xxi QUARTZ PORPHYRIES 309 



But it also contains a number of scattered larger felspar-lathes 

 averaging '2 mm. in length and giving extinctions of 5 when 

 simple, and of 8° to io° when lamellar. There is also some small 

 prismatic augite in the groundmass but often decomposing. The 

 original interstitial glass is represented by numerous reddish-brown 

 patches of devitrified glass. 



In the second type of these trachytes, the rock is more open in 

 texture and is at times vesicular, the specific gravity being usually 

 less than 2*4. The general characters are much the same, but 

 sanidine is better represented among the phenocrysts, and the 

 groundmass is more blurred ; but when the felspar-lathes are 

 distinct they give an extinction either nearly straight or from 4 to 

 8°, according as they are simple or display lamellae. The augite of 

 the groundmass is scanty and more or less decomposed ; whilst 

 the interstitial glass when unaltered is in fair quantity and nearly 

 isotropic. 



The alteration observed in several of these oligoclase-trachytes 

 is restricted chiefly to the interstitial glass in which secondary 

 quartz and at times calcite and viridite are developed. Scarcely 

 any of them are quite free from these changes. 1 



The pitchstone or vitreous form of these trachytes is displayed 

 in the blocks of an agglomerate-tuff between Tawaki and Mount 

 Thuku. It has a specific gravity of 2*36, is dark-brown, and has. 

 a conchoidal fracture. Phenocrysts of felspar, mostly oligoclase,, 

 with extinction-angles of 5 to n°, and often penetrated by the 

 magma, are inclosed in a semi-isotropic groundmass showing 

 incipient development of felspar and other darker microliths. 

 There are also a few small phenocrysts of pale augite. 



Quartz Porphyries and Rhyolitic Rocks 



Wichmann when he wrote in 1882 that no quartz-bearing 

 younger eruptive rocks had hitherto been observed either in Fiji or 

 in the South Sea Islands generally, had apparently overlooked 

 Dana's observations in the Fijian group. The American geologist 2 

 refers to a rock found on the north-east shores of Vanua Levu 

 which exhibited in a greenish base thickly disseminated crystals of 

 quartz (bipyramidal dodecahedrons, £ of an inch in diameter) and 

 glassy felspar, together with a few sphene crystals. 



1 Highly altered rocks of this class are exposed at the base of Mount. 

 Nailotha as described on p. 215. 



2 See work quoted on p. 218. 



