Vol. 55.] EHYOLITES OF THE HAUEAKI G0LDFIELD8. 461 



an irregular fluxion-banding. These bands are occasionally seen to 

 sweep round the porphyritic crystals, a circumstance which tends to 

 confirm the view that the rock is a glassy rhyolite; otherwise 

 it might here and there be mistaken for a pumice-tufF. Portions of 

 the section are, however, quite free from these streaks, and in such 

 parts irregular perlitic cracks frequently appear and often pass into 

 the porphyritic crystals, especially the quartz-crystals (PI. XXXIII, 

 fig. 3), in a manner closely resembling that described by Prof. Watts 

 in his paper 'On the Occurrence of Perlitic Cracks in Quartz,'^ from 

 a porphyritic pitchstone (or obsidian) occurring at Sandy Braes, 

 1| mile north-east of Tardree Mountain. The specimens described 

 by Prof. Watts were pitch-black, and exhibited a brightly vitreous 

 or occasionally resinous lustre. Herein the Irish rock differs con- 

 siderably in aspect from that now under consideration : for the 

 latter is more nearly white than black, and presents scarcely any trace 

 of vitreous lustre. That it has been deprived of this by merely an 

 incipient devitrification is evident, for although the groundmass of the 

 section appears to be isotropic when viewed between crossed nicols, 

 one can, on carefully excluding all extraneous light, perceive a liazi- 

 ness or milkiness pervading those parts of the section where fluxion- 

 banding is most marked. When viewed in ordinary transmitted 

 light, the character of this devitrification is not very clearly dis- 

 cernible, but under an amplification of 400 diameters it appears 

 to be microfelsitic ; in other words, the devitrification is due to the 

 development of fibres, scales, and some longulites and globulites. 

 The more strongly-marked fluxion-streaks can be seen to be a brown 

 glass, apparently more or less made up of globulites and forming small 

 and irregular strings, which sometimes show feeble double refraction 

 when lying at 45° to the principal sections of the crossed nicols. 

 It is interesting, in connexion with the occurrence of opal in this 

 rock, that Prof. Watts mentions the not unfrequent deposition 

 of opal in a greyish-green variety of rhyolite from Connor, Sandy 

 Braes. ^ It may be that heated water, charged with alkaline 

 silicates, accounts to some extent for the development of perlicity 

 in quartz; and, considering the prevalence of hot springs in New 

 Zealand, it seems by no means unlikely that they may have had 

 some influence in the devitrification of glassy rocks in their vicinity. 

 The ' quartz-blows ' mentioned by Mr. Park,^ which are alluded to 

 on p. 463, may be due to a like cause. 



Besides quartz, the other porphyritic crystals in the rock here 

 described appear to be chiefly oligoclase, andesine, and labradorite. 

 Pyrites is also present in small quantity. One or two specks of this 

 mineral occur in a porphyritic crystal which has a reddish-yellow 

 colour in transmitted light, but in reflected light appears white, 

 and is probably kaolinized felspar. 



H^g. "W ai h i. — A pale yellowish-grey lithoidal rock, with numerous 

 dark micaceous-looking crystals, mostly about -^ inch in diameter. 



1 Quart. Journ. Geol. Soc. vol. 1 (1894) p. 367. ^ xbid. p. 368 



3 ' The Geology & Veins of the Hauraki Goldfields,' 1897, p. 101. 



