﻿450 PROF. E. W. SKEATS ON THE GNEISSES [Aug. I9IO, 



19. 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 

 Sxeats, D.Sc, A.R.C.S., F.G.S. (Read January 12th, 1910.) 



[Plates XXXII-XXXIV— Microscope-Sections.] 



Contents. 



Page 



I. Introduction 450 



II. Previous Literature 450 



III. Geological Eelations in the Field 452 



IV. Petrography 455 



V. Chemical Composition of the Minerals and Bocks. 461 



VI. Effects produced by Contact-Metamorphism 463 



VII. Origin of the Gneisses 465 



VIII. Summary and Conclusions 467 



I. Introduction. 



The rocks with which this paper deals occur about 25 miles distant 

 from Melbourne, in a direction a little south of east. The area is 

 all included within the parish of Earree Worran, in the counties 

 of Evelyn and Mornington. The oldest rocks in the district are 

 sediments of Lower Palaeozoic age, either Silurian or Ordovician ; 

 but with these the paper is not directly concerned. 



Intruded into these sediments is a mass of granodiorite, which is 

 now exposed at the surface in the southern part of the area. To 

 the north of the granodiorite lies an extensive area of hypersthene- 

 biotite-dacite and quartz-porphyrite, which forms the Dandenong 

 Hills. This rock series is also younger than the Older Palaeozoic 

 sediments, for in some places it appears to rest upon them, and in 

 others to he intrusive into them. The boundary between the 

 granodiorite and the dacite runs in an easterly direction, and while 

 examining this junction in 1905 I discovered a belt of gneissic and 

 altered rocks which for several miles come between the normal 

 development of the granodiorite and of the dacite. It is with 

 these rocks and their relations to the plutonic and volcanic series 

 adjoining them that this paper deals. 



Hitherto gneissic rocks have been recognized in Victoria only in 

 certain parts of the Western District and in North- Eastern Gipps- 

 land. These, however, are probably of Archaean age, and certainly 

 have nothing in common with the foliated rocks described below. 



II. Previous Liteeature. 



The origin, age, and field-relations of the big volcanic dacite 

 masses of Victoria constitute one of the important problems in 



