188 president's address — section c. 



Quartz-felsophyrites. — This rock is probably yoimger than those 

 just described, it is flanked by breccias, it is intensely hard and flinty 

 in 'character, of a black or greyish black colour, and includes nume- 

 rous fragments of the ceratophyi'es and of the sedimentary rocks 

 which give it a porphyritic appearance. Under the microscope the 

 ground mass is a dark yellow brown glass, showing very marked 

 fluidal structure. In places the glass is replaced by a cryptocrystal- 

 line aggregate. The larger constituents of the rock include angular 

 fragments of glass, fractured and corroded ciystals of quartz, twinned 

 and fractured felspars, and some chloritic material. Howitt suggests 

 that the ground mass of the rock agrees with that of a pitchstone. 

 It seems to me that the marked fluidal character allies it rather with 

 the rhyolites, and I prefer to describe it as a soda-rhyolite. 



LOWER DEVONIAN. 



The Dacite and Quartz-Porphtrite Series. 



Geograj)hical Distributinn and Physiography (See Plate 4). 



Rocks belonging to these petrographic types occur in large masses, 

 ■covering very considerable areas in Central Victoria. About 40 miles 

 N.W. of Melboum© occurs Mount Macedon, a veiy considerable portion 

 of which is composed of dacite. About 20 miles E.S.E. of Melbourne 

 another considerable area of dacite and quartz-porphyrites forms the 

 hilly area of the Dandenong Hills. 



The largest single area of dacites, quartz-porphyrites with some 

 gianite^porphyries, commences at Healesville, about 40 miles N.E. of 

 Melbourne, stretches S. to Warburton, and N.E. past the Black's Spur 

 and Narbethong to Maiysville and the Cerberean Range. The 

 northern parts of the Strathbogie Ranges, south of Violet Town, con- 

 sist of similar rocks, while to the east of this and south of Benalla 

 there is a large area of rocks, including Mount Samaria and Whitfield, 

 which have been correlated with the Snowy River porphyries (74), but 

 which, I think, are more probably related to the dacite, quartz-por- 

 phyrite series. All these areas mentioned are more or less mountainous 

 in character. Mount Dandenong is just over 2,000 ft. in height. Mount 

 Macedon just over 3,000 ft.. Mount Juliet, near Healesville, 3,600 ft., 

 while several of the mountains near Warburton exceed 4,000 ft. in 

 height. In most cases these volcanic and intrusive masses rise well 

 above the levels of the granodiorites and the Lower Palaeozoic sedi- 

 ments with which they are associated. Professor Gregory (83) regards 

 them as volcanic masses, probably of Lower Kainozoic age, poured out 

 over and rising above a platform of granodiorite and Lower Palfeozoic 

 sediments which before the outpouring of the dacites, &c., had been 

 reduced practically to a peneplain. For reasons which will be stated 

 later, I believe the dacites to be a much older series, into which the 

 granodiorite was intruded, and that the present surface relations and 

 the exposure of the granodiorites at the surface is the result of long- 

 continued differential denudation of the sediments and igneous rocks. 

 The level-topped, plateau-like character of the dacites of Mount 

 Macedon, and of the Dandenong Ranges in particular, suggests the 

 possibility that these may be the remnants of a former extensive pene- 

 plain developed by long-continued subaerial denudation of the igneous 

 find sedimentarv rocks before Mid-Kainozoic times. Later movements 



