MoEGAif. — Tlte Igneous Rucks of tJtc WaiJii (J old field. 273 



Porphyritic grains of quartz and feldspar are easily detected in the 

 unweathered wilsonite. Sections examined under the microscope show 

 that the feldspar is mainly oligoclase and oligoclase-andesine. Some of the 

 more basic feldspars may be andesine, or even acid labradorite. A little 

 orthoclase is probably present. Small fragments of ferro-magnesian minerals 

 also appear. 



The greater part of the sections consists of a glassy base nearly, but not 

 quite, isotropic. The bulk of the glassy matter shows a streaky flow struc- 

 ture, but there are also small patches of brownish glass not affected by 

 flow. The dark streaks of the hand-specimen are plainly visible, and from 

 the manner in which they pass into the lighter portions the view that the 

 rock is a lava and not a tuff receives emphatic support. 



(c.) Tridymite-rhyolites. 



The tridymite-rhyolites in the neighbourhood of Waihi appear as a 

 number of small flows that at several points may be seen breaking through 

 wilsonite. They are greyish-white rocks of even grain, but somewhat harsh 

 texture. Fluxion structure is usually more or less apparent to the eye, 

 and on close examination small glassy feldspars may be detected. Frag- 

 ments of andesite, and possibly other rocks, are commonly noticeable. In 

 a flow on the east side of the Martha Hill charcoal has been observed at two 

 points — namely, in the Grand Junction A shaft, at a depth of 110 ft. to 

 112 ft. from the surface ; and in the chamber at No. 3 level adjoining No. 4 

 shaft, Waihi Mine (depth below shaft-collar, about 195 ft.). 



Sections of the tridymite - rhyolites from various localities show that 

 they are composed mainly of a glassy base with a fine corrugated flow struc- 

 ture. The scattered phenocrysts are mostly plagioclase (oligoclase or 

 andesine). Rutley has reported sanidine (5, p. 460). Small broken crystals 

 of green hornblende, augite, and hypersthene (Sollas), probably zenoliths, 

 are occasionally observable. An interesting point is that most of the sec- 

 tions show nests of tridymite, which apparently takes the place of free 

 quartz. Slightly decomposed or altered specimens^ — for example, the 

 charcoal-bearing rock from the Grand Junction shaft — do not show any 

 tridymite, but secondary quartz may be observed. Magnetite and zircon 

 (Sollas) are present in some sections. 



The tridymite-rhyolites near Waihi have been minutely described by 

 Rutley (5, pp. 457-60, Nos. Hg-H.J and Sollas (11, vol. 2, pp. 14-15 and 

 66-67). 



(5.) Andesitic RocJcs of Doubtful Age. 



Under this heading are included some of the younger andesitic rocks 

 that were not suflSciently studied to enable the determination of their relative 

 ages with reference to one another and to the rhyolites to be made. The 

 rocks thus occurring are hornblende-andesites of a variable character. 



The most notable occurrence of such rocks is the andesite forming the 

 Black Hill, east of Waihi. In places it shows well-marked though not 

 perfect columnar structure. The rock is not very uniform in appearance, 

 but in the main is a dark-grey hornblende-andesite, sections of which, 

 besides abundant hornblende, show a little augite, and in places biotite. 

 The feldspar phenocrysts are probably oligoclase-andesine. A little mag- 

 netite and apatite in fine needles are present. A nearly isotropic mineral 

 of very low refractive index was not determined. The base contains nume- 

 rous feldspar microlites, and with a ^ in. objective exhibits a kind of feathery 

 appearance under crossed nicols. 



