Victorian Igneous Rocks. 291 



VI. The Origin of the Victorian Rock Magmas. 



In an earlier poitiuii of this paper evidence has been given for 

 regarding the Heatlicotian diabases and the Kainozoic basalts as 

 being associated with trough faulting, while the granites, grano- 

 diorites, dacites, etc.. were associated with intense earth folding. 

 This means that the basalts, for we may regard the diabases aa 

 greatly altered basalts, were associated with one type of tectonic 

 movement, while the granites, etc., were associated with a quite 

 distinct type of movement. 



If we turn to the generalised variation diagram (Fig. 4) of the 

 differentiation products of the Victorian granitic magma, we find 

 that the theoretical Ijasic differentiation end product differs radi- 

 cally in composition from normal basalt. This means that a 

 basaltic rock, could not be formed as a serial differentiation 

 product from this magma. 



In the case of the Macedon alkaline rocks it was shown that some 

 of the rocks were apparently derived by the splitting up of a partial 

 magma, and that the products so formed bore no serial relation- 

 ship to one another. 



In the Geelong and Heathcote districts we have a certain amount 

 of evidence that some of the rocks were derived in a somewhat 

 similar way by the splitting up of a magma. 



Westward from Geelong and at the foot of the northern slope 

 of the Barrabool Hills is a large outcrop of a basic rock which is 

 best described as an epidiorite. Intruded through this rock is a 

 well-defined dyke of granite porphyry. The field evidence is such 

 that there is no doubt as to the relative ages of the two rocks. 



Northward from this area lies the granite massif of the Dog 

 Rocks. A comparison of sections of this granite with a section of 

 the granite porphyry dyke from the southern area show that except 

 for the difference in the coarseness of grain there is very little 

 distinction between the two, and there is little doubt that they are 

 genetically related. The granite area of the Dog Rocks is roughly 

 triangular in outline, and at the south-eastern corner is an outcrop 

 of epidorite. Microscopically this resembles very closely the epi- 

 diorite from the southern area, and again they appear to be 

 genetically related to one another. In this case, however, the 

 epidiorite is apparently intrusive into the granite, and so is 

 younger than that rock. Unfortunately the field evidence is not as 

 conclusive as could be desired, so that the relationship of tlie two 

 rocks cannot be stated to have been proved. The epidiorite occurs 



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