526 The Dee. 



consisted of a mass of Silurian rocks, great part of 

 which has since been removed by denudation. 



In my opinion this region of North Wales has never 

 been depressed beneath the sea since the beginning of the 

 Permian epoch, excepting in part during a short episode 

 in Giacial times (see p. 413). During that long lapse 

 of geological ages, there was therefore ample time for the 

 action of all the ordinary processes of subaerial denu- 

 dation, the most powerful of which is the action of rain, 

 rivers, and glaciers, and thus it happened that the Dee, 

 a river of very ancient date, wandering hither and 

 thither, by degrees deepened its channel in the same 

 manner that the Rhine and the tortuous Moselle have 

 cut out theirs, as described in my memoir ' On the 

 Physical History of the Valley of the Rhine.' While 

 this process was going on, minor tributary valleys were 

 cut by rain and rivers in the tableland to right and 

 left of the great main channel, and other smaller 

 rivers in adjacent regions playing the same general part, 

 this wide tableland of marine denudation was gradually 

 turned by the scooping out of unnumbered valleys, 

 into a region of hill and dale. 



The Vale of Clwyd is of extreme antiquity, for it 

 was a valley before the deposition of the New Red 

 Sandstone, and it may be that the Clwyd has flowed ever 

 since the end of the Triassic epoch, and the Conwy like 

 the Dee is at least as old. 



I cannot pretend to give a detailed account of the 

 river systems of Scotland. My personal knowledge of 

 the subject is less minute, and however minute it might 

 be, the subject is difficult. 1 Something of the subject 



1 Professor Geikie, who fully realises the difficulty of the 

 subject, nevertheless enters into it and explains it, as far as his 

 present knowledge will allow, in his work, the ' Scenery and Geology 

 of Scotland.' 



