22 E. Wynne Hughes — Geology of 



the boundary of the trap as far as Llanllyfni, and west of that village perhaps 

 throws the slate down against the low Cambrian grits and conglomerates that 

 form the western boundary of the porphyry." 1 



If we accept these slates on the south-east side of Cil-y-Coed as 

 Ordovician, as the Survey imply, the only inference that can be 

 drawn is that the Cil-y-Coed Series is of earlier date than Lower 

 Ordovician. 



(iii) South-West Side. — On the south-west side the rock surface 

 slopes steeply until cultivated land is reached. This is the case all 

 along this side except at the extreme end of the mass. Here the 

 rock disappears in a wooded glen, and although the glen is narrow 

 and deep, with a brook running down its entire length, no exposure 

 is seen anywhere in the glen; but the debris in the bed of the brook 

 suggests the proximity of the rhyolitic series. On the south side of 

 this brook and 30 yards from it is an old "trial" level. This adit 

 cuts into slate. The slate is black and is irregularly cleaved, the 

 cleavage striking E. 10° N. and dipping 80° S. 10 c E. In this adit 

 there are about five hard black bands varying from 1J inches to 

 2 feet in thickness and dipping in the same direction as the cleavage, 

 but at a slightly smaller angle. The black bands are not cleaved, 

 but are somewhat stratified parallel to their edges. They undoubtedly 

 mark the true dip of the beds, which is therefere about 60° to the 

 south. The slate in the adit is undoubtedly identical with the black 

 ferruginous slate of the Ordovician system. It is in the direct line 

 of strike of the grits on Cil-y-Coed. 



The difference in the dip, but the much greater difference in the 

 nature of the exposures on each side of the brook, coupled with the 

 narrow glen separating the two, suggests the presence of a fault at 

 this point. The map accompanying the Survey memoir shows a fault 

 between the Cambrian and the Silurian Series curving round to the 

 west at a point about 100 yards to the north-east of Cil-y-Coed. 

 This fault has already been referred to (p. 21) in the extract from 

 the Geological Survey memoir. A continuation of this fault in its 

 south-westerly direction for another 1,200 yards, before turning to 

 the west, would bring it down the glen separating the Cambrian and 

 Pre-Cambrian rocks from these Ordovician slates in the quarry. 

 Such a prolongation of the fault is shown in Maps I and II. 



(iv) North-East Side. — Following the strike of the Cil-y-Coed 

 rocks in a north-easterly direction we come again to cultivated land, 

 where no rock exposure is visible. A careful search in all the fields 

 oti this side of the hill discloses no rock exposures. The River 

 Desach outs its way through a gap on this side, but even here no 

 rock exposures are found. A little way on, however, at Pentwr, 

 half a mile from the most easterly exposure on Cil-y-Coed, a small 

 quarry was opened in 1912 for building-stone. The rock here 

 exposed is a bluish fine-grained grit, approximating almost to a hard 

 shale. At the south-west end of the quarry is green slate, but no 

 indication of the dip is disclosed in this exposure. Further to the 

 east, 60 to 70 yards away, there is again an exposure of green slate; 

 the weathered surface shows the cleavage to strike E.N.E.-W.S.W., 

 1 Mem. Geol. Surv., 1866, p. 143. 



