Lava Residuals. 131 



ieads," *' high plains," or *' volcanic plateaux," are the result of 

 -differential erosion — the greater resistance to erosion offered by 

 basaltic lavas compared to the lesser resistance of the rocks con- 

 tiguous to theni. The development of a residual is characterised 

 by three readily defined cycles of erosion. In the development of 

 the Older Basalt lava residuals these cycles are a,s follows : — 



(1) Tlie Pre-Older Basalt Cycle, towards the close of which the 

 .stream system had reached a certain development (Fig. 3), with 

 graded valleys probably for the most part of the mountain region 

 type. 



(2) The Older Basalt cycle extended over the period of volcanic 

 .activity during which the valleys of the previous cycle were occu- 

 pied with lava for a considerable distance upstream, and a certain 

 height above the pre-basalt stream. The lava confined between the 

 pre-basalt watersheds of less resistant rocks, is referred to as a 

 ** conhned lava Held." At the locality down stream, where the lava 



-overflowed these Avatersheds, an '* extensive lava field " was formed. 

 The period of volcanic activity was characterised by several short- 

 lived and minor cycles of erosion at the conclusion of each flow, 

 <except that of the last one, which marked the beginning of the 

 Intermediate cycle. 



(3) The Intermediate Cycle inaugurated a new drainage system, 

 for the lava of the second cycle had obliterated all traces of the 

 previous drainage channels. The location of the new streams was 

 guided by the posititm of the least resistant in regard to the more 

 resistant rocks, and the direction of tlie gradient. Both of these 

 conditions were fulfilled at the edges of the confined lava fields; con- 

 sequently *' lateral streams " (Fig. 1) started to cut back along 

 the.^o edges upward from the line of junction of tlie confined and 

 •extensive in lava fields. 



Below this line of junction the new streams were compelled to 

 carve out valleys on the resistant basaltic lava and assumed direc- 

 tions quite irrelevant to the suljmerged watersheds. ^ The water- 

 sheds, however, being covered by the least thickness of lava, and, 

 consecpiently, the liiie.s of least resistance, were eventually exposed 

 by vertical erosion. (Fig. 2.) Tributai'ies commenced to cut back 



!RedsU)neHill Emu C"^ Resutuai S.iltivatcrR fmivpc H.ii 



i ^ 



Sandstones ■ ^^^^rj^f^ 



Fig. 2. 



Section of a residual devel<ipwl from an extensive Iii,v:i field, north of Bulla. 



