Rocks of the Howqua River. Of 



The structure is typically porphyritic, the base is fine grained, 

 and feistic, often showing flow phenomena. The phenocrysts in 

 order of abundance "would appear to be, felspar, 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- 

 magnesian 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. — Ihis 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 Avere 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 uncommon, 

 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 are 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 the rock approaches that of a normal diorite. In general 

 the character of the rock compares closely with tliat of the normal 

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

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

 to be correlated witli the general and extensive one affecting enstorn 

 and south-eastern Australia, and re<^arded as Lower Devonian in- 

 age. 



