Palaeozoic Geology of Victoria. 117 



^ar recognisable luaoroscopically. In the thin section the base is 

 seen to be slightly schistose, and micro-crystalline, consisting of 

 felspar and quartz, and showing som^ secondary silicification. It is 

 set with angular fragments of triclinic felspar, showing fine 

 repeated twinning after the albite type, and occasionally tarls- 

 bad twinning also. Extinction measurenaents on several of tlie 

 most suitable sections indicate that the felspar is probably oligo- 

 clase-albite. A little sericite and chlorite are also present. No. 38 

 is somewhat coarser in grain and of a light green colour. The frag- 

 mental character and slightly i-chistose structure are revealed in 

 .tha thin section; angular quartz fragments are abundant, also 

 altered felspar, and probably some scapolite. Dark, green chlorite 

 and calcite are present as alteration products, and some fine- 

 grained fragments probably represent altered shale. 



One feature worthy of mention in connection with the micro- 

 scopic examination of the above rocks is that all the identifiable 

 felspar fragments noted are triclinic. 



Tracluific and Andesific Rochs. — In contrast to the liiiht 

 coloured siliceous rocks just described, another group of darker and 

 more basic types is met with in Bogg}^ Creek (Specimens 9-14), and 

 so far they have not been traced or recognised' beyond the restricted 

 area, in the region here examined. Tliey occur in definite bands, 

 and appear to represent both effusive and pyroclastic beds. IMost 

 of the rocks of this type are very much altered, and the original 

 ferro-magnesian minerals are almost invariably altered to chlorite?. 



J*fo. 11 is a fairly typical example of the andesitic type. It is 

 a dark rock, with pink felspar phenocrysts. 



Thin sections under the microscope show phenocrysts of triclinic 

 felspar, slightly cloudy through decomposition, of stumpy habit and 

 with broad twin lamellae. No very satisfactory extinction measure- 

 ments were obtained, but it may be oligoclase. Ihe ferro-magnesian 

 mineral was proba.bly augite, but it is completely decomposed to 

 chlorite. Magnetit-e is common. The base is cloudy and altered, 

 but small felspar laths are abundant in it. The rock is regarded 

 as an altered augite-andesite. No. 9 is more altered, showing some 

 -serpentinization, and much chlorite. The felspars are abundant 

 and similar in habit to 11, but are kaolinizecl and carbonated. 



No. 12 is a dark, denfe rock, slightly porphyritic, with patches 

 -of red jasper. The thin section shows a distinct trachytic structure, 

 Avith abundant felspar laths in a .brownish devi trifled base. 



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