﻿608 ME. W. G. EEAENSLDES ON THE GEOLOGY OF [Aug-. 1905, 



29. On the Geology of Aeenig Fawe and Moel Llyenant. By 

 William Geoege Feaensides, M.A., F.G.S., Fellow of Sidney 

 Sussex College, Cambridge. (Read January 18th, 1905.) 



[Plate XLI— Map.] 



I. iNTEODUCTlOlSr. 



The mountains known as Arenig Fawr and Moel Llyfnant 

 are situated somewhat to the north of the centre of the county of 

 Merioneth, and lie immediately east of the main water-parting of 

 Wales. The area studied lies wholly within the quarter-sheets 

 13 S.E., 13 S.W., 21 N.E., & 21 N.W. of the 6-inch Ordnance- 

 Survey maps of Merionethshire. Rising from a plateau some 

 800 to 1200 feet high, they attain heights of 2800 and 2437 feet 

 respectively, and, being entirely above the cultivation-zone, are 

 'practically uninhabited. The Great Western Bala - Ffestiniog 

 Railway crosses the northern end of the district, to which Arenig 

 Station, close under the northern slopes of Arenig Fawr, affords 

 easy access. 



Although to a certain extent glaciated, the mountains are swept 

 clean rather than Drift-covered, and it is only in the lower ground 

 that moraines become inconveniently abundant. The adaptation of 

 surface-forms to rock-structures is exceedingly complete, and few 

 are the places where the ribs of harder rocks do not protrude 

 through the scanty soil or turf. Being open to the full blast of 

 the westerly and south-westerly gales from the ocean, the district 

 receives its full share of weather ; and, in consequence, although the 

 eastern slopes are often grass- or heather-covered, the steeper parts 

 of the western slopes are rugged and bare, and afford magnificent 

 exposures of the component rocks. Farther, the structure of the 

 district is fairly simple, and the cleavage, though present, is not so 

 strongly developed as to prevent one from breaking the rock along 

 the bedding-planes. 



Ever since the time when Sedgwick applied the name of 

 Arenig Ashes and Porphyries to the lower series of North 

 Welsh volcanic rocks, the Arenig district has been a source of 

 interest to geologists. Yet, and notwithstanding that the name 

 of Arenig is so constantly upon the lips of all who study or teach 

 the geology of the Lower Palaeozoic rocks, no complete or detailed 

 account of it has yet been published. 



The only connected general account of the district which I have 

 found, is that contained in the first edition of Sir Andrew Eamsay's 

 ' Geology of North Wales.' 1 A good deal of additional information 

 is contained in the second edition of that work (1881), but is somehow 



1 Mem. Geo! Surv. vol. iii (1866) 



