150 Eobert A, Kehle : 



In the bottle-neck, however, the lava is flanked on both sides by 

 hard rocks — actually harder tlian the infilling lava; consequently, 

 all streams developed later than the lava have to seek an outlet 

 over it. The resulting retardation of the development of the- 

 system further back makes them easy captures to adjoining systems. 



A good example of the retarding effect of a bottle-neck and its- 

 consequences is the sequence of events leading up to the capture of 

 the eastern lateral — Woori Yallock River (so-called) of the Woori 

 Yallock residual. Two confined lava fields converged southwards-, 

 towards the Gembrook bottle-neck, the western one at the contact 

 of the Mt. Dandenong dacite massif and the Dandenong granitic 

 series, and the other or eastern one along the Pakenham granitic- 

 series. The converging contacts were only about a mile apart a 

 little to the south of Gembrook, where the two confined lava fields' 

 joined. The gradients of the converging lateral streams were 

 regulated by the lava in the bottle-neck over which they had to- 

 pass. Their development was comparatively slow and their 

 general levels were always higher than the adjoining system of the- 

 Middle Yarra. When the streams of the latter system were re- 

 juvenated, a tributary of the eastern lateral of the Lilydale resi- 

 dual breached or cut back round the lava occupying the western- 

 most valley of the Upper Yarra system and captured its eastern 

 lateral. (Fig. 8.) The Watts River — the upstream, portion of the- 

 captured lateral — was diverted through the breach, and the down- 

 stream portion was reversed as far as the Gembrook bottle-neck, 

 and likewise diverted through the breach. Moreover, the lateral 

 formed along the easternmost confined lava field converging 

 towards tlie Gembrook bottle-neck, has been captured by the- 

 reversed lateral of the western confined lava field. 



Ultimate confif/nration of res\<duals. 



The frequent rejuvenation of the laterals and the consequent 

 widening of the breaches tends to increase the isolation of the 

 residuals, and at the same time reduce their extent and bulk. The 

 shape sliows the degree of encroachment of erosional forces, and 

 may be illustrated by a fairly complete set of examples. (Fig. 9.) 



The greater axis of the residual almost invariably lies between 

 S 15°E. and S 15°W, suggesting that the trend of the Pre-Older 

 Basalt valleys coincides Avith a system of erosion governed by the 

 strike of the Palaeozoic sediments. The few exceptions may be 

 explained by local irregularities due to unequal resistance or the 



