are 

 light green to colourless granules with high refractive index and 

 strong double refraction, the latter apparentlj^ stronger than that 

 of the augite. These are suggestive of olivine,* but are of too 

 minute dimensions to be at all positive. Magnetite in small 

 idiomorphic grains, and apatite in very slender crystals are also 

 abundant. 



Besides these constituents, there are very numerous hair-like 

 microlites which do not appear to penetrate any of the other 

 minerals, and are therefore probably the last minerals to 

 crystallise. 



Felspar cr3'stals of the first generation are comparatively very 

 large and abundant. They are perfectly clear and colourless, and 

 free from decomposition products. Zones of growth are indicated 

 in ordinary light by marked increase of refractive index from the 

 periphery towards the centre. Both the principal cleavages are 

 strongl}^ developed, the cracks being numerous and sharpl}^ defined. 

 The refractive index for each zone is well above that of Canada 

 balsam. Like the porphyritic felspars in all the volcanic rocks 

 before described, they are simply packed with interpositions, 

 zonally arranged. These phenocrysts have suffered considerabl}^ 

 from corrosion, in many cases being reduced to rounded grains. 

 On the outside of these grains there has been subsequently added, 

 in optical continuity with the central mass, a la3'er of felspar of 

 much more acid composition, which has partialh^ or completely 

 built up the original crystalline shape of the mineral. 



* Kosenbusch states that two generations of olivine are very lare. 



