Part of Carnarvonshire. 1 7 



drift-covered, and in consequence well cultivated. As a result 

 only three rock-exposures occur in this area — Cil-y-Coed, Craig-y- 

 Dinas, and Caer Engan. Cil-y-Coed and Caer Engan are two small 

 hills, whilst Craig-y-Dinas, with Pentwr to one side of it and Bryn- 

 mawr to the other, are on the slopes of rising ground. It will be 

 convenient to consider each of these exposures separately, commencing 

 with Cil-y-Coed, which is the largest and also the most south-westerly 

 of the three. 



1. Cil-t-Coed. (See Map II. 1 ) 



A. Surface Features. — This hill, situated about three-quarters of 

 a mile from the village of Clynnog Fawr on the north-west coast of 

 Carnarvonshire, attains an elevation of about 480 feet O.D., and 

 stands out prominently from the drift plain which stretches for 6 or 

 7 miles along Carnarvon Bay. It is the first hill of a series 

 increasing progressively in height towards the south, and it covers 

 an area of nearly a quarter of a square mile, being about 1,000 yards 

 long and 900 yards wide at its broadest part. The northern slopes 

 of the hill are everywhere gentle, except at the extreme north- 

 west, where crags are exposed, making the slope somewhat steeper. 

 On the south-west and west sides, however, the ground drops very 

 abruptly from 480 to 200 feet, below which level there is a gentle 

 slope down into the plain. On the north side the rocks are smoothed 

 by glaciation, and the lower flanks of the hill on every side are 

 covered with a thin deposit of drift. The contour of the hill was 

 undoubtedly carved out by the Irish Sea ice-sheet, which traversed 

 the hill from the north. There is also a small capping of drift on 

 the actual summit near the south-west extremity. 



B. Pre-Cambrian", the Bhtolitic Series. — The hill in part 

 undoubtedly owes its prominence to an igneous rock which is 

 exposed more or less continuously in a line of crags on the west side 

 extending from the roadway to the top of the hill, a distance of about 

 650 yards. 



(i) Stratigraphy (see Map II). — It will be convenient to begin our 

 description with the most northerly rocks exposed. These are to be 

 seen on the north-west side of the hill, 20 yards away from the small 

 quarry that is situated here. In hand specimens the rock appears to 

 be a dark felsite, noticeable for the presence of numerous porphyritic 

 crystals of quartz and of pink felspar. The groundmass is fine- 

 grained and compact, hence the rock is very hard and tough. 



The next exposure examined is in the quarry itself , where the rocks 

 appear in hand specimens to be rather different from those just 

 described. Although still felsitic-looking the rocks here are light- 

 coloured, and only on close examination are any porphyritic crystals 

 to be seen. In the mass the rock appears to be a rhyolite, and it is 

 probably to the rock exposed in this quarry that Tawney and Harker 

 refer. The rocks are much jointed and sheared, and readily break 

 into angular chips. The main joints strike E. 10° N. and dip 

 IS". 10° W. at an angle varying from 60° to 80°. Other joints strike 



1 The area here dealt with is shown on Map II in dotted lines. 



DECADE VI. — VOL. IV. — NO. I. 2 



