Lava Residuals. 149 



«of the Goulbuin are the only rivers in Victoria that have valleys 

 with a distinctly westerly trend. The Latrobe, on the other hand, 

 is the only river with a decidedly easterly trend. The three 

 streams mentioned undoubtedly owe this peculiarity to their com- 

 posite character ; they consist of portions of laterals and cross 

 tstreams pi-eced together. As evidence of the great strength of the 

 breaching streams, it is suggestive that the Mt. Leckie residual 

 trends east and west parallel to the cross streams of the Upper 

 Yarra and Latrobe. The ''high level lead" beneath, I am in- 

 formed by a miner who has prospected it, falls to the south, which 

 is to be expected from the trend of the confined lava field. 



Residuals with resistant rocks on one side. 



It has been observed under the heading of resistance, that the 

 .more resistant rocks of the areas are those altered at the contact of 

 the Palaeozoic sedimentary and igneous series. The relative resist- 

 .ance of the altered rocks decreases aw^ay from the actual contact 

 until the normal sediments are reached ; there is therefore in the 

 metamorphic zone, a gradation from the most resistant to the least 

 resistant rocks. Where two contacts converge and are in close 

 proximity a bottle-neck is formed, and all streams between the 

 •converging contacts are forced to seek an outlet through this 

 bottle-neck. The Flinders bottle-neck was the main outlet for 

 the Pre-OIder Basalt streams of the Western Port system. It was 

 formed by the converging contacts of the Arthurs Seat and Pyra- 

 mid Point granite series. Tlie Gembrook bottle-neck was formed 

 by the converging contacts of the Dandenong and Pakenham 

 'granitic series. 



The tendency of some Pre-Older Basalt streams was to cut back 

 along the normal sediments immediately outside the aureole of 

 metamorphism; as a consequence one side of their valleys was 

 flanked by rocks that increased in resistance until they were 

 ■among the most resistant rocks of the area, w^hile those on the 

 other side were among the least resistant. At other localities hard 

 •dacites are on one side of the valley and soft sediments on the 

 other. In the development of laterals after a valley of this kind 

 has been occupied by lava, the erosion of the lateral on the hard 

 rocks is retarded or suppressed, but tlie lateral formed on the 

 •soft rocks on the other side develops quickly. This is due to the 

 fact that the latter is the outlet for the whole of the drainage of the 

 infilled valley. 



