508 THE CONTINENTAL ORIGIN OF FIJI, II., 



fully developed; the}^ are apparent 1}^ opaque. They are for the 

 most part straight, but an occasional geniculate twin indicates 

 that they consist of rutile. 



Quartz is much less abundant than felspar, and is entirely 

 interstitial. The ragged areas and isolated grains are in some 

 cases optically continuous over comparatively wide areas. On 

 the other hand, the larger grains have suffered very considerably 

 from crushing. As is usual, the other minerals show little or no 

 sign microscopically of the crushing of the rock: while quartz, 

 the hardest mineral present, has been very considerably splintered. 

 Usually the effect is the j^roduction of shadowy extinction, or of 

 areas whose positions of extinction are very close; but in some 

 cases the action has proceeded further, and a coarse mosaic has 

 been produced. 



The individualised inclusions in the quartz are similar to those 

 above described for felspar. In addition, fluid-cavities with 

 moving bubbles are very abundant. In the smaller ones the 

 bubbles move spontaneously. These cavities are distributed 

 along roughly parallel planes, with intermediate clear spaces. 

 The directions are fairly constant in each quartz grain, but the 

 planes do not pass from grtiin to grain as is often the case. 



Hornblende is less abundant than either of the above minerals. 

 It is subidiomorphic to allotriomorphic in habit, and has suffered 

 very much from decomposition, passing into a chloritic product. 

 Refractive index and cleavage are quite normal. Double refrac- 

 tion is, perhaps, not quite so strong as usual. In sections parallel 

 to the clinopinacoid the extinction is 18° from the trace of the 

 vertical axis. Where undecomposed the pleochroism is strong : 

 a = light brownish-yellow. 

 b = dark brownisli-bronze. 

 C =^ dark bronze-green. 



Absorption being : ii<b<f. 



Incipient decomposition is marked by a change in the character 

 of the pleochroism. All the colours become more bluish, particu- 

 larly those for the directions of maxinmm and minimum elasticity; 

 C in some cases is quite greenish-blue. Up to this stage the 



