xxi QUARTZ PORPHYRIES 311 



The quartz crystals are rounded and penetrated by the magma, 

 and contain numerous strings of fluid-cavities. The groundmass 

 was originally spherulitic; but this structure is more or less 

 disguised by the development of a mosaic of chalcedonic quartz. 

 It shows some micro-porphyritic patches of viridite and calcite. 

 A singular altered white rhyolitic rock is exposed on the north 

 coast of Natewa Bay between Natasa and Sangani, where it is 

 associated with altered tuffs. It is compact with a conchoidal 

 fracture and has a specific gravity of 2'48. The hand-specimen 

 has a banded appearance. Under the microscope it appears as a 

 rhyolitic glass for the most part devitrified and rendered opaque 

 by the formation of secondary silica. Much of it presents a micro- 

 felsitic structure, the bands appearing as semi-opaque streaks. 



Glassy forms of the quartz porphyries or intrusive rhyolitic 

 rocks are extensively represented in the pumice -tuffs of the Undu 

 Promontory and of the coasts between the Langa-langa river and 

 Lambasa. These tuffs will be found described on page 336. 

 Fragments of a grey rhyolitic glass looking like perlite are inclosed 

 in the pumice-tuffs near the mouth of the Wainikoro River. Under 

 the microscope it is displayed as a colourless glass inclosing 

 phenocrysts of sanidine, oligoclase (ext. 4), and quartz, the last 

 with rounded outlines and a fused-like outer surface. The glass 

 shows in places perlitic cracks ; but it is mainly characterised by a 

 vacuolar structure, the minute cavities being lengthened out in the 

 direction of the flow and displaying eddy-currents around the 

 phenocrysts. The elongated steam-cavities sometimes contain 

 water, but are usually more or less filled with granular materials. 



