390 PROCEEDINGS OF SECTION C. 



You Yangs lies mainly in its dimensions, wliicli are mucli larger than 

 those of any basin having a similar origin which has come under my 

 notice. 



PETROLOGICAL DESCRIPTION OF THE GRANITIC AND ASSOCIATED 

 METAMORPHIC ROCKS OF STATION PEAK. 



The Granite.— Many of the rocks which have been called granites 

 in Victoria have been shown to be in reality more basic in composition 

 than true granites. Some of these have been described by Dr. Howitt 

 as quartz-mica-diorites(6), while Professor Gregory, on account of the 

 considerable proportion of alkali felspar, has suggested the name grano- 

 diorite.(c) A short examination of rock sections of the You Yangs 

 granite is sufficient to show that is does not belong to the grano-diorite 

 division, but is an alkali granite. No detailed description of the rock 

 has, I believe, been hitherto attempted. On the geological survey 

 map it is described as " coarse-grained, with large crystals of felspar. "^ 



Megascopic Examination. — Its porphyritic character is certainly 

 its most prominent characteristic, some of the felspar crystals being 

 Sin. or 4in. in length. The base is also fairly coarse-grained, and con- 

 sists to the eye of felspar, quartz, and biotite. The specific gravity of 

 a specimen of the rock is 2.60. The granite shows the usual dark 

 segregation patches, and is traversed by veins or dykes, some of which 

 are more acid, others more basic, than the normal granite. The 

 microscopical character of these will be described below. 



MICROSCOPICAL EXAMINATION, [SECTION 393 {d) (See Plate I.. 



Fig. I). 



The Structure of the rock is porphyritic, owing to the development 

 of large early-formed felspars, while the structure of the later-formed 

 ground mass is medium-grained and hypidiomorphic to panidiomorphic. 

 (d) These numbers refer to sections in the collections at the University of Melbourne. 



The Minerals present, in order of relative abundance, are alkali 

 felspar, quartz, oligoclase, biotite, hornblende, with small amounts of 

 the accessory minerals, apatite and iiiagnetite. 



The Felspars : Alkali Felspar. — The phenocrysts consist of an alkali 

 felspar which, for the most part, shows either no twinning, simple twir- 

 ning, or an obscure and very fine twin lamellation. A perthite inter- 

 growth, Avith albite, occurs in places. A blowpipe examination gave 

 marked flame reactions for both sodium and potassium. The sections 

 have been mounted in liquid balsam, with a refractive index of between 

 1.52 and 1.53. The refractive index of this felspar is in places equal 

 to that of the balsam, and in places slighter higher. The optical and 

 chemical evidence available makes it probable that the felspar is the 

 soda-potash felspar anorthoclase, at least in part ; while part is 

 ordinary orthoclase. The felspar also shows a certain amount of un- 

 dulose extinction — a common feature with anorthoclase ; but, as the 

 quartz i)i the rock behaves similarlv, the effect mav be due to strain. 



(b) Roy. Soc, Vic, vol. xx., 1B94, p. 31. 

 (r) Roy. Soc, Vict., vol. xiv. (New Series), 1902. 



