HISTORY OF THE DEE, WALES. 227 



in connexion with the angle of its fall, the more powerful is its 

 erosive force ; and this may account for the proportionately much 

 greater amount of time taken by our little river in the work of 

 excavating its valley as compared with the work done by the 

 Rhine. 



This great lapse of time also accounts for what I may call the 

 ruinous state of the old tableland that lies on either side of the 

 valley through which the Dee flows ; for in the long geological ages 

 that the region has remained as land there has been ample time, as 

 the river gradually excavated its valley to lower levels, for the 

 drainage from the north and south to cut out tributary valleys, 

 which by watery erosion got deeper and deeper in proportion to the 

 gradual excavation of the main artery of the valley of the Dee ; and 

 thus it happens that what was once a long slightly undulating 

 inclined plane has by degrees been converted into a very hilly country, 

 simply by the power of rain and running water, and, in a minor 

 degree, for a time by glacier ice. 



According to these views the sum of the whole matter is : — 



1. That after the last important disturbance of the pre-Permian 

 rocks, North Wales was carved by slow degrees and by subaerial 

 agencies into its present mountainous form, chiefly between Permian 

 times and the present day. 



2. That by far the greater part of that valley-excavating work 

 was performed between Permian and pre-Glacial times. 



3. That the work of the glaciers of the latter period some- 

 what deepened, widened, smoothened, and striated the outlines of 

 the mountains and valleys, and excavated many rock-bound lake- 

 basins, but on a grand scale did not effect any great changes on the 

 preexisting larger contours of the country. 



4. That a minor submergence of part of Britain during part of 

 the Glacial epoch produced no important effects on the large out- 

 lines of the rocky scenery. 



5. That the effects of subaerial waste subsequent to the Glacial 

 epoch have been comparatively small, simply for lack of time. 



I may also remark that the method of analysis employed in this 

 paper is, I am convinced, applicable, with variations, to the reading 

 of the history of all the more important rivers of Wales, to Cum- 

 berland, and to much of Scotland. 



Facts of a broad kind, setting forth the general principles of 

 watery erosion, have been well known ever since the days of Hutton — 

 though for long, with a large class of geologists, his views fell into 

 undeserved neglect. The subject, indeed, with some definiteness, is 

 far older than Hutton ; for it was insisted upon by Ray, who, while 

 the fourth edition of his ' Physico-Theological Discourses ' * was 

 passing through the press, was surprised and delighted to find that 

 his opinion had been anticipated by Josephus Blancanus, in his book 

 'De Mundi Fabrica,' in which he stated, among other physical 

 matters bearing on modern geology, that " our Appenine was at first 



* Edit. 4, p. 359. 



