in North-West Carnarvonshire. 263 



of about half a mile to near Glynllifon House on the south-west. The 

 width of the outcrop so exposed is only some 150 yards, but I am 

 informed that the rock was found in excavations for cellars at the 

 back of the house, which shows that it extends at least 300 yards 

 further westwards. At one exposure it is seen that grits coarse and 

 fine lie directly on the felsite ; these rocks are apparently continuous 

 for a mile or more to the west. 



At the quarry called Grallt-y-Prif a narrow band of the quartz- 

 felsite is found among the grits, brought in apparently by faults. 



The greater part of Glynllifon is covered with drift, and there are no 

 other exposures of the felsite or of the so-called Cambrian Grits, either 

 to the east or south of those just described, except in the locality first 

 mentioned. This is near the southern boundary wall, in the wood 

 named on the 6 inch map, Coed Penbrynmawr. Here in several 

 openings the quartz-felsite is again exposed. The quartz-felsite of 

 Glynllifon varies from cream-coloured to grey and reddish-grey in 

 colour, and contains many quartz phenocrysts much corroded and 

 broken ; in places it is much cleaved, in others it is massive. 



(6) Brynmator Farm. — A few yards south of the southern boundary 

 wall of Glynllifon, continuing the line of the exposures in Coed 

 Penbrynmawr, there is a small opening where the quartz-felsite is 

 again to be found, and further on for half a mile to the south-west there 

 are other exposures in the somewhat steeply sloping ground on which 

 stands the farm called Brynmawr. Judging from the Survey Maps 

 these rocks were considered to be metamorphosed Cambrian Sandstones. 



(7) Llanllyfni. — On Sheet 75 N.W. of the 1 inch Geological 

 Ordnance Map, the boundary between the quartz-felsite and the 

 adjoining rocks, the so-called Cambrian Slates and Grits, is drawn as 

 passing from JS^antle Railway Station south-westwards to a point just 

 north of the village of Llanllyfni. The stratified rocks are marked as 

 forming a continuous band, a mile or more wide, wrapping round the 

 felsite. As a matter of fact much of this area is formed of the felsite 

 faulted against the slates and grits. It is exhibited at the following 

 places, so-named on the 6 inch ordnance map or so known locally : 

 Penglog,^ Tyddyn Agnes, Gwynfaes, Taldrwst isaf, Taldrwst ganol, 

 Taldrwst ucha, Singrig Quai'ry, and Tanyrallt Quarry. The area 

 over which the rock is exposed is rather more than half a mile from 

 south-west to north-east and about one-third of a mile from north- 

 west to south-east. Probably the felsite is present over a still larger 

 area, but deep drift obscures the outcrops. 



In this locality the felsite is extremely schistose, the felspars having 

 been in most cases completely broken down. In some places the 

 quartz grains are very numerous, while elsewhere they are few and* 

 small. 



The surface of the ground between the exposures numbered (2) to (6) 

 inclusive is largely formed by Drift, so that one cannot actually trace 

 the course of the quartz-felsite from exposure to exposure by means of 

 the surface features. There cannot, however, be any reasonable doubt 



1 This is probably the locality just referred to by J. H. Blake, Q.J.G.S., xlix, 

 p. 463. 



