Yo]. 55.] RHYOLITES OF THE HIUEAKI GOLDPIELDS. 463 



the felspars from Waihi have undergone a dry fusion, while these 

 in the rock from Mataura have probably been subjected to the 

 action of steam. The rock contains minute crj^stals of magnetite, 

 apparently a considerable amount of tridymite, and possibly some 

 pyrites. 



H^g. Waihi. — A rocktao hard to be scratched by steel, and con- 

 sisting of rather dark and lighter bands of bluish-grey, chert-like 

 material. The specimen has a subconchoidal fracture, and is 

 translucent on thin edges. The cut surface, from which the thin 

 section was sawn, has taken a brilliant polish, more so, indeed, than 

 that of the silicified rhyolite from Mataura, last described. This 

 specimen is from one of the ' quartz-blows ' described by Mr. James 

 Park as ' covering an area of several acres/ aad often standing 

 6 or 8 feet in height.^ 



Under the microscope the section is seen to consist essentially of 

 chalcedony, with some opaque white grains which may be kaolin, 

 and fragments af a brown, vesicular, glassy lava (PI. XXXIII, fig. 4), 

 There are also numerous long, thin, brown, hair-like streaks in 

 the section. At first one might be disposed to regard them as very 

 irregular cracks in the chalcedony, stained, or occasionally filled, 

 with minute granules of limonite, but their lash-like character 

 and the apparent absence of any approximate parallelism or lines 

 intersecting at right angles seem to negative the supposition that 

 they are cracks. The question is whether they represent any 

 organic structure. Even what, in the absence of more precise and 

 special knowledge, have here been provisionally termed fragments 

 of brown, vesicular, glassy lava, might possibly, in the eyes of a 

 palaeontologist, be recognized as the chalcedouic vestiges of some 

 organism. They have been compared with sections of the vesicular 

 glassy basalt-lavas or tachylyte of the Sandwich Islands, but in the 

 latter the vesicles are usually much larger, while the olivine- 

 crystallites, so prevalent in the Honolulu rocks, are absent in these 

 fragments. Whether the material of this so-called ' quartz-blow ' 

 was brought from below and deposited from heated water charged 

 with silica (and the section in pi. xiv of Mr. Park's ' Geology & 

 Veins of the Hauraki Goldfields ' seems to indicate such a possi- 

 bility) there is no distinct evidence to prove, and it appears 

 safer, so far, to refrain from any such conjectures. Whatever 

 interpretation may be put on the included fragments, their general 

 appearance is fairly indicated in PI. XXXIII, fig. 4. 



H^g. Waihi. — A bluish-grey rock, mottled with deep reddish- 

 brown. The fracture is subconchoidal, and the lustre feebly sub- 

 vitreous. This specimen is also stated to have been derived from 

 a ' quartz-blow.' 



Under the microscope, the thin section appears to consist of 

 fragments of an altered eruptive rock cemented with chalcedony. 

 The fragments of rock are probably those of an andesite, although 

 no ferromagnesian minerals are to be recognized in them, any 



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



