286 PROF. W. MORRIS DAVIS ON [Aug. I9O9, 



methods of stream-analysis. The peculiar indifference of topo- 

 graphic form to the trend of formation-boundaries is only another 

 means of exhibiting the insequent arrangement of the valleys. 

 The insequent stream-arrangement is, however, precisely what 

 might be expected as the result of prolonged erosion upon a mass 

 of complicated and generally resistant structure. Neither the 

 ridges nor the streams exhibit any close obedience to structural 

 control ; the ridge-crests run in all directions in relation to rock- 

 structure : — with, oblique to, or square across the strike of the 

 rocks ; the valley-lines are similarly indifferent ; the cwms are 

 opened in various formations ; their head-cliffs, their floors and 

 their front steps seem to occur about as often within a single 

 formation as at the passage of one formation into another, and to 

 stand across the strike of the rocks about as often as along it. This 

 generalization will be of importance in the attempt, made on a 

 later page, to restore in a general way the pre-Glacial forms of the 

 Snowdon district. It is not, however, intended to assert that no 

 relation is to be found between structure and form. A close 

 acquaintance with the district would probably discover many minor 

 examples of coincidence between strong structures and ridges, and 

 between less strong structures and valleys ; yet, on the whole, the 

 Snowdon area shows these coincident relations only in a subordinate 

 degree. It is, therefore, in strong contrast with such examples as 

 the Jura Mountains, or still better the Pennsylvanian Appalachians, 

 where the alternation of strong and weak formations has been 

 absolutely dominant as a guide in the erosion of existing forms, 

 and where geological structure is largely revealed in topographic 

 relief. It seems, therefore, safe to look upon the Snowdon district 

 as consisting on the whole of resistant rocks, and through such 

 an introduction the main question of this paper may be approached. 



VI. General Erosion of the Snowdon District. 



There is.no longer any dispute as to the conclusion that the hills 

 and mountains of Wales are the result of long-continued erosion 

 upon a once larger mass. The detailed evidence so forcibly 

 presented in favour of such a conclusion by Sir Archibald Geikie 

 in his ' Scenery of Scotland ' for the Scottish Highlands may 

 be applied with equal force in the case of the Welsh mountains, 

 where the way was long ago made ready for it by Ramsay's great 

 essay on the 'Denudation of South Wales' (1846). Indeed, both 

 in Scotland and in Wales, the evidence in favour of several more or 

 less complete cycles of erosion is so convincing, and the proof of 

 deep denudation is so complete, that little is gained in the study 

 of the present or of the pre-Glacial forms of either country by 

 trying to restore the long-lost earlier forms due to deformation. 

 One may here appropriately quote Playfair, who, in describing the 

 coast of South-Western England, recognized that the present outline 

 resulted from the action of ' wasting and decay ' on the original 



