Victortiin Igneous Jiocks. 259 



fliurac'ter uf tlie uortlifrn Suatlil)ugie rucks tends trt sliow that they 

 were entirely intrusive in cliaructer. One might picture first an 

 intrusion of the porphyry, followed at a short interval by a more 

 deep-seated intrusion of adamellite. Subsequent cooling and con- 

 traction left an opening at the junction for the intrusion of the 

 still molten, somewhat more acid residuum. Except for a slight 

 schistocity there is very little evidence of contact metamorphism in 

 the porpliyry. such as has been noted at Macedon, Belgrave and 

 elsewhere in the dacite. This might be explained by the fact that 

 the temperature of the aplite at its intrusion would be relatively 

 low. and the resultant metamorphism would be almost negligible. 



Thu adamellite of the- St rathbogies can be traced continuously 

 from the neighbourhood of Seymour to tlie Broken River, near 

 Xillahcootie. I have shown (22) that during the ascent of Mount 

 Samaria, in this neighbourhood, granite, granite porphyry and 

 quartz porphyry are successively met wnth, and there is little doubt 

 that the three types belong to the one intrusion, the structural 

 variation being due to the different pressures under which the rocks 

 ci-ystallised. The above rocks would be better described as adamel- 

 lite, adamellite porphyry and quartz porphyrite. Eastward from 

 Mount Samaria the prevailing type is a quartz porphyrite similar 

 to that found at the summit of the mount. 



The granular groundmass so typical in the dacites and the 

 Strathbogie quartz porphyrite is generally aVjsent in the Mount 

 Samaria and Tolmie porphyrites, its place being taken by a fine- 

 grained cryptocrystalline aggregate. 



Hypersthene is absent in most sections examined, but several 

 sections of the rock from Burn's Track, near Tatong, showed this 

 mineral, and in these sections the granulitic groundmass was pre- 

 sent. The field evidence shows that the quartz porphyrites of the 

 Tolmie Highlands and of the Strathbogies belong to the one great 

 period of intrusion, and this is supported by the general similarity 

 of the micro-structure of the rocks in the two areas. This relation- 

 ship has been given in some detail because the age of the dacites 

 and allied rocks largely hinges on the evidence obtainable in this 

 area. 



The field evidence in the Macedon District shows that the dacites 

 are post Upper Ordovician, and are younger than the Kainozoic 

 igneous rocks of the area. The dacites of the Dandenong Ranges 

 and Healesville are post-Silurian. The Strathbogie and Tolmie 

 adamellites and quartz porphyrites are intruded into the Silurian, 

 and these rocks in the southern portion of the Tolmie area ar^ 



5 



