508 I'EOF. \V. J. PUGlt ON THE GEOLOGY OF THE [vol. Ixxix, 



19. The Geology of the Disteict around Coeeis and Abeblle- 

 fenni (Merionethshire). By William John Pugh, 

 O.R.E., B.A., F.G.S., Professor of Geology in the University 

 College of Wales, Aberystwyth. (Read May 16th, 1923.) 



[Plate XXVII— Map.] 



Contents. 



Page 



I. Introduction; Physical Features 508 



II. History of Previous Research 509 



III. General Succession and Structure 512 



IV. Detailed Description of the Beds 513 



V. Detailed Description of the Structure 531 



VI. Comparison with other Areas 537 



I. Introduction. 



The area described in this paper lies south-east of the main 

 mass of Cader Idris, about the villages of Corris and Aberllefenni. 

 The north-western boundary is the great fault- valley of Tal-y-llyn, 

 lying at the foot of Cader Idris, and along which runs the road 

 from Towyn to Dolgelly, over the picturesque pass called Bwlch- 

 llyn-bach. Tbe south-eastern boundary lies parallel, at a distance 

 of some 3 or 4 miles. The area extends from the high ground 

 about Taren y Gesail in the west to Mynydd Dolgoed and Waen 

 Oer in the east, a distance of about 6 or 7 miles. 



The district lies mainly within the county of Merioneth, but a 

 part extends over the border into Montgomeryshire. It is included 

 in the 1-inch Ordnance Survey Maps,- Sheets 149 & 150, in the 

 6-inch maps of Merionethshire, Sheets XLII, ISLE. & S.E., XLIII, 

 N.W., XXXVII, S.E., XXXVIII, S.W., and in the Geological 

 Survey Map, Old Series, 1-inch Sheet 59, N.E. 



The area is drained almost entirely by tributaries of the River 

 Dyfi (Dovey), which lies some 4 or 5 miles south of Corris. The 

 most important stream traversing the area is the River Dulas, 

 which passes through the villages of Corris and Aberllefenni. It 

 is along this valley that the main road from Machynlleth reaches 

 Corris and Aberllefenni, and it is also followed by a light railway 

 which affords alternative communication between the above- 

 mentioned localities. 



The region is one of some economic importance, and contains 

 important slate- and slab-quarries, which have been worked for a 

 great number of years ; it is stated that the quarries at Aberllefenni 

 were opened about the year 1500. 1 The area forms part of an 



1 D, C Davies, ' A Treatise on Slate & Slate- Quarrying ' 1S78, p. 64. 



