Rucls of tJie Hotuqua River. GT 



The structure is typically porphyritic, the base is fine grained 

 and feist ic, often showing flow phenomena. The phenocrysts in. 

 order of abundance would appear to be, felsj^ar, ferroniagnesian 

 mineral, quartz and occasional garnets. The felspars are more 

 or less decomposed, but in general the twinning and cleavage 

 can be recognised. There appears to be about an equal amount 

 of repeated and untwinned forms, but undoubted orthoclase has not 

 been recognised for the refractive index is invariably higher than 

 that of the Canada balsam. 



The quartz is often rounded, cracked and embayed. The ferro- 

 niagnesian minerals have been almost completely chloritized, but 

 their outlines are marked by black outlines due to iron oxides. 

 Biotite is indicated, and hypersthene and hornblende are also 

 suggested. The amount of ferromagnesian mineral indicates a 

 rock related to the Dacites (Slides 29, 30, and 32.) 



2. Granodiorite. — This rock is very abundant in the boulders of 

 the Howqua, but it is only found in situ within the area mapped 

 in the extreme north-eastern corner on the slopes of Mount Buller. 

 Its intrusive character is shown by the contact alteration of the- 

 adjoining Lower Palaeozoic strata. All the sections prepared 

 were from boulders in the Howqua, as these were the freshest speci- 

 mens obtainable, and their source was known with fair certainty. 

 The prevailing rock has a tj-pical granitic structure, and is of a 

 grey colour, but very fine grained varieties are not xmcommon,. 

 showing a tendency to porphyritic structure. 



It is worth recording that a small splash of molybdenite was 

 noted in one of the boulders. 



Hornblende, biotite and triclinic felspar ai'e readily recogniz- 

 able in hand specimens. In thin sections, felspars appear to be 

 slightly more abundant than quartz. Twinned and untwinned 

 forms are about equal in amount. The repeated twinning 

 is very minute, with occasional fine cross twinning, suggesting- 

 anorthoclase. 



Biotite and hornblende, both green in colour, are invariably 

 present, but in varying quantities in the different slides. In the 

 basic segregation patches, the hornblende predominates, and the 

 nature of tlie rock approaches that of a normal diorite. In general^ 

 tlie character of the rock compares closely with that of the normal 

 granodiorites of the State, and its association with the rocks of a 

 dacite type is also similar. Tbis intrusion, therefore, is probably 

 to be correlated with the general and extensive one affecting eastern 

 and south-eastern Australia, and regarded as Lower Devonian in 

 as:e. 



