Igneous Rocks. 27:^ 



Section No. H30, xenolith in tuff, one-third of a mile above 

 Wade's Look-out. — A leucocratic, porphyritic rock, consisting of 

 phenocrysts of quartz, microperthitic, ortboclase, acid plagioclase 

 and niuscovite in a cryptocrystalline ground mass composed of the 

 same minerals. Orthoclase is in excess of plagioclase. The pheno- 

 crysts of niuscovite usually show included needles and sagenitic 

 M'ebs of rutile, and occasionally include a little epidote. They un- 

 doubtedly replace original biotite. Quartz grains are plentiful. 

 They remain clear, although often greatly corroded. Leucoxene is 

 fairly abundant, and one or two twinned sections of epidote are 

 also present. The rock may be described as a quartz porphyry or 

 rhyolite. The presence of the microperthitic orthoclase, so charac- 

 teristic of the rhyolites, is interesting. 



Section No. H15, xenolith in tuff, one-third of a mile above 

 Wade's Look-out. — The rock is an altered quartz porphyry consist- 

 ing of large phenocrysts of quartz, highly sericitised and kaolinised 

 felspar, and occasional biotite crystals, in a microcrystalline 

 ground mass. The idiomorphic outlines of the felspar and the 

 absence of twinning suggest orthoclase. A little yellowish-brown 

 opal occurs in places, and brown iron oxides are rather common,, 

 showing that the rock is much weathered. 



Track to Malory's falls. — Tuffs are well developed near the 

 northern foot of the Blacks' Spur, and they can be seen at 

 numerous points along the track from near Lindt's Hermitage ta 

 Malory's falls. They are generally light coloured, white or grey, 

 but are occasionally stained with hydrated oxides of iron. Certain 

 of the tuffs still remain loose and cavernous, but the majority have 

 been secondarily silicified and rendered more compact. Cubes of 

 pyrite are common. The lapilli present in the tuffs are usually 

 small, being rarely more than half an inch in diameter. Examined 

 microscopically, these tuffs are seen to consist chiefly of fragments 

 of rhyolitic rocks. Lapilli of sedimentary rocks are very rarely 

 present. Especially characteristic, in these pyroclastics, is the 

 microperthitic orthoclase so abundant in the rhyolite lavas. 



Specimen No. 114, track to Malory's falls.— Numerous fragments 

 of banded rhyolite are seen in hand specimens of this rock. 

 Crystals of quartz and altered orthoclase are also visible mega- 

 scopically. Examined microscopically phenocrysts of altered micro- 

 perthitic orthoclase, acid plagioclase, and colourless quartz, are 

 immediately recognised. Chloritised biotite occurs in small 

 amount. The matrix of the rock appears to consist largely of 

 microcrystalline silica and sericitised felspar. That secondary 



