Vol. 56.] ERUPTIVE ROCKS FROM NEW ZEALAND. 497 



and minerals are embedded in rhyolite, the fluxion-banding of 

 which sweeps around them. 



Everything seems to indicate that the rock is a tufaceous 

 rhyolitic lava, which has either taken up fragments of andesite 

 and rhyolite in its lower portion, or else has been erupted during a 

 shower of lapilli upon its surface. The rhyolite of the lava-stream 

 is practically isotropic. 



H 29 . Waitekauri. — A brown semi-vitreous rock, with small dark 

 &nd light crystals or fragments : the groundmass, where weathered, 

 appearing pale-grey, with small white and brown crystals. 



Under the microscope, this is seen to be a rhyolite with a 

 damascened or corrugated fluxion-banding, and containing many 

 crystals and fragments of felspars — chiefly oligoclase and andesine, 

 augite, rhombic pyroxene, olivine, and pyrites. The marked 

 difference between the unweathered and the weathered portions of 

 the hand- specimen is not very clearly seen in the section when it is 

 viewed in ordinary transmitted light ; but in reflected light it is 

 found to be due to a selective alteration (kaolinization) of certain 

 streaks, so that, in the most weathered part of the rock, a very 

 large amount of kaolin is present : while in the comparatively un- 

 weathered part much less opaque white matter is visible, since only 

 ,a limited portion of the fluxion-banding has been kaolinized. This 

 clearly shows that certain bands in a rhyolite may be more readily 

 attacked by atmospheric agency than others, not necessarily because 

 they differ in chemical composition, although some slight differences 

 may exist, as pointed out by Prof. Iddings x ; but rather because it 

 may often be noted that there are differences in grain or texture 

 in the alternating fluxion-bands. It seems only reasonable to infer 

 that the same kind of difference in solubility would exist between 

 such bands as exists in the solubility of any mineral substance 

 when coarsely and finely powdered. 



The rock may be called a tufaceous rhyolite. The crystals 

 and fragments in it have evidently been derived from andesites 

 which occupy a considerable tract north of Waitekauri. 2 



H 30 . Mataura. — A pale-grey rock, with very delicate banding, 

 and showing small colourless scales. It is sufficiently hard to resist 

 the point of a penknife. 



A section of this rock, when examined under the microscope, 

 does not seem to present any distinct characters suggestive of a 

 lava, but rather those of a laminated tuff, composed of exceedingly 

 fine volcanic dust in which a considerable amount of tridymite 

 occurs, mostly in irregularly-segregated plates, forming bands and 

 patches. Where free from tridymite the section is nearly isotropic. 

 It appears to be a much-altered rhyolitic tuff. Microscopically 

 it bears some resemblance to certain earthy volcanic tuffs met with 



1 'Obsidian Cliff' 7th Ann. Kep. U.S. Geol. Surv. 1885-86 [1888] p. 274. 



2 See map of the Hauraki Goldfields, by James Park, Quart. Journ. Geol. 

 Soc. vol. lv (1899) pi. xxxi. 



