Part of Carnarvonshire. 21 



D. Summary of the Succession — "We have then at Cil-y-Coed the 

 following descending sequence : — 



(i) A quartz grit 6 or more feet thick, 

 (ii) A fine felspathic grit 8 feet thick, 

 (iii) A conglomerate 18 feet thick. 



All with a strike N. 60° E. and a dip of 50° to S. 30° E. These 

 rocks rest on 



(iv) A volcanic series consisting of rhyolitic lavas and tuffs which 

 appear to dip 60-80° N. 10° W. 



The pebbles in the conglomerate are undoubtedly derived from the 

 underlying rhyolitic series. 



This apparent discordance of dip suggests the presence of an 

 unconformity between the sedimentary rocks and the volcanic series 

 of rocks. The exposures, however, are not sufficient to actually 

 prove the existence of this unconformity, and it would require 

 considerable trenching to make it apparent. 



E. Relation to the Surrounding Areas. — On attempting to 

 trace the beds into surrounding areas we find that Cil-y-Coed is 

 somewhat isolated. 



(i) North and West Side. — On the north and west side the mass 

 is flanked by glacial drift, which stretches as a continuous sheet 

 right to the seashore, 500 yards distant. 



(ii) South- JEast and East Side. — On the south-east and east side 

 the ground is under cultivation, and no semblance of solid rock is 

 to be seen except in a field near Garregboeth, distant 30 yards from 

 the nearest exposure of the grits. Here an arenaceous slate is seen, 

 but it is doubtful whether this rock is really in situ. Assuming that 

 it really is in place, then it undoubtedly forms a continuation of 

 a considerable mass of arenaceous slate which is exposed at Pen-y- 

 garreg some yards further on. Between these two exposures there 

 is an intrusive mass of picrite. 



This band of arenaceous slate gives rise to a prominent feature at 

 Pen-y-garreg, and it can be traced for at least two miles to the 

 south-west, the outcrop generally running in a north-east to south- 

 west direction. 



In the Survey memoir this arenaceous slate is considered to be of 

 Silurian (Ordovician) age, but no fossils have as yet been found in 

 the neighbourhood. The slate is very different from the blue and 

 purple slates of the Cambrian Series, which are so persistent in the 

 Nantlle Valley and were traced by the Survey as far as Llwyd-Coed, 

 three miles north-east of Cil-y-Coed. The Survey assume the 

 existence of a fault in this neighbourhood throwing down the 

 Silurian beds against Cambrian rocks. The memoir states : — 



" South of Llanilyfni the strike of the Cambrian rocks changes to east and 

 west, and the purple slate does not occur south or south-west of Mynydd 

 Llanilyfni and Ty Coch near Clynnog Fawr. The drift-covered district further 

 south is composed of black and ferruginous Silurian shales and grey sand- 

 stones. The point farthest south where purple slate has been found is at 

 Llwyd-Coed about a mile south of Llanilyfni, and to the west of the last-named 

 place the ground is so obscure that the reason for the disappearance of the 

 slate is unknown. The fault which throws the (Cambrian) slate against the 

 quartz porphyry ridge (St. Annes-Llanllyfni ridge) is probably continued along 



