part 3] lower paleozoic of arthog-dolgelley. 279 



It is a curious Coincidence that there should be a slate-band intercalated in 

 the Cefn Hir Ashes and occupying the same relative position with respect to 

 the ash-group, as the slate-band in the Bryn Brith Beds does to the latter 

 beds. Such a coincidence might make it appear that the Cefn Hir Ashes are 

 merely a faulted repetition of the Bryn Brith Beds, were it not that a detailed 

 ■comparison shows that the two groups of ashy beds are lithologically quite 

 distinct. 



The slate-band is followed by the main mass of the ash-group, the ashes of 

 which are very variable in character. The fine-grained andesitic or kerato- 

 phyric type reappears, sometimes passing into ash of the ' china-stone ' type, 

 blue-hearted and white weathering, and with the usual splintery jointing. 

 These fine-grained types are interbanded with, or overlain by, massive 

 bands of coarser material, often well laminated. Among these latter beds, in 

 the centre and upper part of the main ash-group, massive agglomeratic 

 .beds appear. These consist of pebbles, lajDilli, and small chips of fine- 

 grained, white-weathering, porcellanous material, probably an andesitic or 

 rhyolitic ash, set in a more or less abundant dark-grey matrix. In some beds 

 the pebbles are of large and variable size, up to 1 foot in diameter, and 

 sporadically distributed ; in others the pebbles are of more uniform size, 

 averaging about three-quarters of an inch in diameter, and making up the 

 greater part of the stratum, thus giving rise to a rubbly-looking rock that 

 weathers quite white, like a normal rhyolitic rock. 



The ash-group is terminated irpwards by a zone of slaty ashes and ashy 

 elates. One of the lower of these slate-bands is full of curious pebbles, half 

 an inch to an inch in diameter, of a porcellanous ash, giving rise to a very 

 •characteristic band which, although quite thin, can be picked up at intervals 

 over a wide stretch of country. The thickness of these uppermost slaty ashes 

 is rather variable at different localities, varying from 40 to 100 feet. The 

 •differences are probably due to a certain amount of strike-faulting, which 

 might naturally be expected to select a thin zone that forms a plane of 

 weakness between the two sets of hard massive beds, the main Cefn Hir 

 Ashes below, and the Lower Basic Series above. 



The Cefn Hir Ashes, following on the soft Crogenen Slates, 

 determine the position of a craggy ridge beyond which comes a 

 small hollow determined by the outcrop of the thin slaty zone 

 that occurs at the top of the ash-group. Where the massive ashes 

 have been somewhat hardened by the granophyre, the feature due 

 to them becomes especially prominent, resulting in the fine pre- 

 cipitous scarp and jagged ridge of Pared-y-Cefn Hir, 1200 feet, 

 which presents so striking an appearance, as viewed from Llynau 

 Crogenen (PI. XVII). The scenery on the opposite, or south side, 

 •of the lake presents a remarkable contrast : for, on this side, the 

 great craggy feature has disappeared, or, more strictly speaking, 

 has shrunk to a sex'ies of tumps. The sudden diminution of the 

 feature is due to the fact that on the south side of the lake the 

 granophyre has transgressed on to beds above the ashes, which 

 latter have consequently not suffered contact-alteration. Owing to 

 the presence of the intrusion on the north side of the lake, the full 

 sequence of the ashes is not seen on that side. The full sequence 

 is present south of the lake where, however, owing to the above- 

 mentioned diminution of the features, exposures are not so good as 

 •on the northern shore. 



The Cefn Hir Ashes are only seen in the west of the area covered 

 by the map (PI. XX). Followed eastwards along the ridge of 

 Pared-y-Cefn Hir, they gradually disappear against the granophyre- 



