Lava Residuals. 



147 



rsliowing that the lava has lost its grip on the soft, underlying sedi- 

 ments. The minor laterals are also the channels for the drainage 

 flowing down the steep slopes of the residual; the combined source 

 gives a supply of water that lasts throughout the driest seasons. 



When a residual has one lateral approaching maturity more 

 rapidly than the other, the weaker one is sooner or later captured 

 by the stronger one. For a certain distance downstream from the 

 breach, the weaker residual is reversed. The eastern residual of 

 ,the Lilydale residual is a case in point. (Fig. 5.) The hard 

 .quartzites in the vicinity of Cave Hill so retarded the foi-mation of 

 the eastern lateral, that Brushy Creek, the western lateral, cap- 

 :tured it near where the Yarra flows across the north end of the 

 residual. The eastern lateral valley is now occupied by three dis- 

 .tinct streams, namely, Steel's Creek, flowing south through the 

 breach, Olinda Creek (wliich at one time had its source in the 

 t€ave Hill quartzites) flowing north, also through the breach, and 

 the Mooroolbark Creek, flowing both north and south on the south 

 side of the Cave Hill (juartzites. It is probable that the eastern 

 lateral of the Lilydale lesidual never at any time assumed the 

 •characteristics of a strong lateral, owing to the hard flanking 

 .dacite rocks of the Mt. Dandenong massif. 



^MIDDLE/ 



/ UPPER YARRA 





CemHrook Bott/e <Vcc^ 



Fig. 8. 



The accession of strength by breaching and capture to the head- 

 waters of what was originally the weak lateral, is consideral)le ; 

 the stronger lateral also benefits by the capture. The breacli l)e- 



lU 



