hfuc.oiis Rocks. 21 h 



tisual, but are often greatly fractured and corroded. The rock is a 

 metasomatieally altered rhyolite of quartz porphyry. 



Section No. H91, xenolith in tuffs, S.S.W. of xMount Riddell. is 

 & slightly luetaniorphosed granodiorite. Biotite is greatly replaced 

 by chlorite (pennine) and rutile has separated out as sagenitic webs. 

 Abundant pyrrhotite occurs throughout the section, replacing much 

 of the original ilinenite'. The felspars are greatly altered, but 

 plagioclase appears to be in excess of orthoclase. 



Section No. H94, andesite tuff, from same locality as preceding 

 specimens. Under the microscope the rock is seen to consist largely 

 of fragments of andesite showing fine pilotaxitic texture. The 

 original biotite crystals have been replaced by strongly pleochroic 

 chlorite (pennine) and epidote (pistacite), and leucoxene has sepa- 

 rated out. Quartz phenocrysts are rare, but the bulk of the 

 matrix of the rock appears to be silica, some of which is chalcedonic. 

 Occasional crystals of colourless isotropic garnet are seen in the 

 section. The felspar phenocrysts and ground mass laths appear to 

 be almost entirely plagioclase, often well zoned. 



Track to Marooiidah weir. — Doubtful fragmental rocks occur near 

 Maroondah weir. Specimen No. Jl, metasomatieally altered 

 rhyolite tuff or rhyolite, one mile south-west of Maroondah weir, 

 is typical of these rocks. Hand specimens are brecciated, consist- 

 ing of fragments of a light-coloured rock in a darker coloured 

 matrix. Thin sections of the rock examined microscopically show 

 that silicification, carbonation and sei'icitisation have greatlv 

 affected the original rock. The primary minerals include simply 

 twinned orthoclase, plagioclase, corroded and fractured quartz, 

 garnet, zircon, sphene, apatite and a (?) femic mineral which has 

 been entirely replaced. Muscovite, carbonates ('I dolomite), chal- 

 cedony, quartz pyrites, (?) opal and brown iron oxides are 

 secondary. Most of the plagioclase has a refractive index less than 

 that of quartz, and appears to be albite, but certain zoned sections 

 give symmetrical extinction angles of 22° from the albite lamellae, 

 indicating andesine. Massive cleaved carbonates and flakes of seri- 

 cite replace much of the felspar. A little secondary, radially 

 arranged, albite has been developed in one place in the section. 

 Yellow-brown aggregates and double wedge-shaped sections of 

 sphene, showing bright pinks and greens of the third order under 

 crossed nicols, are not uncommon. The garnet is rarely idio- 

 morphic, occurring more often in corroded and irregular-shaped 

 grains. Microspherulitic chalcedony, showing a black cross under 

 crossed nicols, occurs throughout the section. Minute veins of 

 quartz traverse the rock. 



