Lava Residuals. ] 55- 



(b) The least thickness was usually along the pre-basalt ridges- 



and watersheds. 



(c) When the line of least resistance was reached cutting back 



and sapping commenced along it, and a new valley was^ 

 initiated — that of a lateral of a prospective residual; the 

 old stream, by domestic piracy, became a. tributary to this 

 new stream. 

 It is apparent, therefore, that the valleys formed on an extensive 

 lava field, though at first seemingly arbitrary as regards direction, 

 eventually carve out valleys parallel to the sub- basalt (pre-basalt)- 

 valleys, but on the watersheds between them. This is the begin- 

 ning of a process of isolation. The residuals are formed by the 

 operation of the same erosional factors as in the case of the con- 

 fined field and the evolution proceeds on precisely similar lines. 



Junction of Confined and Extensive Lava Fields. 

 Cross Lateral. 



Arising from the fact that the lateral streams of a confined lava- 

 field cutting back on the soft rocks at the edge of the infilling lava, 

 are relatively close compared to the lateral streams formed on an 

 extensive lava field, the vicinity of the junction of a confined and 

 extensive lava field is characterised by typical and important 

 changes in the conditions of erosion. It is the critical locality 

 where, when erosion has reached a certain stage, one class of lateral 

 crosses over and assumes the characteristics of the other. This is 

 usually effected by its cutting round the spur of softer rocks flank- 

 ing the confined field, ^ to the line of least resistance on the exten- 

 sive field, that is, in the direction of this spur under the lava. 



This critical locality affords perhaps the most advantageous- 

 conditions for piracy in the whole drainage system. The lateral 

 streams of both classes are in juxtaposition, and the distance- 

 between them has in places been halved — in fact, a portion of any 

 possible cross stream with piratical tendencies has been formed by 

 the crossing over of the lateral. Such a stream precipitated, say, 

 on the relative upthrow side of n fault parallel to the trend of the 

 lateral streams, or encroaching from an adjoining more powerful 

 system, finds this locality a vantage point. iVs streams of con- 

 siderable length are usually involved, the piratical stream receives 

 a powerful accession of strength. 



1 At this critical point gradient sometimes overcomes resistance and the lateral streams of 

 the confined lava fields maintain their directions. 



