192 president's address — section c. 



as Pre-Carboniferous. If, as is generally believed, the bulk of the 

 Victorian granodiorites are of Devonian age, it is probable that the 

 dacites are Devonian also, and may belong to the lower part of the 

 Devonian and be practically contemporaneous Avith the acid lavas of 

 the Snowy River poi'phyries. 



Petrological and Chemical Evidence. 



Practically no microscopic or analytical work had been done on 

 these rocks prior to Prof. Gregory's paper. The recognition of these 

 rocks as dacites and their first description are due to Prof. 

 Gregory. With his description of the minerals present and of the 

 structures of the Macedon dacites, Mr. Summers and myself are in 

 almost complete agreement. He distinguished, however, the Macedon 

 rock from those of other areas in Victoria on account of its supposed 

 higher percentage of alkalies, as indicated by an analysis quoted in his 

 paper. On this account he grouped it with the alkali-trachytes of the 

 district, and gave it the specific name of geburite-dacite after the 

 native name for Mount Macedon. The minerals quoted by Prof. 

 Gregory as present in the rock do not, however, indicate alkaline 

 affinities, and in fact are precisely similar to those occurring in other 

 Victorian dacites. There can be no doubt that its separation from 

 the other dacites is not justified, and a later analysis of the same rock 

 by Messrs. Lewis and Hall, of the Mines Department (266), shows that 

 the composition of the rock is quite normal. 



The Macedon dacite as shown by Gregory is commonly poi'phy- 

 ritic when examined microscopically, due to the presence of hypers- 

 thene and plagioclase ranging from oligoclase to bytownite. There 

 is a fine granulitic base of quartz, acid felspar, and minute biotite 

 crystals. Ilmenite occurs both as phenocrysts and in the base of the 

 rock. 



Prof. Gregory refers to the occurrence of dykes and of agglo- 

 merates and ashes on Mount Macedon. Our observations have failed 

 to substantiate this. There occur on the mountain above Upper 

 Macedon what are described by Gregory as dykes, agglomerates and 

 ashes. Mr. Summers and I refer these to the recent surface dis- 

 integration and weathering of the dacite on the steep hillside. 



On the north foot of the mountain west of Hesket, however, we 

 found an example of a rock which appeared to be fragmental, but 

 which may be due to differential flow of coarser and finer parts of a 

 lava. The Dandenong dacite differs from that of Macedon not at all 

 in chemical composition, but with a similar ground mass generally 

 contains phenocrysts of quartz and biotite, as well as h}'persthene and 

 plagioclase. Where the rock becomes gneissose near the contact with 

 the granodiorite, structural and mineralogical changes of an interest- 

 ing character are seen under the microscope. A gradual banded struc- 

 ture is developed by reciystallisation, hypersthene passes into bastite 

 with a marginal fringe of secondary biotite developed partly from the 

 hypersthene, partly from the quartz and acid felspar of the ground 

 mass. The ilmenite, too, develops secondary biotite as a marginal 

 fringe. A little blue soda-hornblende is also developed. Near the 

 granodiorite these changes become complete, so that the gneissic rock 

 consists entirely of biotite, plagioclase, quartz, and ilmenite. It still 

 shows a ground mass which is granulitic, but much coarser in grain 



