94 Proceedings of the Royal Society of Victoria. 



ground mass. The felspar microliths are frequently twinned 

 accoi'ding to the Carlsljad law, give straight extinction and have 

 very often the rude transverse parting so characteristic of sani- 

 dine. Large pellucid phenocrysts of felspar occur, mostly of 

 tabular habit and showing rectangular cleavage lines. As a rule, 

 the crystal boundaries are well marked, though occasionally, 

 when bunches of crystals occur, (me or more of the terminal 

 faces may be wanting. In certain of the crystals small irregu- 

 larly-shaped inclusions, either opaque or of deep brown colour, 

 may be seen. This alteration product appears to start along 

 cracks in the crystal, and is probably caused by the infiltration 

 of iron oxide. The phenocrysts are on the wliole very free from 

 included matter ; in some the phenomenon of strain-shadows is 

 exhibited. The few crystals of columnar habit which appear in 

 the slide show Carlsbad twinning, with occasionally the broken 

 divisional line due to interpenetration. I have been unable 

 with the means at my disposal to make a suitable measurement 

 of the axial angle of the felspar, so as to at once settle whether 

 it is sanidine or orthoclase, but after comparing it with the sani- 

 dine of trachyte from the Drachenfels, Monte di Vetta, Scarru- 

 pata and Laach, and of phouolites from many disti'icts, and 

 noting how many of the peculiar characteristics of sanidine it 

 possesses, I see no reason to doubt that the monoclinic felspar of 

 the Macedon rock is sanidine, and that the rock itself must be 

 classed as a tracliyte. The other minerals which occur in the 

 rock, magnetite and sphene, do not call for any special notice. 



The second type of trachyte which is found in this area differs 

 slightly, both macroscopically and microscopically from that 

 just described. It outcrops on the Turritable Creek near the 

 residence of Mr. R. Harper and may be traced for some short 

 distance up the creek. What exact field relation it bears to 

 the trachyte already described and the igneous rock which 

 constitutes the main rock-mass at the western end of the 

 Macedon Range, I am unable at present to say. It is grey- 

 brown in coloui*, fine-grained in texture, and shows numerous 

 porphyritic crystals of glassy felspar. Under the microscope 

 the ground mass appears to be made up of felspar microliths, 

 grains of augite, and brown and opaque specks of matter. The 

 felspar microliths are not so perfectly formed as in the rock just 



