Physiography of Yarra, etc 499 



pear to have beheaded tlie upper portions of the Yarra does 

 not invalidate the conchisions stated as to the cause of the 

 limitation of the Yarra Basin, but suggests that this basin was 

 at one time a little larger than at present, but for some pos- 

 sibly local reason it has been slightly reduced by capture. 



Whether the Yarra will extend its territory at its head 

 waters is difficult to answer. It has the great advantage of 

 the shortest route to the sea, so that ultimately at any rate its 

 upper waters should have a greater fall than those of the 

 Tliomson or the Goulburn. By this means it may possibly 

 increase its boundaries. 



As already noticed, the areas of the basins have been deter- 

 mined by the positions of the hard old acid igneous rocks ; 

 and the larger the basins, the greater the volume of water 

 passing into the main river, and consequently the greater the 

 vertical erosion of the main river and the headward erosion 

 of its tributaries. The divides of the smaller basins are thus 

 generally forced backwards, and such basins contracted, even 

 in the areas of the igneous rocks. This is especially notice- 

 able in the southern boundary of the Yarra Basin, and the 

 northern boundary of the Upper Goulburn Basin.' In the case 

 of the former, the rivers of the great basin of Gippsland ap- 

 pear to have forced the divide to the north ; whilst in the 

 latter, the rivers of the Murray Basin — practically all tribu- 

 taries of the Goulburn and Ovens Rivers— appear to have 

 driven the divide almost to the southern edge of the igneous 

 rocks. 



A southward-shifting divide between the Upper Goulburn 

 and Yarra Basins may explain (unless continued far to the 

 north) the southerly slopes of the ridges north of the divide, 

 which Prof. Gregory states there exist, and which slopes may 

 not necessarily be evidence of old south-flowing streams from 

 the north. 



The Relations of Gapdiner's and Main Creeks. 



Gardiner's Creek is a tributary of the Yarra, and enters 

 that stream south of Hawthorn after following a north- 

 westerly course. Main Creek runs south-westerly from Black- 



18a 



