﻿76 



EIYEE-GORGES IN CORNWALL AND DEVON, [vol. lxxii. 



in the glaciated and in the unglaciated districts. Respecting the 

 mature upland topography, the speaker thought that in Cornwall 

 the substantive evidence of the cliff-line at the 430-foot level, and 

 the marine accumulations at St. Erth's and other places below that 

 level, went far towards establishing the platform of marine erosion 

 to which the Author had referred. Farther north along the coasts 

 of Merioneth and Carnarvonshire, of which the speaker had special 

 knowledge, the evidence was not so clear, and it seemed to him 

 that there the Late Tertiary marine platform must be looked for 

 at a much lower level. Nevertheless, in that same district of 

 North Wales, the level to which the new coastal streams have cut 

 back and rejuvenated their courses rises from about 200 feet along 

 the coast of the Western Lleyn to 600 feet at the head of the Vale 

 of Ffestiniog. To the speaker this suggests that the mature 

 topography above the angle of rejuvenation is the surface of sub- 

 aerial denudation, the planation of which has never been completed. 

 In his opinion, neither the 300-foot nor the 500-foot level, sug- 

 gested by Mr. Greenly for the platform in Anglesey or Northern 

 Carnarvonshire, could be established as a surface of marine plana- 

 tion, but both must be considered as incidental in tbe general rise 

 of the margin of the rejuvenated region as the streams have worked 

 their new ravines back into the hills. 



In endeavouring to express growth and maturity of topograph} 7 

 in terms of stratigraphical chronology, it would be well to bear in 

 mind how much the rate of denudation depends upon the hardness 

 of the rocks. Certainly the rocky ravines of North Cornwall are 

 voung, almost in their infancy ; but if, instead of the hard 

 Palaeozoic killas and volcanic rocks, the rejuvenated streams liad 

 worked in such unconsolidated sands and clays as those that 

 compose the Tertiary and Mesozoic formations of the East Coast 

 of England, it might well be that ahead} 7 the district would again 

 have become degraded almost to a peneplain. 



Mr. G. M. Part drew attention to precisely the same phenomena 

 of gorges dissecting the edge of a marine plane of denudation that 

 are to be seen on the West Coast of Scotland, and expressed the. 

 hope that it would one day be possible to correlate the planes 

 of Cornwall, those of Wales mentioned by Mr. Greenly and 

 Prof. Fearnsides, and others traceable all round the coasts of 

 Britain. 



[May 8th, 1917.] 



