2 8o S. S. BUCKMAN [april 



area of secondary rocks drained by the Thames appears to have been 

 carved out of an original plain which cut across the edges of the 

 strata. It would be a plain of diverse stratal composition. 



But when the area was first upraised, there must have been a 

 plain of uniform stratal composition. For short these two plains may 

 be called " a uniform plain " and " an outcrop plain." The outcrop 

 plain has been formed by the removal of the superincumbent strata. 

 But how ? 



The usual idea is marine denudation. Professor W. M. Davis 

 argues against this (4). He considers the outcrop plain to have been 

 formed from the uniform plain by subaerial denudation — that the 



South Carney 



Fig. 6. — Restoration of Thames drainage before capture by the Frome. Elbow of capture of 



Churn by Upper Thames at A. 



rivers cut the country down to a low level : he calls this the first 

 cycle of denudation. The low level plain was subsequently upraised 

 to become a new high level plateau. It is about 700 feet in the 

 Chilterns and about 1000 feet in the Cotteswolds. After the upraising 

 the rivers commenced to cut down afresh — it was the second cycle of 

 denudation. 



To me the difficulty seems to be this. Before an area could be 

 worn by river-denudation to a low level of little relief, the third phase 

 of river -development would have reached its possible maximum: a 

 most important system of obsequent streams would have been 

 developed, and they should be as long or even longer than the be- 

 headed consequent streams. 



These important obsequent streams are not found. Even at the 

 present day the obsequent streams are not nearly so long as they may 

 be expected to become in order to establish that drainage equilibrium 



