Lieut. Scrivenor — Origin of Clays and Boulder-clays. 157 



a region diversified with ridges and valleys, with its own drainage 

 system independent of a mountain drainage system adjoining it. 



4. This upland plain terminates at a height of 430 feet above 

 sea-level and is of widespread occurrence. 



5. A similar plain forms conspicuous features in Cornwall and in 

 Devon, and terminates in those two counties at precisely the same 

 height as that of North Wales. This plain is not more recent than 

 Pliocene, but may be older. 



6. There is evidence that these upland plains of IS^orth Wales and 

 Cornwall and Devon were formed contemporaneously and by marine 

 erosion . 



EXPLANATION OF PLATE VII. 



Fig. 1. — Llyn Ogwen. The view shows the landlocked waters of the lake 

 with the amphitheatre of great mountains encircling it. In the middle 

 distance is the low barrier through which the waters escape in a gorge. 

 It is deeply grooved by glacial striae, which also extend up the lower spurs 

 of Carnedd Dafydd to the right. On the left of the barrier rises a valley- 

 step that separates Llyn Idwal and its plain from Llyn Ogwen. The 

 mountains in tbe background are Y Garn and Foel Goch. On both of 

 these mountains are characteristic cwms, all facing to the north-east. 



Fig. 2. — Nant Ffrancon, looking south. The valley-step is seen across the 

 head of the valley, over which the waters of Pvhaiadr Ogwen fall. In the 

 middle distance is a " roche moutonnee " ; on the right are cwms and 

 aretes, while the lower slopes of the mountain show truncated spurs. 

 The foreground consists of glacial detritus and peat which has accumulated 

 on the floor of the valley. The mountains in the distance are Y Glyder 

 Fach and Y Glyder Fawr ; on the right are Y Garn and Foel Goch. 



Fig. 3. — The " Pliocene " plain of North Caernarvonshire. This view is taken 

 from Anglesey and shows the extensive upland plain diversified with deep 

 valleys and flat-topped hills. In the foreground lies the Menai Straits, 

 here about 70 feet deep. The Snowdon range of mountains forming the 

 background rise abruptly from the plain. 



II. — The Origin of the Clays ani) Eouldku-clavs, FEDERATKn 



Malay States.^ 



By Lieutenant J. B. SCKIVENOR, M.A., F.G.S. 



SINCE the earlier edition on the geology of Kinta Avas written 

 much fresh evidence has been brought to light on the subject of 

 the origin of the clays and boulder-clays and the tin-bearing deposits 

 showing bedding at Gopeng. The effect of this evidence has not 



^ The subjoined note which accompanied this article from the author to the 

 Editor of the Geological Magazine was received on January 8, 1918, when 

 Mr. Scrivenor was leaving for France : — 



Sir, — With reference to Mr. W. K. Jones's paper in No. 287 of the 

 Quarterly Journal of the Geological Society, pp. 16.5-94 {issued November 23, 

 1917), on the "Secondary Stanniferous Deposits of the Kinta District", 

 I shall be grateful if you will publish the following article on the ' ' Origin of 

 the Clays and Boulder-clays ". This was written before I left the Malay 

 States and before I had seen Mr. Jones's paper. I note that on p. 176 of his 

 paper the latter .says that at Kacha, Tanibun, Lahat, and Papan, clays and 

 boulder-clays can be traced into partly decomposed phyllites exhibiting distinct 

 foliation. I do not remember Mr. Jones offering to show me these occurrences. 

 The section at Siputeh mentioned in the fourth paragraph of p. 177 is that 

 described by myself, and I took Mr. Jones to the mine to see it. 



J. B. Scrivenor. 



E.E. Depot, Baldock, Herts. January 7, 1918. 



