president's address SECTION c. 197 



by Dennant (63), and later work in the area of the Grampians has 

 been done by Herman (82). The larger area outcrops near Hamilton 

 and stretches northwards to near Cavendish (See Map), while a 

 second area of apparently similar rocks occurs in the neighbourhood 

 of Balmoral. 



The district is one of comparatively low relief, and the volcanic 

 or intrusive rocks are exposed at the surface owing to the denudation 

 of the Lower and Upper Kainozoic sediments which formerly covered 

 them. The rocks consist mainly of quartz-porphyries, and their 

 stratigraphical relations are by no means clear. They are certainly 

 older than any of the Kainozoic rocks of the district. East of 

 Cavendish they appear to come into close juxtaposition with the 

 Grampian sandstones of the Victoria Range, while east of Balmoral 

 the other mass probably has similar relations to the sandstones of the 

 Black Range. *It would appear as if the quartz-porphyries underlie 

 these sandstones. No fossils have been foimd in the sandstones, and 

 their age is to some extent a matter of conjecture. They are usually 

 regarded as probably L. Carboniferous. If so, it would appear that the 

 quartz-porphyries of Cavendish and Balmoral may have the same 

 relations to the Grampian sandstones as the rhyolites and quartz-por- 

 phyries of Mount Welling-ton have to the Upper Palaeozoic sediments 

 of that area. While the field evidence, scanty as it is, suggests that 

 the igneous rocks underlie or are interbedded with the sandstones in 

 one or two places, as at Hall's Gap in the Grampians, dykes of an 

 acid character penetrate the sandstones. They are mineralogically 

 similar to the quartz-porphyries and make it possible that some at 

 least of the quartz-porphyries may represent intrusive rocks rather 

 than lava flows. I have only examined these rocks at Grange 

 Burn near Hamilton, where they appear rather massive in character. 

 Dennant states, however, that near these the poi^plmy shows good 

 prismatic structure, and that on the east side of the Dundas Ranges 

 they are seen in a small creek and present a laminated appearance as 

 if they had been poured out in sheets. 



Petrological and CJiemical Characters. 



The Mount Wellington area. The acid series of volcanic rocks. — 

 The only description of these rocks is by Howitt, included in Murray's 

 description of the area (46). The rocks are described as felsites, 

 and quartz-felsites. In some notes on L. Karng, on Mount Welling- 

 ton, Howitt (59) refers to the acid series as quartz-porphyries with 

 possibly varieties of ceratophyre. I have examined some of Howitt's 

 sections and others which I have had cut from specimens collected by 

 Thiele. Two types appear to be represented. 



The one type well represented on the southern plateau of Mount 

 Wellington is a beautifully banded rhyolite. The accompanying 

 analysis by Thiele (92) shows its veiy acid character. The very high 

 silica percentage is partly due to secondary silicification. Under the 

 microscope corroded and cracked phenociysts of quartz and orthoclase 

 are present; a small amount of radiating crystals of biotite of a drab- 

 brown colour also occur. The ground mass is cryptocrystalline to 

 microcrystalline, shows fiuidal structure particularly well and in 

 places a tendency to perlitic cracks. The rock is in part fragmental 



