GEOLOGY OF CENTRAL WALES. 



151 



Little need be added to what we have said on the alternating- 

 argillaceous beds. The raft-typo is best seen upon tho Allt-wen 

 cliffs, south of Aberystwyth, where also an elongated type, serving 

 very well as rough pencils, forms the " pencil-rab these beds 

 commonly appear much more solid in the heart of the rock, the 

 fragmentary structure only becoming developed after exposure to 

 weathering. 



The cleavage phenomena here are often of peculiar interest from 

 their frequently incomplete, half-developed character. The dark shaly- 

 looking rock of Constitution Hill, Aberystwyth, would not at first be 

 suspected of a slaty structure ; but specimens may be gathered with 

 the cleavage seen distinctly cutting across the stripe-lines ; and many 

 of the dark, soft, shaly rock-beds of the neighbouring cliffs, when 

 seen in sitic, will be found to have their divisional lines running 

 across the bedding. 



A pale, homogeneous, hardened mud, or pale mudstone, splitting 

 with curved, conchoidal surfaces into 2-3-inch blocks, is of additional 

 importance, as probably marking a zone ; I have found it some half 

 a mile north-east of Nant Eos, at Pen Craig, Llanilar, on the road-side 

 N.N.E. of Llanbardarn, and at five miles east of Aberaeron. This 

 appears to indicate a distinct zone near the top of the Aberystwyth 

 grits. The same kind of rock occurs at Llyn Eyrddyn Fawr, and at 

 Ehos Ehydd, near Llantrisant, where it is again not far removed 

 from the thin-flag series — another zone at the top of the Abery- 

 stwyth grits. 



The Large-flag series. — This series, which belongs to the upper 

 part of the Aberystwyth group, is best marked to the east, in the 

 neighbourhood of the Devil's Bridge and Llantrisant, where its 

 presence is soon made manifest in the construction of piggeries and 

 other low huts, whose sides and roofs are covered with the flags. 

 These are grit-beds, about 1 inch thick, but little jointed, so that 

 large slabs, frequently 4 and 6 feet square, are commonly extracted 

 for use in the neighbourhood. The rock is fine-grained, and usually 

 exhibits a complete contortion in the lines of laminse ; altogether 

 it much resembles some of the grit-beds of the middle Lingula- 

 flags of North "Wales. The surfaces are generally undulated, and 

 often show true ripple-marks, over which we find Graptolites and 

 other fossils spread out: the Devil's-Bridge Dendroid Grraptolites 

 occur in this series. 



The further extensions of these beds, north and south, are seen 

 around Pont Erwyd and south of Llantrisant ; and the same series 

 seems to be brought up to day by folds, on the east at Eglwys 

 Newydd, and to the west near the head of Cwm Symlog. Bands of 

 rottenstone (decomposed limestone) occur at the Devil's Bridge, near 

 Eglwys Newydd, and again in the hills some four miles east of Tal 

 y bont. 



Around Aberystwyth these flags are not so well developed ; but 

 they may be recognized at Cefn Hendre, as already described, where 

 they are put to the usual purpose of hut-making for the work- 

 men's shelter. 



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