Lava Residiiais, - 153 



•due to differential erosion. Although, for example; the Dande- 

 nong-Cape Schanck fault may have been, to some degree, respon- 

 sible for the encroachment of the streams belonging to the Port 

 Phillip system into the Western Port area in the vicinity of Narre 

 Warren and Cranbourne, by far the greater factor was the differen- 

 tial erosion of the two systems. The hard lava in the Flinders 

 bottle-neck, over which all the drainage of. the Western Port system 

 had to pass, so retarded its development that the comparatively 

 rapid development of the Port Phillip system made capture rela- 

 tively easy. The Wurunjerri Rangs'i being compos -d of soft sedi- 

 ments in the vicinity, Dandenong was the most probable locality 

 for breaching. 



411. — Evolution of a Residual from an Extensive Lava 



Field. 



The Western Newer Basalt Lava Field. 



On the extensive and comparatively young Newer Basalt lava 

 field of Western Victoria, many of the drainage channels still fol- 

 low the courses assumed by them at the cessation of volcanic activity 

 and their disposition in relation to the softer rocks beneath the 

 lava is quite arbitrary. Thei'e are areas, however, where the 

 .softer sediments along the Pre-Newer Basalt watersheds have been 

 ■ exposed by vertical erosion and where sapping has commenced 

 .along the line of least resistance, that is along these watersheds. 

 A portion of the lava plain north of Melbourne affords a typical 

 example. The Saltwater River has sapped its valley along the old 

 watershed between the Pre-Newer Basalt valleys coming from the 

 • directions of Rom,sey and Wallan, and joining in the vicinity of 

 EuUa. Both the Saltwater River and Emu Creek will ultimately 

 become laterals (Fig. 2) to a residual, the protective covering 

 of which is lava belonging to the Newer Basalt. In the central 

 Avestern area of Victoria there are many examples of confined lava 

 fields belonging to the newer volcanic series, where laterals are 

 ^cutting back on the softer rock along their edges, and all the 

 erosional factors are operating to produce the residual just as they 

 have in the Western Port area from the older lavas. The confined 

 lava plains of the Avestern area extend northwards for many miles 

 but southward for a limited distance to where they join the exten- 

 sive lava fiekl of Western Victoria south of the present Divide. 

 The circuitous line marking this junction is where what is here 



Vide p. 157, post. 



