Vol. 55.] EHTOLITES OF THE HATJRAKI GOLDFIELDS. 459 



several porphyritic felspar-crystals, which, except along cracks and 

 in a few irregular spots, are completely isotroxjic: they have, in fact, 

 been fused and converted into felspar-glass. The fragment of 

 spherulitic rhyolite in which these fused felspars lie has been less 

 affected by the action of heat, being merely fused at the surface, 

 from which a few small streamers or threads of brown glass are 

 seen to emanate and to become mixed with the fluxion-streaks of 

 the surrounding colourless rock (PI. XXXI V, fig. 4). Other included 

 rock-fragments are in some cases lithoidal, but the majority are 

 vitreous and brown. The rock in which the fragments and crystals 

 already mentioned lie is essentially an obsidian with well-defined, 

 wavy, corrugated fluxion-structure, but sufficiently devitrified by 

 globulites to have a thoroughly lithoidal aspect in the hand-specimen. 

 It is only by the darkness of the thin section between crossed nicols 

 that the slightness of the devitrification can be realized. Close 

 examination then shows that the section is not uniformly dark, but 

 that it exhibits a nebulous milky haziness in those parts where 

 cumulites occur and in those streaks in which globulites are crowded. 

 The section is interesting when compared with the specimen from 

 which it was cut, as showing what slight globulitic devitrification 

 suffices to impart a lithoidal aspect to a glassy rock. This rock is 

 not a rhyolite-tuff, but it might be termed a tufaceous rhyolite 

 or devitrified obsidian. 



Hj., [182]. Waihi. — A pale grey lithoidal rhyolite, showing very 

 small lath-like felspars with a vitreous lustre. 



In thin section under the microscope it is seen to be a devitri- 

 fied rock with a corrugated or damascened fluxion-structure. It 

 contains porphyritic felspars, often showing zonal structure. From 

 their extinction-angles they appear in some cases to be oligoclase, 

 in others andesine. Examined by Becke's method, no felspar 

 with a refraction less than that of Canada balsam seems to be 

 present among the porphyritic crystals. Some crystals and grains 

 of pyrites are visible. 



The rock much resembles that from which the previously-described 

 section H,^ [181] was made, but it has undergone much greater 

 devitrification by globulites. The darkness of the section, however, 

 between crossed nicols indicates that a large amount of unaltered 

 vitreous matter yet remains. 



H^3 [187]. Adit-level, Waihi. — A pale pinkish-white to 

 cream-coloured lithoidal rock, with rather numerous dark specks. 



Under the microscope the section shows a very delicate spherulitic 

 structure, which in places assumes an axiolitic character. This is 

 best seen in ordinary transmitted light. Between crossed nicols it 

 becomes very obscure. Occasionally a few bright spicules or 

 crystalline rods of a microlitic nature may be discerned among the 

 radiating isotropic fibres of the spherulites ; and that, like the fibres, 

 they have an approximately radial disposition, may now and then be 

 made out. These rods api>ear to give straight extinctions, but they 



