Lava Residuals. 141 



(a) Residuals due 'to the erosion of rocks of varying resist- 



ance flanking the protective lava beds, the rocks on one 

 flank being a little more resistant than the lava. One 

 lateral valJey has formed, as in the Gembrook residual 

 (Fig. 4'^), but the other will subsequently form, or is in 

 process of formation, as in the Woori Yallock residua 

 (Fig. 43). 



(b) llesiduals where the flanking rocks are more resistant on 



one side, but somewhat less resistant than the lava. 

 Erosion is facilitated on one side and retarded on the 

 other ; consequently one lateral valley approaches maturity 

 more rapidly than the other. The Upper Pakenhanx and 

 Lilydale residuals are examples (Fig. 4^ & 6). 



(c) liesiduals due to the erosion of relatively feebly resistant 



rocks flanking the hard lava. If the flanking rocks are of 

 relatively uniform resistance the lateral valleys are of 

 equal importance. The Berwick residual is typical of 

 this stage. (Fig. 4.ii). 

 From the fact that the resistance of the flanking rocks is always 

 variable, even in the case of a residual developed under the con- 

 ditions outlined for stage (c), all pass successively through the 

 above development stages. Concisely the three stages may be taken 

 to represent the orderly evolution of a residual from a confined 

 lava field, and are successive stages in the process of denudation, 

 the object of which is to reduce the lateral streams to base level,, 

 and, incidentally, remove by lateral planation the lava protection. 



Relative Resistance to Erosion. 



In descending order of decreasing resistance to erosion, the- 

 rocks of the Western Port and Port Phillip areas 'may be tabulated 

 as follows : — 



Firstly — Metamorphic rocks at their actual contact with. 



igneous rocks of Palaeozoic age. 

 Secondly — Volcanic rocks. 



(a) Dacite, andesite, etc., of Palaeozoic age. 



(b) Lavas of the Newer Basalt. 



(c) Lavas of the Older Basalt. 

 Thirdly — Plutonic rocks. 



Granites, granodiorites, syenites, etc., of Palaeozoic- 

 age. 



