154 Robert A. Kehle : 



termed the " cioss-laterar'i will form, and is the probabler 

 direction of a great valley to be developed perhaps under the same- 

 condition as that much earlier valley referred to by Prof. Grejjory'^ 

 as the Great Valley of Victoria.. 



The residual is the outcome of the erosional processes on both 

 the extensive and confined lava fields, but peculiar considerations^ 

 afiteot the transition from each type of lava field. In the case of 

 the confined lava field (Fig. 1) the lateral streams are formed on 

 the softer rocks at the edge of the lava, and relatively near the- 

 infilled stream.; but on the extensive lava field (Fig. 2), the lateral" 

 streams may form along uncovered watersheds anywhere between? 

 the infilled streams. In other words, from a confined lava fields 

 laterals form comparatively close parallel valleys, and the result- 

 ing residuals are restricted in size, while the laterals developed orr 

 an extensive lava field are usually far apart, and extensive resi- 

 duals are the result. Moreover, the flanking softer j-ocks exposed 

 by a lateral evolved from an extensive lava field are always below^ 

 the lava covering of the residual, which is not the case in the- 

 development of a residual from a confined lava field, where, on the- 

 opposite side of the lateral valley to the residual, they are both 

 above and below the level of the lava. 



Apart from these and other minor distinctions, striking paral- 

 lels in development are forthcoming by a comparison of the stilF 

 youthful erosion of the Avestern cycle from the newer volcanic 

 series, and the mature erosion of the eastern cycle from the older 

 volcanic series. 



Development of a Residual from an Exfensive Lava Field. 



Streams that formed wholly on an extensive lava field flowed in- 

 the direction of the gradient which, according to Harker,'^ is very 

 slight. Although their initial directions were dominated by this' 

 gradient, their orce seemingly aimless courses were due to such 

 factors as sag, the edges of flows, faults, etc. When, however, one- 

 of these streams had cut down to the underlying less resistant rock, 

 the factors that governed its development may be summed up in the- 

 following : — 



(a) They cut back along the line of least resistance, i.e., along- 

 the line of least thickness of overlying lava. 



1 Vide p. 155, post. 



2 Bibliog.. No. 4 



3 Vide Biblioy:., No. 6. 



1 



I 



