494 ./. T. J at son: 



is well to bear in mind the possible service that physiography 

 may render. 



The Gpowth of the Yappa. 



A glance at a map of the Yarra and its tributaries will show 

 that the present main stream consists of various parts which 

 have not always formed sections of the principal river of the 

 area. From the nature and disposition of the rocks and the 

 general configuration of the countrj^ the growth of the Yarra 

 would appear to be as outlined below. 



As far up stream as Healesville, the Yarra is simple, and 

 since the inception of the present topography — i.e., since the 

 formation and uplift of the Nillumbik Peneplain — it has ap- 

 pai'ently been the main stream. At Healesville there is a 

 sudden break. From a dii'ection east and west, it changes 

 to one north and south. Tbe latter is not its natural continua- 

 tion. The Watts River is really this. Tlie present Yarra from 

 Healesville to its junction with the Woori Yallock Creek, to- 

 gether with the latter, was no doubt at one time a tributary, 

 but as it developed more rapidly than the Watts, the latter be- 

 came the tributary and the former the principal river. 



The reason of such greater development is obvious. The 

 Watts River had the hard rocks of the dacite and granodiorite 

 area to the north and north-east of Healesville to erode, and 

 the Acheron, a strong affluent of the Goulburn, cutting back 

 towards the Watts through soft silurian strata. Tte southern 

 tributary of the Yarra had a wide and long area of similar 

 «ilurian rocks to work upon. Hence erosion here was rapid, 

 and the stream quickly cut its course backward and sovith- 

 ward. Tlie present Woori Yallock Creek was one of the upper 

 arms of this stream, the other being the existing Yarra. east of 

 the Woori Yallock junction to the Yarra Rivulet (or Little 

 Yarra) and the latter itself. Most of this second arm is also 

 in silurian country. 



The present Yarra for some distance east of the Yarra 

 Rivulet would be a weak tributary of this second arm, its 

 weakness being due to the bar of hard igneous rocks at War- 

 burton through which it would take long to cut. 



J 



