3io A NATURALIST IN THE PACIFIC chap. 



Quartz porphyries, akin to the rhyolites, are especially charac- 

 teristic of the north-east part of the island, to which in fact they 

 are entirely confined. They perhaps are best represented in the 

 vicinity of Mount Thuku and in the neighbourhood of the mouth 

 of the Wainikoro River. None of my specimens have the fresh 

 appearance of the Lipari rhyolites and all are more or less altered. 

 Their specific gravity does not exceed 2*4, and they are for the 

 most part intrusive in character. 



A rock frequently exposed between Tawaki and Mount Thuku 1 

 contains abundant phenocrysts of glassy felspar (oligoclase and 

 sanidine) and quartz in a greenish opaque groundmass having a 

 blurred microfelsitic structure. There appears to have been a 

 secondary devitrification of the groundmass since consolidation. 

 The porphyritic quartz crystals are rounded and about 2 mm. in 

 diameter. 



Another rock displayed in the coast-cliffs on the north side of 

 Natewa Bay, a mile east of Mount Thuku, has a somewhat banded 

 appearance. It shows crystals of quartz, more or less rounded and 

 3 to 4 mm. in diameter, together with phenocrysts of glassy felspar 

 (oligoclase with lamellar extinction of 5 and sanidine). The 

 groundmass displays traces of spherulites and is in places semi- 

 isotropic ; but for the most part it is microfelsitic. 



The type of rock found in the Wainikoro district and in the 

 adjacent sea-border, where it may be observed forming dykes in 

 ^he pumice-tuffs, is light-grey and loose-textured with a specific 

 ^gravity of 2T. It exhibits small phenocrysts of quartz and of 

 ■glassy felspar (oligoclase 5° to 12 , and sanidine), with, in one 

 locality only, a scanty amount of dark green hornblende yielding 

 •extinctions up to 20 . The quartz crystals, which are 1 to 2 mm. 

 in size, are sometimes bipyramidal ; but are often rounded and 

 have fused-like outer surfaces. The groundmass is semi-isotropic 

 with a blurred aspect, and shows traces of spherulites and numerous 

 crystallites, with occasional felspar-lathes giving a nearly straight 

 extinction. 



An extensively altered quartz porphyry of a different type is 

 associated with other altered rocks at the base of Mount Nailotha. 

 It has a specific gravity of 2*54, and displays large opaque 

 crystals of plagioclase, 2 to 5 mm. in size with small quartz crystals 

 1 to 2 mm. across, in a grey compact matrix. The first-named 

 shows the felspar to be mostly replaced by alteration products ; 

 but occasionally a lamellar extinction of 6° or f can be observed. 



1 See p. 230. 



