[ 917 ] 

 XOVTIL Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from p. 823.] 



January 12th, 1910.— Prof. W. J. Sollas, LL.D., Sc.D., F.R.S., 

 President, in the Chair. 



r pHE following communications were read : — 



]. 'On the Igneous and Associated Sedimentary Rocks of the 

 Glensaul District (County Galway).' By Charles Irving Gardiner, 

 M.A., F.G.S., and Prof. Sidney Hugh' Reynolds, M.A., F.G.S.; 

 with a Palaeontologieal Appendix by Frederick Richard Cowper 

 Reed, M.A., F.G.S. 



2. ' On the Gneisses and Altered Dacites of the Dandenong 

 District (Victoria), and their Relations to the Dacites and to the 

 Granodiorites of the Area.' By Prof. Ernest Willington. Skeats 

 D.Sc, A.R.C.S., F.G.S. 



The area described lies about 25 miles south-south-east of 

 Melbourne. The earlier literature is discussed, and it is shown that 

 the early geological surveyors regarded the dacites as Palceozoic 

 ' traps ' passing gradually into the granodiorites. Prof. J. W. 

 Gregory first described the rocks as dacites, probably of Lower 

 Tertiary age, resting upon the denuded surface of the granodiorites 

 and of the adjoining Lower Palaeozoic sediments. The author 

 describes the field-relations of the rocks, and shows that gneiss 

 occurs between the dacite and the granodiorite in places. Else- 

 where at the contact the dacite appears slightly altered. The contact 

 with the plutonic rock is everywhere abrupt. Xo foliation or 

 banding occurs in the granodiorites, but acid veins pass from the 

 junction into the altered dacite and also cut across the foliations 

 of the gneiss. The field-evidence, therefore, shows that the dacites 

 are older than the granodiorites, and also that the gneiss was 

 formed before the intrusion of the acid veins. Chemical analyses of 

 the rocks and of the coloured minerals of the dacites are recorded. 



The chemical evidence indicates that slight differentiation of a 

 magma took place : the dacite was first erupted, and, following 

 shortly on that, the granodiorite (of slightly more acid composition) 

 was intruded into the dacite. The microscopic characters of the 

 granodiorite, the dacite, the altered dacites, and the gneiss are 

 described. In the altered dacites a slight banding or schistosity 

 occurs near the contact, ilmenite is changed to secondar}' biotite by 

 reaction with the felspar in the microgranular ground-mass, biotite 

 is corroded by the attack of the ground-mass, and hypersthene is 

 altered at its margin to secondary biotite and secondary quartz. 



