Vol. 55.] RHYOLITES OF THE HATJRAKI GOLDFIELDS. 465 



II3. [194]. Waitekauri. — 1 pale grey rock with dark green 

 or blackish crj^stais, resembling 192, but of somewhat paler colour. 



Under the microscope this rock shows a microcrystalline ground- 

 mass with some brown devitrified glass. jSTumerous porphyritic 

 crystals of oligoclase and andesine are present, often with well- 

 marked zonal structure. Pseudomorphs of hoematite after hornblende 

 also occur porphyritically. There are, moreover, a very few small 

 crystals of biotite, sometimes only represented by pseudomorphs. 

 Small cubes and grains of pyrites occur in this section, while 

 here and there irregular vesicles may be seen, filled with chlorite 

 in fan-like groups of crystals. This rock, like the preceding, is a 

 hornblende-andesite. 



Hgg [195]. Waitekauri. — A rather dark-grey, compact rock, 

 with numerous blackish-green spots and a few very small dark 

 crystals which have a brilliant lustre. 



The section shows a microcrystalline groundmass with numerous 

 porphyritic crystals, mostly of labradorite, porphyritic crystals of 

 altered pyroxene, and specks of pyrites ; some of the pyroxene may 

 be rhombic, but the alteration does not admit of a definite conclusion 

 upon this point. The rock may be regarded as an altered 

 augite-andesite. 



Hg^ [196]. "Waitekauri. — A rather dark-grey compact rock 

 with numerous small, lath-shaped, colourless crj'stals, having a 

 vitreous lustre, and some small dark green or blackish crystals. 



The section shows a crystalline granular groundmass, with 

 many felspar-microlites. In this occur porphyritic crystals of 

 felspar and hornblende, the former somewhat corroded. From their 

 extinction-angles they may in some cases be referred to labradorite, 

 in others to andesine. The hornblende-crystals are mostly frag- 

 mentary. Magnetite and some small specks of pyrites are present. 

 The rock is a hornblende-andesite. 



Summary. 



Eeviewing the rhyolites of the Hauraki Goldfields, they are 

 found to present many points of interest, and to supply examples of 

 almost every structural peculiarity known among rocks of this class. 



Perlicity appears to be of rare occurrence, even in the obsidians, 

 but although many of the lithoidal rhyolites possibly assumed that 

 character on solidification, there are others which doubtless became 

 lithoidal through subsequent devitrification and, in a large number, 

 comparatively unaltered or quite fresh vitreous matter is still 

 present. 



Eeheating of already solidified lavas has been, as we have fre- 

 quently had occasion to remark, a by no means uncommon feature 

 in the history of these rhyolites, and in some cases the temperature 

 has been sufficient to render felspars isotropic : in other words, to 

 reduce them to the condition of felspar-glass.^ 



^ Descriptions of somewhat analogous cases may be found in M. A. Lacroix's 

 work, ' Les Enclaves des Roches Volcauiques,' ISOS. 



Q. J. G. S. No. 219. 2 h 



