Physiography of Varra, etc. 495 



Both the arnis referred to ultimately reached in their head 

 waters large areas of igne»ous rocks (with the exception of a 

 narrow strip of silurian at the head of one of the branches of 

 the Woori Yallock Creek, and which has apparently not 

 favoured the latter, jDrobably on account of a struggle with a 

 south-tlowing stream). Tbese igneous rocks, from their great 

 hardness, would act as a check upon the erosive activities of the 

 Woori Yallock Creek and Yarra Rivulet. Meanwhile the War- 

 burton tributary was steadily cutting its way between the 

 dacite and granodiorite of that district. Behind (to the east 

 of) these igneous rocks was a great area of silurian country, 

 which if completely tapped, would grea"tly add to the volume 

 of the stream. Tlie igneous bar at Warburton is very narrow, 

 compared with the width of the other areas of igneous rocks 

 in the Yarra Basin. This gave the Warburton tributary an 

 immense advantage over the other streams that were strug- 

 gling for the predominant position. The bar was cut through, 

 leaving the present narrow gorge at Warburton, and the river 

 then rapidly eroded backwards. AYhile the gorge was being 

 eroded in the igneous rocks, lateral erosion was proceeding 

 in the silurian area farther up-stream. This has continued, 

 until, at the present time, there is the narrow gorge in the 

 igneous rocks, with the short torrential mountain streams, 

 whilst up-stream we have the more open valley with long, 

 well-graded tributary streams. 



The Warburton tributary therefore gained such an acces- 

 sion of strength that it rapidly became the present main 

 stream, with its outpaced competitors subordinated to it. 



The history here outlined probably commenced in the cycle 

 of erosion prior to that originated since the uplift of the 

 Xillumbik Peneplain. 



Prof. Gregory has suggested that the Acheron and Watts 

 Rivers were originally one stream, passing through the Beenak 

 Gap, and that the Acheron part was captured by the Goulburn, 

 whilst the Watts and its southern continuation were captured 

 by the Yarra. Tliis question is outside the scope of the present 

 paper, but it may be remarked that this could only have taken 

 place when the whole country was at least as high as the 

 Beenak Gap. This is probably what Prof. Gregory refers to 



