MM. Cole & Holland— Structure of Rhobell Faicr. 449 



rocks of the actual summit, which consist of light-coloured ash 

 without distinct stratification. On its western side the grit is 

 exposed along a north and south line for about 80 yards, and is seen 

 to dip slightly to the east. Beneath it is an extremely characteristic 

 slaty ash, dark and well bedded, with white or grey bands of 

 coarser felspathic material. Intrusive dolerites come up from below 

 into the series and locally affect its members. 



The grit itself contains quartzite pebbles, sometimes three-quarters 

 of an inch in diameter, together with numerous rounded fragments of 

 black shale. Here and there rolled lumps of andesite occur, resem- 

 bling those of the less basic tuffs of Rhobell Fawr. A thin parting 

 of shale underlies the grit at places, dividing it from a layer of 

 cemented sand. The shale-pebbles frequently lie with their longer 

 diameters at a high angle to the planes of stratification, as though 

 they had been rapidly hurried along by a current and banked up in 

 the surrounding sand. The larger examples of these black lumps, 

 which may have been only slightly consolidated when washed into 

 the grit-bed, have developed a delicate concentric shell-structure by 

 contraction, and often, in form and in the markings on their 

 fractured surfaces, present a curious resemblance to Nummulites. 



Lower down on Moel Cors-y-garnedd, in a hollow to the north- 

 west of the summit, grit may be seen again, containing rather coarse 

 pebbles of andesite ; and on the bank rising to the east of this 

 hollow there is a further exposure of grit in a bed having the 

 thickness and characters of that occurring on the summit of Rhobell 

 Fawr. The bed, which dips south-east, is exposed along a line 

 running north-east and south-west. We have thus probably two 

 distinct beds of grit on Moel Cors-y-garnedd, one of which resembles 

 the grit of Rhobell Fawr, whilst both, curiously enough, coincide 

 iu thickness and relations with the two bands of grit which we shall 

 mention as occurring on Allt Lwyd. 



The summit of Rhobell Fawr is similarly encircled by exposures 

 of quartzose grit, which correspond in character with the lower bed 

 on Moel Cors-y-garnedd. This grit is best seen to the north-west 

 of the 2408 point, where it has an easterly dip. The dip, however, 

 varies, and, by crossing the adjacent boundary-wall, several outlying 

 exposures are encountered, much disturbed by the invading dolerites. 

 On the south-west of the summit the grit is replaced by a sandy 

 ash ; and it graduates clown into a similar bed along its best defined 

 outcrop. Below it are the characteristic slaty ashes, precisely as 

 at Moel Cors-y-garnedd ; while above are bedded but coarser ashes. 

 Hence the two conspicuous crests of the denuded volcanic area 

 appear to owe their origin to the presence of these relics of a once 

 continuous gritty layer. The metamorphic action of the intruding 

 dolerites may have given the bed at both points local powers of 

 resistance. 



In order to compare the grits of Rhobell Fawr with those recog- 

 nized as of Arenig age, we examined the steep front of Allt Lwyd 

 ('•Rallt Llwyd" of the one-inch map), which rises close at hand, 

 facing the volcanic area. The typical grey quartz-grit occurs here 



DECADE III. VOL. VII. — NO. X. 29 



