Part of Carnarvonshire. 23 



but the dip of the beds is not clear in such a small exposure. 

 Continuing from here in a N.N.E. direction we come to a much 

 larger exposure of rocks at Craig-y-Dinas, separated from Pentwr by 

 three small exposures of purple slate. The several exposures in this 

 neighbourhood will now be described in greater detail. 



2. Pentwr, Craig-y-Dinas, and Bryn-mawr. 



These three places are on the slopes of a ridge which stretches 

 practically from Glynllifon to Cil-y-Coed. The crest of this ridge 

 along almost its whole length attains an altitude of 200 feet. At 

 Craig-y-Dinas the River Llyfnwy cuts through the ridge in a 

 roughly semicircular sweep exposing precipitous rocks at several 

 localities on both banks of the river. 



A. Cambrian, Sedimentary Rocks, (i) Conglomerate. — This rock 

 is best exposed on the right bank of the river at the end of the gap 

 farthest from the sea. The pebbles in this conglomerate are com- 

 parable in size with those at the north-east end of the exposures at 

 Cil-y-Coed, being seldom more than an inch in length. They consist 

 essentially of rhyolitic and felsitic chips. In addition occasional 

 chips of a slaty nature occur, and in this respect the rock differs 

 from the Cil-y-Coed conglomerate. The matrix of the conglomerate 

 is felspathic, and approximates both in hand specimens and under 

 the microscope to that already described at Cil-y-Coed. The rock is 

 considerably sheared, and the pebbles have arranged themselves in 

 the direction of shear, which dips generally at an angle of 70° S.S.E. 

 The true dip of the conglomerate is obscured by cleavage. 



(ii) Grit. — Overlooking the conglomerate, to the south-east of it, 

 is a coarse grit in which there are several bands of a much finer grit. 

 Some of these bands, though only 1 inch thick, are very persistent. 

 In one locality the river runsfor 100 yards in a N.E.-S.W. direction. 

 Here the bands are perfectly horizontal, suggesting that the strike of 

 the rock is approximately in the same line. Further down the river 

 runs east and west, and here the dip of the beds can be observed 

 owing to the alternation of fine and coarse bands. A lateral gap, at 

 another locality further east, shows that the banding is persistent on 

 both sides of the gap. The dip given by this banding is 50° S.E. 



At the extreme southerly bend of the river the grit becomes much 

 less felspathic, approximating more nearly to a quartzite. The rock 

 weathers almost white, and on breaking shows crystals of opalescent 

 quartz. This type of rock is predominant on the south-east bank of 

 the river. Near the farm of Pen-y-bont (also on the left-hand side 

 of the river) there is another exposure of quartz grit. It forms 

 a small island bounded on one side by the river and on the other side 

 by a "cut out" or old overflow channel. The line of outcrop here 

 trends 20 N. of E., and the beds are very nearly vertical. This does 

 not agree with the observations in the main mass. 



(Hi) Purple Slate. — For some distance beyond this grit the land is 

 completely under cultivation and no exposures are visible. However, 

 in a well that was dug in 1913 in the school playground at Bryn- 

 eurau (500 yards south-west of Craig-y-Dinas) purple slate was 



