Lava Residuals. 163 



The relative resistance of the rocks of the area is tabulated, and 

 the governing factors referred to. 



Short cycles of erosion during the Older Basalt cycle are con- 

 sidered, and the possibility of their having continued for a rela- 

 tively long period is negatived. The possibility of an Intermediate 

 phase of volcanic activity between the Older and Newer Basalts is 

 .also negatived. 



The initial stage of the Intermediate cycle involving the develop- 

 ment of the lateral streams is described. Their development is 

 shown to be governed by the concomitant factors of gradient and 

 resistance; the gradient was regulated for the most part by the 

 -extensive lava field and infilled tributaries. 



The formation of minor laterals is explained. The action of 

 their head-waters is the prelude to the breaching of a residual. 



The development of the cross stream is accompanied by the cap- 

 ture and reversal of the opposite lateral. The development of 

 cross streams is adduced to explain the composite character of the 

 Yarra. 



The development of residuals w4th resistant rocks on one flank 

 is enlarged on. The profound effect of bottle-necks on the subse- 

 quent development of the Western Port system is indicated. 



The ultimate configuration of residuals is described and illus- 

 trated. 



The '^ uncovered residual" or the spur or range left after the 

 removal of the lava is shown to afford a physical connection between 

 -existing residuals. 



Residuals are shown to be isolated by the width of the valleys 

 of the cross streams. 



The piracy of certain streams belonging to the Western Port 

 system -by the adjacent Port Phillip system is considered to be due 

 more to differential erosion than to faulting. 



The development of a residual from, an extensive lava plain may 

 be more conveniently studied on the Newer Basalt lava field. 



The stream development on an extensive lava field is primarily 

 governed by such factors as sag, the edges of flows, faults, etc., 

 and their trend is quite arbitrary as regards the submerged water- 

 sheds. 



The distinguishing differences between a residual developed 

 from a confined and one developed from an extensive lava field are 

 indicated. 



12a 



