254 PROF. A. H. COX AND ME. A. K. "WELLS ON THE [vol. lxxvi,. 



The Lower Palaeozoic Bocks of the Arthog-Dolgellet 

 District (Merionethshire). By Prof. Arthur Hubert- 

 Cox, D.Sc, Ph.D., F.G.S., and Alfred Kingsley Wells„ 

 B.Sc, F.G.S. (Read March 10th, 1920.) 



[Plates XVI-XX.] 



Contents. 



I. Introduction 254 



(a) Physical Features of the Area. 



(b) History of Previous Research. 



II. The Stratigraphical Succession 257 



(ft) Upper Cambrian. 

 (b) Ordovician. 



III. The Intrusive Rocks 280 



(a.) The Diabases. 



(b) The Granophyres and Related Rocks. 



(c) Age of the Igneous Rocks. 



IV. The Tectonics 305 



V. Comparison with other Areas 315 



VI. Summary and Conclusions 318 



I. Introduction. 



The district to be considered in this paper is situate on the 

 southern flank of the Harlech Dome, and is underlain by strata of 

 Upper Cambrian and Lower Ordovician age. The area is bounded 

 on its northern side by the Mawddach Estuary, which separates it 

 from the Harlech-Barmouth area of Lower Cambrian rocks. On 

 its southern side the whole district is overlooked by the great 

 escarpment of Cader Idris, from which, however, it is separated by 

 a long straight fault-valley — the Llyn Gwernan Valley. The 

 western boundary is formed by the valley of the Afon Arthog, 

 while the eastern boundary lies near the town of Dolgelley. The 

 area thus defined is about 8 miles long by 2 miles broad. The 

 ground is represented on the Ordnance Surve} r maps, 1-inch (large- 

 sheet series) 59 (Barmouth), and on the 6-inch quarter-sheets 

 Merioneth 36 N.E., S.E., 37 N.W., with portions of sheets 33 S.W. 

 and 37 N.E., S. W., and on the Geological Survey Map, Sheet 59 

 N.E. 1-inch (Old Series). 



(a) Physical Features of the Area. 



The ground between the two boundary- valleys, the Mawddach 

 Estuary and the Llyn Gwernan A r alley, is occupied by a hill-range, 

 or rather by a series of ranges, made by the scarps of the various 

 harder beds, and having a general west-south-westerly trend. The 

 hills average 900 to 1000 feet in height, attaining their greatest 

 elevation on Bryn Brith (1259 feet). They fall into two main 

 groups, which are separated one from the other by a broad upland 



