Victorian Igneous Hocks. 257 



branch is fundamentally associated with tectonic movements charac- 

 teristic of the Atlantic coast type, and the latter is associated with 

 the Pacific coast type of earth movements. Evidence will he ad- 

 vanced in opposition to this belief, and to show that in Victoria the 

 non-calcic granites are genetically related to the calcic dacites and 

 granodiorites. 



Professor Skeats and the author have already expressed tlie Ijelief 

 that the alkaline rocks of the Macedon District represent a frac- 

 tional differentiation product from the calcic basalt magma. 



If these two contentions be upheld, then it follows that the correla- 

 tion of alkaline rocks with subsidence due to faulting and the cor- 

 relation of calcic rocks with folding, due to lateral compression, is 

 open to grave objection. 



The differentiation of rock magmas will be discussed at some 

 length, but more with the view of determining the relationships of 

 the end products to one another than to attempt any explanation of 

 the means whereby these end products are derived from the parent 

 magma. 



II. Victorian Igneous Rocks. 



(a) T/ie Macedon Granodiorite-dacite Ser/e.s. 



In tiie Macedon District there are extensive developments of rocks 

 belonging to two very distinct cycles of igneous activity. The older 

 of these series has been shown (19) to consist of dacites, granodiorites 

 and granodiorite porphyries. The dacites were in part extrusive 

 and in part intrusive, and were closely followed by the intrusion of 

 the granodiorites and granodiorite porphyries. It is quite impos- 

 sible to definitely fix the age of these rocks, as they were intruded 

 through and into rocks of Ordovician age, and are overlain in part 

 by sands and lavas of Kainozoic age. However, for reason given 

 elsewhere it is highly probable that these rocks are of Devonian age. 



(b) Victorian Vyinr Palaeozoic Sei-ies. 



Dacites similar in every respect to those of the Macedon District 

 occur in the Dandenong Ranges and in the neighbourhood of 

 Healesville. Mineralogically and texturally these dacites are entirely 

 comparable with those of Macedon. Phenocrypts of plagioclase, 

 hypersthene, biotite, and occasionally quartz, are set in a granu- 

 litic groundmass consisting mainly of quartz and felspar, with some 

 biotite, ilmenite and minor accessories. Naturally there are textural 

 variations, but the above brief description can be applied to tlic 

 majority of sections examined from Macedon, Dandenong Ranges, 

 and Healesville. 



