Lava Residuals, ] 39' 



It is not known, however, whether the clays between the lava 

 flows Avere residuaU or transported, or whether they represent the 

 Avliole or only part of them. These and other pertinent considera- 

 tions arise, but the inference seems tolerably safe that the period 

 of volcanic activity was comparatively short compared with that 

 of the Intermediate cycle between the Older and Newer Basalt. 

 From such a point of view these phases of volcanic activity are of 

 great stratigraphical and physiographical value, marking, as they 

 do, the termination and inception of three great cycles of erosion 

 — the Pre-Older Basalt, the Intermediate, and the Post Newer- 

 Basalt. 



Some stress has been placed on the use of such terms as '' older 

 basalt " and '' lower newer basalt," but I venture to think, especi- 

 ally as the same flow is designated by both symbols, that only one- 

 period is meant. The use of the two symbols has probably arisen 

 from the points of view of the two observers, one of whom con- 

 sidered it to belong to the close of the Miocene and the other to- 

 the beginning to the Pliocene. 



Mr. Jutson2 thinks that there is some reason, on lithological 

 grounds, for establishing an intermediate period of volcanic 

 activity. The complications arising from the assumption of an 

 intermediate flow are referred to in another part^ of this paper. 



||._Evolution of a Residual from a Confined Lava Fields 



Stages of Evolution. 



At the beginning of the Intermediate cycle the valleys towards 

 their headwaters contained long and narrow^ lava fields flanked by 

 watersheds of less resistant rocks. The streams of the two previous 

 cycles were submerged beneath the Older Basalt so that the drain- 

 age of the Intermediate cycle was forced to seek new outlets. The 

 development of the new drainage system was guided by two factors, 

 namely, the direction of highest gradient, and the line of least 

 resistance, conditions that were at once fulfilled by the compara- 

 tively unresistant rocks at one or both edges of the lava. 



The resistance of the rocks flanking a residual affords a means 

 of classifying them. They severally belong to one of the following- 

 stages : — 



1 Mr. Ower, Assistant Boring: Engineer of the Geological Staff, assures me that the inter- 

 calated clays in the P'linders and Cape Schanck Lores were residual. 



2 Vide Bibliog., No. 13. 



3 Vide p. 143, post. 



