[Proc. Eot. Soc. Victoria, 35 (N;S.), Pt. II., 1923.] 



Art. XII. — The Relationship between Dacite andl 

 Granodiorite in Victoria. 



BY H. S. SUMMERS, D.Sc. 

 Associate Professor of Geology, University of ^Melbourne. 



(With one text figure.) 



[Read 10th August, 1922.} 



The association and lield relations of dacite and granodiorite^' 

 at Macedon, Mt. Dandenong, Warburton and Healesville, together 

 with the somewhat related occurrences in the Cerberean Ranges, the 

 Strathbogie Ranges, and the Tolmie Highlands, furnish interesting 

 material for students in petrogenesis, and in the mechanics of igneous 

 intrusion. 



In all cases the evidence proves that there was an extrusion of 

 lava, followed by the intrusion of granodiorite or adamellite in such 

 a manner that in every area the plutonic and volcanic rocks are 

 brought into contact with one another. As a result of this relation- 

 ship the dacite, etc., show the effects of contact metamorphism. 



The Macedon occurrence is the best known, and may be taken as 

 more or less typical of the relations that exist in the areas named. 



The general field relations of the rocks in this area are shown 

 in the map and section (fig, 1). It will be seen that the dacite is- 

 shown resting on the granodiorite over a considerable area. As the 

 dacite has been proved to be the older rock, it follows that it must 

 have been piled up on a platform of Ordovician sediments, and that the 

 present justaposition of the dacite and granodiorite has been brought 

 about subsequently. 



The relationship of the batholith of granodiorite to the overlying 

 dacite does not seem explainable by Suess'ss conception that a batholith 

 occupies a space formed during the dislocation of the lithosphere. 



Iddings'2 modification of this idea, viz., " that as the lithosphere 

 in places tended to part, molten magma might enter the fracture and 

 because of its density and hydrostatic pressure might permit the frac- 

 tured parts to separate, the magma supporting its share of the over- 

 lying load," also finds little support in the occurrence under dis- 

 cussion. Iddings considers that the tendency to part is the result 

 of fracturing by thrusting and flexure. It is difficult to picture any 



1. Richards, H.C., Proc. Roy. Soc. Vic, Vol. 21 (n.s.), p. 528, 1908. 

 Skeats, K. W., Q.J.G.S., Vol. 66, p. 450, 1910. 



Skeats, E. ^V.. and Summers, H. S., Geol. Surv, of \'ic.. Bull. 24, 1912, 

 Summers, H. S.. Proc. Roy. Soc. Vic, Vol. 26, (n.s.), p. 256, 1914. 

 Morris, M., Proc. Roy. Soc, Vic, Vol. 26 (n.s.), p. 331, 1914. 

 Junner, N. R., Proc. Roy. Soc Vic, \"'ol. 27, (n.s.), p. 261, 1915. 



2. Suess. E., The Face of the Earth, Vol. 1, page 163, 1904. 



3. Iddings, J. P., The Problem of Volcanism, p. 197, 1914. 



