Lava Residuals. 



145 



broken with falls, rapids, and shallows, particularly where the 

 tributary confined lava fields joined the main confined lava field. 



The added force to the main lateral, once an infilled tributary 

 fiad been breached, was considerable, and accelerated breaching- 

 further back. The breach soon widened, and the detached tribu- 

 tary field became more and more isolated. Many detached and 

 isolated residuals of small dimensions have had the relation of an 

 infilled tributary to the main confined lava field. The best examples? 

 are to be found to the east of Pakenham and the north of Gem- 

 brook. They have not, as far as I am aware, any particular 

 names. 



Where a tributary of relative importance joined the main 

 stream, particularly tow^ards its head waters, the lateral streams 

 on the outer edges of the lava became the main laterals, and those 

 on the inner edge were suppressed or became tributaries. (Fig. 7.) 

 This has probably occurred in the vicinity of Broadmeadows, where 

 two Pre-Older Basalt streams had their confluence, one from the 

 direction of Romsey and the other from Wallan. 



Fig. 7. 



Lateral valleys at the confluence of infilled valleys. 



Minor laterals. 



The erosion of minor laterals on the talus slope between the main 

 lateral and the receding edge of the lava is an important and 

 characteristic feature. They commence when and where the talus- 

 slope has assumed such a low gradient that underground and 

 meteoric waters cut in a diagonal direction across it; they are 

 particularly apt to form where the residual is altering its general 

 trend, and have a tendency to attack the residual on both its con- 



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