PRESIDENTS ADDRESS SECTION C. 



19$ 



than that of the original dacite, and quite unlike the typical hypidio- 

 morphic fabric of the granodiorite, although composed of the same- 

 minerals. Somewhat similar structural and mineralogical changes 

 appear to be characteristic of sections of the rocks of the gneissic 

 fringe near Warburton. This view of the mineralogical changes in 

 the dacite is strengthened by H. C. Richards' work (94) on the 

 separation and analysis of the minerals of the dacite of Upwey. 



At Macedon, Nyora and Marysville, no gneissic structures are 

 developed at the contact of the two series. The alteration appears to 

 have been much less intense and complete, and consists in production 

 of a tendency to parallel arrangement in the minerals and the produc- 

 tion of secondary biotite at the expense of the hypersthene. Some 

 of the rock sections of the dacites show flow structure as well as a 

 granulitic ground mass. These can be referred to a superficial origin 

 as lava flows with considerable probability. Other types are probably 

 intrusive, but no microscopic criteria are available for recognition, 

 except that the rocks with a coarser-grained ground mass are 

 probably of deeper-seated origin. The nearest approach to a plutonic 

 tjrpe occurs at Aura, on the Gembrook Railway line, where a com- 

 paratively coarse-grained holocrystalline dacite comes into relation 

 with the granodiorites. 



At Dandenong Hills, Narbethong, Marysville, and the Strathbogie 

 Ranges, the dacites in places appear to pass by increase of quartz and 

 diminution or disappearance of hypersthene into quartz-porphyries 

 and quartz-porphyrites. No sharp junctions have been noticed, and it 

 is inferred that the change is a gradual one. 



Chemical Analyses. 

 Analyses of four dacites are recorded here, and with them an 

 analysis of a granodiorite from Braemar House, Macedon, for com- 

 parison with that of a dacite from the same locality. The close 

 agreement in composition suggests a close genetic relationship in the 

 two rock types. 



1. Dacite, BlHck'sSpur N. of Healesville. An«i,vist — H. C. Jenkins. 



2. Dacite, Upwey. Analyst— H. C. Richards (94). 



3. Dacite, Willimigonggong Creek, Macedon. Analysts — Lewis and Hall (266). 



4. Ducite, Braemar House, Macedon. Analysts — Lewis and Hall. 



5. Granodiorite, Braemar House, Macedon. Analysts — Lewis and Hall. 



