68 E. Greenly — Denudation in North Wales. 



to trace any of the principal nervui-es to their source, but the wing; 

 is otherwise in a good state of preservation. 



It will be placed, provisionally, in the genus Fouquea, Ch. Brongn.^ 

 and, in order to note the principality in which the wing was found, 

 I propose the name Fouquea cambrensis. 



The specimen has been presented to the Geological Survey 

 Museum, London, by Mr. Eoblings. 



VI. — Kecent Denudation in IS ant Ferancon, Nokth Wales. 

 By Ed-vvakd Greenly, F.G.S. 



READERS of this Magazine may remember that early last 

 August there were descriptions in many provincial and even 

 in some London newspapers of an extensive ' landslip,' which had 

 occurred on the side of the mountain called Carnedd Dafydd, on 

 the eastern side of the valley of Nant Ffrancon, in North Wales. 

 The impression conveyed was perhaps somewhat exaggerated, and 

 yet the phenomenon was on a scale quite large enough to be of 

 geological importance. 



In a brief but vividly written article in the Geological Magazine 

 for January, 1900, my friend Mr. J. R. Dakyns described a number 

 of cases of denudation on an important scale that had come under 

 his observation. It may be well, therefore, in the same way, and 

 under a similar title, to preserve a record of this landslip. There 

 was almost incessant rain from the 5th to the 10th of August, with 

 streams all in heavy spate and floods in many districts, and on 

 August Gth (I believe) at about 4 p.m. two torrents broke out on the 

 side of Carnedd Dafydd, carrying with them a great deal of debris, 

 and blocking the road in the valley for many yards. The spot is 

 on the eastern side of the valley, nearly opposite the house called 

 Pentre, shown on the Geological Survey and old Ordnance Maps. 



The mountain side here is composed of the Bala volcanic series, 

 alternations of various igneous rocks with hard grits, resting, with 

 a south-easterly dip, upon a thick mass of softer and rather homo- 

 geneous black slates. The volcanic series form a great range of 

 crags along the brow of the mountain some hundreds of feet in 

 height, cut into huge buttresses and deep recesses, while the dark 

 slates give rise to long uniform steep slopes extending from the 

 foot of these crags to the bottom of the valley, and along theni 

 the road is carried at a height at this point of about 200 feet above 

 the alluvial plain. These slopes are covered with great sheets of 

 scree, resting upon loose glacial debris, which, though rising here 

 and there into moraine-like mounds, have for the most part, and at 

 the point where the landslip occurred, a pretty uniform slope. 



As we pass up the valley from the north we see nothing but two 

 streams of stones, grey and fresh-looking, near the road. They do not 

 extend far up the slopes, and appear, indeed, rather insignificant; but 

 this is due to the great depth of the valley, and when we arrive at 

 the place where they cross the road they are much more imposing 

 in appearance. From this point we see that they are fans of debris 

 spread out at the ends of two long channels, which, light grey and 

 evidently quite newly cut, are conspicuous features all down the 



