118 Proceedings of the Royal Society of Victoria. 



Appendix. 



Note on the Microscopic Structure of Some Rocks 

 from Dandenong, 



By professor J. W. GREGORY, D.Sc, F.R.S. 



In connection with Mr. Sutherland's paper on the lower 

 palaeozoic and granite rocks of Dandenong, I have given a few 

 of the rocks a microscopic examination. One of the lower 

 palaeozoic rocks from allotment 61, Narre Warren, collected by 

 the contact, proves, on microscopic examination, to have been 

 altered into a very fine grained biotite hornstone ; it closely 

 resembles some of the ordinary typical rocks formed by contact 

 metamorphism around our granitic masses. The ordinary 

 granitic rock of the district is connected with a series of dykes, 

 the examination of which was of interest, owing to the possibility 

 of some of them having been connected with the Dandenong 

 dacites. The dykes examined, however, liave no connection 

 with that series, and may be all derived from the much older 

 grano-diorite massif. 



The dykes, of which the best occur in the Police Paddock and 

 some adjacent allotments, belong to two groups. The first is a 

 diorite-porphyry (No. 11), which is composed of phenocrysts of 

 pale, partially leached, hornblende, and of andesine in a coarsely 

 granular holocrystalline base. The second series of dykes is 

 better described as quartz-biotite-porphyrite. Biotite is abund- 

 ant, but has now been mainly altered into chlorite, the larger 

 crystals containing granules of epidote, surrounded by the green 

 chlorite. There are abundant corroded and embayed phenocrysts 

 of quartz, and also of plagioclase. Tliese phenocrysts are widely 

 scattered in a very fine-grained felsitic base, which was, no doubt, 

 originally glassy. In some cases the felspars have undergone 

 considerable decomposition, and the dull, dusty crystals, under 

 polarised light, are lightened up by the bright granules of zoisite. 



The following analyses of these rocks may be conveniently 

 repeated^: — 



1 Gregory, J. W. : The Geology of Mount Macedon Victoria. Proc. Roy. Soc. Vict., 

 vol. xiv., U.S., 1902, p. 201. 



