458 MESSRS. J. PAEK AND F. ETJTLEY ON THE [Aug. 1 899, 



with numerous minute colourless crystals, which occasionally are 

 lath-shaped and exhibit a brilliant vitreous lustre. Seen under 

 the microscope, the section shows numerous porphyritic crystals 

 and fragments of crystals of oligoclase and andesine, and in 

 one instance a sharply-defined crystal of labradorite included in 

 another of less basic constitution. The rock has apparently been a 

 glassy lava, but is now completely devitrified by globulites. It 

 contains bands and irregular patches of yellowish-brown matter 

 resembling microfelsite. Numerous nests of tridymite are present, 

 and the section shows a delicate, but not strongly-marked, wavy 

 fluxion-structure and a moderate amount of magnetite-dust. 



H^Q [179]. Waikino, near Waihi. — A pale yellowish-grey or 

 buff- coloured lithoidal rock, containing minute crystals, some of 

 which are colourless and glassy, while others are dark green or 

 black. The section, under the microscope, is seen to contain corroded 

 porphyritic crystals and fragments of oligoclase, and occasionally of 

 labradorite and andesine. There are also a few fragments of crystals 

 of pale greenish hornblende, but the pleochroism is extremely feeble 

 or barely perceptible, and it is mainly on account of the extinction- 

 angle that the crystals cannot be mistaken for pjToxene.^ Unfortu- 

 nately the slide shows no transverse section of these prisms, and thus 

 the angle of intersection of the cleavages cannot be ascertained. The 

 crystals are crossed by transverse fissures, as in actinolite. The rock 

 itself is a lithoidal rhyolite, with rather poorly-defined corrugated 

 fluxion-structure, and containing small nests of tridymite and occa- 

 sional crystals and irregular aggregates of magnetite and pyrites. A 

 few small rock- fragments are present in this lava. Some of them con- 

 tain a large amount of vitreous matter, often with numerous micro- 

 lites and grains of magnetite. It seems probable that these small 

 fragments of rock, as well as the larger fragments of crystals of the 

 more basic plagioclastic felspars and those of hornblende, were 

 derived from andesites. These included fragments may be perhaps 

 regarded as sufficient warrant for terming the rock a tufaceous 

 rhyolite, but they are not very numerous, and probably indicate a 

 mere sprinkling of volcanic dust. 



H^^ [181]. Waihi. — A pale greyish - white lithoidal rock, 

 speckled with minute dark crystals and fragments, and with a very 

 few larger dull white ones. 



Under the microscope, the section is seen to contain porphyritic 

 crystals of felspar, mostly fragmentary and much corroded. These 

 in some instances are oligoclase, in others andesine, and occasionally 

 labradorite. One or two of the more basic felspars are twinned on 

 both the albite- and pericline-types. There are also some fragments 

 of rock in this lava which have undergone more or less fusion on 

 their boundaries. In one instance such a fragment shows, in 

 ordinary transmitted light, the tolerably well-defined forms of 



^ c: f=r about 15°, measured from the cleavage. The mineral appears to be 

 partially altered into a scaly subttanee, possibly talc. . 



