president's address SECTION c. • 199 



A section from the handsome red quartz-porphyry of Grange Burn 

 near Hamilton shows that the rock is porphyritic in structure. The 

 phenociysts consist of cloudy orthoclase and of quartz, both of which 

 are typically corroded. Ferro-magnesian minerals appear to be 

 absent. The ground mass varies from cryptocrystalline to micro- 

 crystalline and is drusy in places. Lining the walls of the druses, 

 quartz and orthoclose crystals are iutergrown in an approach to the 

 granophyric habit. Minute opaque areas of iron oxide occur in the 

 ground mass, which is reddish m colour owing to iron staining. 



A section from an acid dyke penetrating the Grampian sand- 

 stones, at the junction of Stony and Fyan's Creek, at Hall's Gap in 

 the Grampians, shows considerable resemblance to the rock described 

 above, ^he phenociysts consist mainly of cloudy orthoclase with a 

 little chlorite secondary after biotite. The ground mass is micro- 

 graphic to granophyric in texture and consists of an intergrowth of 

 quartz and ortlioclase. The orthoclase, both the phenociysts and that 

 in the ground mass, is iron-stained, and the rock may be described as 

 a micrographic or granophyric orthoclase-quartz-porphyry. 



Jurassic. 



No volcanic rocks of Jurassic age are known in Victoria. The 

 bulk of the Jurassic freestones, mudstones, or sandstones from the 

 Otway Ranges and from S. Gippsland, however, are rocks composed 

 larg-ely of the detritus of igneous and probably volcanic rocks. 

 Whether they derive their material from contemporaneous or from 

 older igneous rocks isi a matter of conjecture. Prof. Gregory has 

 suggested (9) that they may have derived their material by denuda- 

 tion of a former northward extension of the Tasmanian diabases. 

 The age of these is generally stated to be Mesozoic, but various 

 authorities have referred them to the Permo-Carboniferous on the 

 one hand, and to the Kainozoic on the other. If the latter view of 

 their age should be correct another source for the volcanic materials 

 of the Victorian Jurassic sediments would have to be sought. It is not 

 impossible that the denudation of the dacite series may have pro- 

 vided the material for these rocks. 



LOWER KATXOZOIC. 

 The Older Basalts. (See Map, Plate i). 

 Geographical and Geological Eel at ions. (See Section, Plate III., 

 Fig. 2). — Any attempt to distinguish precisely tlie age and strati- 

 graphical relations of all the Victorian basaltic rocks is attended with 

 very great difficulties. In a general way the early Geological Survey 

 of Victoria recognised two main geological horizons in the Kainozoic 

 rocks among which basic lavas and pyroclastic rocks occur. S'trati- 

 graphical evidence sufficient to fix their age is wanting in the case of 

 many of the occurrences, and attempts made to distinguish between 

 older and newer basalts by the petrological characters of the rocks 

 or by the degree of alteration they have suffered have met with 

 indifferent success. So much is this the case that even now, except 

 in the comparatively few cases where the relations of the lavas to the 

 Kainozoic sediments serve to define their stratigrapliical horizon, the 



