Igneous Rocks. 279 



Section No. H77, from Meyer's creek road, soutli-we.st of Mount 

 Jit. Leonard, is typical of these rocks. It consists of phcnocrysts of 

 <}uartz and occasional highly sericitised crystals of orthoclase and 

 plagioclase, in a microcrystalline ground mass of quartz, biotite 

 and altered felspar. The structure of the rock is homogeneous. 

 Radial aggregates of secondary biotite have been developed in 

 association with mosaics of secondary quartz and a little pellucid 

 felspar. 



Section No. H72, from same locality as the preceding specimen. — 

 Microscopically it consists of occasional phenocrysts of pellucid very 

 rounded quartz, and a few panidiomorphic phenocrysts of highly 

 altered felspar in a micromosaic of quartz, felspar, sericite, iron 

 oxides, biotite and pyrite. One rather rounded and broken crystal 

 of slightly pink garnet occurs associated with a little biotite and 

 muscovite. Irregular shaped grains of pyrite are seen replacing 

 felspar in places in this section. 



F. — Metasoinatically altered rocks. 



The dacites in the Healesville area are remarkably fresh, and 

 -except for the occasional presence of chlorite, epidote, sericite and 

 very rarely tourmaline, they remain unaltered. It has been pre- 

 viously suggested that the dacitic lavas were very viscous and poor 

 in mineralisers; hence, perhaps, the minor alteration of these 

 rocks. The basal volcanic rocks, — the rhyolites, and tuffs, — how- 

 ever, have been greatly altered in places. These metasomatically 

 altered rocks are best seen along the aqueduct from the Badger 

 river, south-west of Mount Riddell. Here, a greenish-coloured, 

 compact rock, veined with carbonates and quartz are locally greatly 

 pyritised, outcrops. Examined under a lens, some of the limon- 

 itised pyrites appears to contain native gold. In the absence of 

 assays of the pyrites, however, one cannot assert definitely that 

 gold is present in these rocks. The fact that gold has been won 

 from the Badger creek, near by here, is perhaps significant. Ac- 

 cording to Professor Skeats, gold has been obtained from creeks 

 passing only over dacites near Marysville and Gcmbrook. but its 

 mode of occurrence is not known. 



Specimen No. H80, from south-west of Mount Riddell, is typical 

 of these propylitised rocks. Hand specimens are compact and 

 aphanitic, and of a greenish-grey colour. The specific gravity of 

 the rock is 2.80. Examined microscopically it is seen that the 

 original rock has been greatly r^laced by carbonates, chlorite, 



