Vol. 56.] SILURIAN SEQUENCE OF EHAYADEE. 97 



up the magnificent crags of Craig-y-Foel ; they may be seen in the 

 new road-cuttings round the hill, and can be followed past Careg- 

 ddu np to Dol-Folau farm. They constitute the main mass of the 

 wooded hills behind. Cwm Elan, Cwm Coel, and Want Gwyllt ; 

 while above Dol-y-Mynach the Cerig Gwynion Grits (Aa) 

 emerge in the form of a huge dome, extending over a vast area of 

 moorland to the westward. 



The slates are harder, more granular, and more micaceous than 

 their equivalents in the Rhayader Valley ; and, in addition, are very 

 highly cleaved. This cleavage appears to have been made use of, 

 for several quarries along the banks of the Claerwen have been 

 opened for the purpose of extracting roofing-slates. Where it is 

 possible to obtain recognizable fossils from the rocks the forms are 

 always found to be Diplograptidae or Climacograptidae — genera 

 characteristic of this division. 



In the Rhayader Valley the Dyffryn Flags are well exposed 

 in the sides of the Builth road from Gigrin Prysyg to the Glyn. 

 In the last-named locality they are best seen in the river-course 

 east of the railway-bridge, and will be found to contain very well- 

 preserved specimens of Diplograptus modestus and CUmacograptus 

 normalis. 



In their lowest division, where the flags are interbedded with 

 shales, a series of quarries have been opened out along the slope 

 of Gwastaden. The grits, which may be extracted with little 

 difficulty, have been used in building the various farmsteads in the 

 immediate neighbourhood ; and, in combination with the stone 

 from Allt-y-bont, have been largely employed in the construction of 

 many of the buildings in the town of Rhayader. 



At Pen-y-bank the Rottenstone Beds (Ab 2 ) form the crest of 

 the ridge above Upper Downfield Farm ; but they have been 

 so faulted that they are now turned at right angles to their 

 original strike, and dip westward. The isolated block, which has 

 been pushed up through the overlying strata, can be followed to the 

 road below to within 150 yards of the Zone of Monograptus 

 fimbriatus (^c 3 ), and is here cut out by the strike-fault already 

 mentioned. 



A few patches of Dyffryn Flags are visible on tbe flanks of 

 Castle Hill, and in the sides of the slope above Great Vaynor Farm ; 

 but their general appearance and lithological characters have altered 

 to such an extent that it is almost impossible to separate out the 

 subdivisions. 



(3) Ddol Shales. 



The lowermost portion of this group, the Zone of Mono- 

 graptus tenuis (Ac x ), is exposed in three places only — on the 

 River Wye, in the Builth road, and in the banks of the lane leading 

 to Cae-newydd. In every case the section is a poor one, but as the 

 topmost beds closely resemble those at the base of the subdivision, 

 and as Monograptus tenuis is absolutely confined to them, we can 

 have no hesitation in making this a distinct zone. 



Q. J. G. S. No 221. h 



