134 A\ R. Jimner: 



usually sub-angular in shape, and larelv more than three inches 

 in diameter. In the absence of field evidence to the contrary, it is 

 possible, but not probable, that these supposed volcanic l)reccias and 

 agglomerates have originated Ijy faulting. The rock is intrusive, 

 and the flow structure, which is characteristic of similar rocks 

 from Liljdale, Steel's Creek, and north of the Black Spur, is absent 

 in this case. Probably the rock fills what was once a volcanic con- 

 duit extruding rhyolitic lavas, which are now entirely denuded aAvay,^ 

 Hand-specimens of the rock are leucocratic and porphyritic. and 

 show phenocrysts of quartz and altered felspar. A little pyrite is 

 also present. Examined microscopically, the rock is seen to have 

 been a quartz-felspar-porphyry, but its pristine characters are now 

 greatly masked l)y extensive sericitisation, carbonation and occa- 

 sional secondary silicification. 



Section No. I':i2. 'J'he rock consisted originally of large clear 

 panidiomorphic phenocrysts of quartz, fairly large euhedral crystals 

 of felspar, and numerous crystals ot a femic mineral, probal)ly 

 biotite. in a microcrystalline ground-mass of the same minerals. 

 Apatite and zircon are accessories. As in the similar rocks 

 described from Diamond Creek, quartz is the only minei-al which 

 has withstood the action of the hydrothermal solutions which have 

 caused the alteration, to any mai-ked extent. It is generally clear. 

 a)id is often rounded and embayed by the ground mass. Such coi- 

 rosion, however, is pr()bal)ly magmatic, and occurs in many volcanic 

 and intrusive rocks not hydrothermally altered — e.g., Healesville 

 dacites. Isolated inclusions of the ground mass, and numerous 

 glass inclusions containing liquid bubbles, are present in the quai'tz 

 phenocrysts. Cracks occur about some of these glass inclusions 

 exactly as figured in Idding's " Rock Minerals," fig. 37b, p. 71. 

 Pyiamidally tern\inated crystals of zircon are also seen in the grains 

 of quartz. The felspar phenocrysts have been extensively metaso- 

 matically replaced by dusty carbonates and microscopic flakes of 

 .sericite. with a little quartz. The alteration of the felspar has 

 been so great in many sections that it can only be recognised by 

 its well-preserved idiomorphic outline. Where the alteration has not 

 been extieme, it can be generally proved without doubt that the 

 felsi)ar is predominantly orthoclase. Numerous included crystals of 

 apatite, zircon, and muscovite occur in the felspar. The original 

 femic mineral was probably biotite. It is now replaced by mus- 

 covite, and rutile has separated out,i however, not in the form of 



1 Biotite frequeiitl.v contains a considerable percentajfe ot TiOj. A hiot.ite from tlie l)an- 

 ilenons: (iacite, analysed by H. C. Kic-liards, contained TO.'i", Ti02 



