Physiography of Yarra, etc. 497 



country farther north, thus throwing that basin into that of 

 the Murray. The term Upper Goulburn Basin is here restricted 

 to that portion of the general basin, east of Trawool, bounded 

 by the two prongs mentioned above. 



The Yarra and Upper Goulburn Basins are largely bordered 

 by old igneous rocks (dacites, granodiorites and porphyries), 

 generally of much greater height than the surrounding coun- 

 try. These rocks are arranged roughly in three prominent 

 parallel east and west bauds, a feature which does not occur 

 elsewhere in Victoria. These bands form respectively the 

 northern boundary of the Upper Goulburn Basin, the boundary 

 between the latter basin and that of the Yarra, and the 

 southern boundary of the Yarra Basin. 



It will I think be admitted that the present basins of the 

 Yarra and Upper Goulburn and their divides are clearly deter- 

 mined by the outcrops of the igneous rocks mentioned, and 

 that such basins have been blocked out by differential denu- 

 dation, as suggested by Prof. Skeats (3, p. 189). Faulting 

 may possibly have assisted ; but denudation was probably the 

 chief factor. 



Assuming this to be correct, the size and form of the 

 drainage areas become intelligible. The Yarra Basin is much 

 smaller than that of the Upper Goulburn Basin, but this is 

 not due to some mere chance. The hard rocks have deter- 

 mined the areas, subject to some favourable circumstance in- 

 creasing one at the expense of the other. 



Coming to greater detail, the Yarra Basin is bounded on 

 the north by the granodiorite of Mt. Disappointment, by 

 Silurian rocks, and by the great mass- of dacite and grano- 

 diorite forming Mts. St. Leonard, Arnold, Juliet, etc. The 

 dacite of the Dandenong Ranges and the granodiorite stretch- 

 ing eastwards from these hills towards Baw Baw, form the 

 southern boundary. The igneous rocks of the two boundaries 

 mentioned are connected by the narrow bar of similar rocks 

 at Warburton. 



The effect of this bar has already been discussed when 

 treating of the growth of the Yarra, and it seems reasonable 

 to maintain that had no such bar existed the basin of the 

 Yarra eastward would probably have been greater in area than 



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