Lava Residuals^ 143 



Fig. 5 shows the erosion succeeding, three consecutive lava flows 

 with a comparatively short period between them. The sam.e 

 amount of lava is supposed to be represented by each flow\ INote 

 the " turtle shape " (to use Button's! term) of the residual. 



Now let us suppose that another series of lava flows has welled out 

 .and occupied graded lateral valleys initiated at the close of the 

 ^Older Basalt. This is so as regards the Melbourne residual (Fig. 

 49), where the Newer Basalt has occupied valleys belonging to the 

 Intermediate cycle and initiated a new cycle — the Post Basalt 

 '<?ycle. This new cycle has proceeded on the same lines as the Inter- 

 mediate cycle and lateral streams, namely the Merri and Moonee 

 Pondsi Creeks, have been formed at the edges of the confined Newer 

 Basalt field. The western edge of the residual is flanked by tlie 

 Newer Basalt extensive lava field, and the Saltwater River is in the 

 unique position of being a lateral to an extensive lava field formed 

 •on the little resistant decomposed lava of the Older Basalt. 



If an intermediate phase of volcanic activity had occurred the 

 Newer Basalt instead of occupying the lateral valleys of the Older 

 Basalt residuals, would be occupying the lateral valleys of this 

 •supposed intermediate lava, and a complicated system of erosion 

 would have resulted at the inception of the NeAver Basalt cycle. 



Main lateral streams. Beginning of the Intermediate cycle. 



Erosion commenced simultaneously on the softer rocks flanking 

 the confined lava field and on the lava field itself. The resistance 

 -of the latter, however, soon threw (Fig. 6) the incidence of erosion 

 'On to the less resistant flanking rocks, where it will remain until 

 the residual disappears by lateral planation. The streams that 

 formed on the lava field were quickly captured by the lateral 

 streams on the less resistant rocks; the indented configuration, ex- 

 hibited by many residuals in plan, is due to the cutting back of 

 these captured streams. Until the lateral streams assumed a 

 mature aspect, the talus boulders broken away from, the lava by 

 sapping had a powerful abrasive effect in corrading and gouging 

 out the lateral valleys. As, however, the talus slopes became 

 graded, much of their force was expended on the reduced talus 

 slope. Talus bouldei's may be seen at the edge of the lava at 

 Hai-kaway wliere, from large angular ones at the edge and smaller 

 ■i^ubangular ones down tlie slope, they ultimately become small 

 and rounded still further away. 



1 Vide Bibliog., No. 'I. 



