148 WALTEK KEEPING ON THE 



belonging to the Large-nag series. The presence of Fucoidal mark- 

 ings, including the Net Algal (Retiofucus extensus, Keep.) and Nema- 

 tolites at one mile and a half from the Devil's Bridge, a] so serves to 

 indicate our proximity to the Aberystwyth -grit series. The general 

 apparent dip is clearly eastward, with some foldings. 



Descending beyond the Arch towards Eglwys Newydd the only 

 matter of special interest is the occurrence of a thin seam of rotten- 

 stone (a decomposed limestone) which may perhaps correspond with 

 the crinoidal zone in the Devil's Bridge. Such rocks are of very 

 rare occurrence in Mid Wales, so that it is a popular saying that 

 " there is no lime in Cardiganshire." 



The rocks around Popty, near Eglwys Newydd, are again seen to 

 be very much of the Devils-Bridge type ; and I am inclined to think 

 that a set of rock-folds with reversals have here brought that fossil 

 zone near to the surface again. We now reach a region of manifest 

 great rock-foldings ; and as we ascend Cwm Ystwyth a number of 

 exposures display an important westerly fold in the rocks. The 

 beds appear still to belong to near the junction of the Aberystwyth 

 grits with the Metalliferous-slate group. 



Next we pass through the rich metalliferous district of Cwm 

 Ystwyth with its network of mineral veins in the Metalliferous- 

 slate group, beyond which, at Blaen y Cwm, thicker grits (about 

 2 feet) with east and south-east dips come in amongst the slates ; 

 and these latter become replaced over a considerable area by dark 

 shaly slate, rab, pencil-rab, and other softer forms of the argillaceous 

 rocks. 



Thick grit-beds, which are cleaved, next form a prominent feature 

 at Craig Lluest (fig. 2) ; and these I regard as belonging to the 

 lower part of our upper grit-series — the Plynlimmon group. This 

 series is better developed in Plynlimmon itself, to which we must 

 refer (infra, p. 156) for a more detailed description. The dip is 

 eastward. 



Some three quarters of a mile beyond the top of the pass the dip 

 changes, a westerly slope setting in ; and this remains well sustained, 

 though with some east foldings, on to the hills west of Rhyader. 

 Here, then, we see the eastern half of the great axial synclinal of 

 this part of W r ales — a great fold in which Plynlimmon lies; but 

 its apparent magnitude is greatly exaggerated by the phenomena 

 of reversed dips. The lithological details of this part of the section 

 in its frequent slight variations, but general monotony, till we 

 reach Ehyader, would occupy much space and be of little value. 

 Some pale shaly slates approaching the character of the Tarannon 

 shales, and thin gritty bands, are seen on the dreary bog- and moor- 

 land of the higher plateau and on the east of the pass, where also 

 some of the grits exhibit irregular fucoidal markings and the Net 

 Algal (Retiofucus extensus, Keep.) upon their under surfaces. 



One good anticlinal fold in "Metalliferous slates" with some 

 grits has its axis about one mile and a half east of Aber Gynwy ; and 

 a synclinal in the slaty series is indicated lower down on the road 

 to Pont ur Elan. The well-sustained general westerly dip of the 



