Vol. 55.] RHrOLITES OF THE HATJEAKI GOLDEIELDS. 451 



forest producing, when viewed from a distance, a very pleasing and' 

 picturesque effect. 



Up to the present time no direct evidence has been found in the 

 Hauraki area to fix the exact age of the rhyolites, which are found 

 resting indifferently on the gold-bearing andesites, of probably 

 Lower Miocene age, and on some volcanic tuffs and breccias to 

 which an Upper Miocene age has been ascribed by Sir James Hector, 

 Capt. F. W. Hutton, and other Xew Zealand gc3ologists. The rhyolites 

 are followed by Pleistocene and recent deposits, but, as already stated, 

 they appear to be continuous in extension, and contemporaneous with 

 the rhyolites of the King Country and Wanganui regions, which 

 have been shown by the Author, from their association with the 

 younger marine Tertiaries there, to range from older to newer 

 Pliocene.^ 



Part II. — Ihs Microscopic Characters of the Rhyolites. 



Ey FrAKK E.UTLEY. 



[Plates XXXII-XXXIV.] 



The specimens were collected by Mr. James Park, F.G.S., and are 

 numbered consecutively, but their sequence indicates nothing more 

 than the order in which they were examined. In some cases the 

 specimens, when received, bore numbers, which are placed ia 

 brackets. 



H^. Omahu. — This is a pale lithoidal rock, consisting of parallel 

 alternating pink or pinkish-grey and very delicate white bands, 

 which are seen under the microscope to consist almost exclusively 

 of spherulites. The latter vary from yV inch in some bands to 

 only -g^Q- inch in diameter in others. These spherulites give a 

 negative sign when tested with the teinte sensible No. 2. The 

 edges of the spherulitic bands often consist of divergent fibres, the 

 divergence proceeding from points on the margins of the bands. 

 Viewed under a fairly high power, the fibres constituting the 

 spherulites appear to be margarites. The spherulitic bands are 

 sometimes separated by nearly isotropic bands, in which some very 

 slightly altered vitreous matter appears still to exist, while in 

 places irregular anisotropic patches and a hazy spherulitic structure 

 can occasionally be discerned. Small aggregates of quartz and 

 felspar sometimes occur in the spherulitic bands, the former mineral 

 being clearly recognizable in certain sections by its positive, uniaxial 

 interference-figure. The felspar, when examined by Becke's 

 method, is seen to have a lower refraction than Canada balsam, 

 while its extinction-angle, 21° with c in a section parallel to (010), 

 clearly shows that it is sanidine. Very diminutiv^e cubes of pyrites 

 occur here and there in the section.^ (PI. XXXIV, fig. 5.) 



^ N. Z. Geol. Surv. Reports : Explorations, 1886 [1887]. 

 ^ An analysis of this rock, made by Mr. Philip Holland, is tabulated on 

 p. 467. 



2g2 



