Yol. 56.] SILURIAN SEQUENCE OF RHAYADER. 91 



the Pale Shales rest on Pale Grey Mudstones (Ad 3 ). In the 

 Castle-Hill section this thickness has diminished to about 250 feet, 

 and in the Vaynor section the Upper Pale-Shale Group rests upon 

 the lowermost beds of the Calcareous - Nodule Group — the Pale 

 Grey Mudstones (Ad x ) and the Zone of Monograptus convolutus 

 (Ad ) having disappeared. It would seem then, if no fault inter- 

 venes to cut out these missing beds, that the Ehayader Group 

 rests irregularly upon the Gwastaden Group. Such indeed is 

 the case, and, as will be shown later, no fault is likely to exist, 

 a marked unconformity separating the two series. The missing 

 beds are not faulted out, but are merely overstepped by those of 

 the overlying group. 



(c) Exposures in the Western Areas. 



East of the Eiver Elan, in the central and eastern portions of 

 the district already described, the interpretation of the sections 

 becomes a simple matter, for, as we have seen, the confirmatory 

 sections show the same general succession in the zones of the 

 Gwastaden Group as in the typical section itself. But as we 

 cross the Elan into the western areas matters wear a very different 

 aspect ; and bearing in mind the comparative simplicity of the 

 geology of the area already examined, one is at first perhaps 

 somewhat startled by the introduction of a new set of phenomena, 

 strikingly contrasted with everything hitherto encountered. 



(1) Allt-y-bont. 



In dealing with this portion of the Ehayader District, perhaps no 

 better method can be adopted than that which has already been 

 used in the western areas ; namely, of sketching a series of parallel 

 sections, in order to show the arrangement of the beds as we 

 advance from point to point along the strike. 



Commencing at the eastern limit and working westward, the 

 first section is shown through Allt-y-bont Hill (fig. 6, p. 88). Here 

 the Cerig Gwynion Grits (Act) are exposed in the form of mag- 

 nificent crags, which swing down from the summit of the ridge 

 into the gorge of the Wye : the total thickness of these beds at 

 this point is about 300 feet. They rest, as before, upon cleaved 

 Blue-black Shales, and are followed by the overlying Micaceous 

 Elags and Grits (Ab x \ a few patches of which flank the northern 

 side of the hill. 



On crossing the stream, however, and ascending the rise below 

 Pen-y-rhiw, one is suddenly confronted with a vertical cliff formed 

 of massive conglomerate (Ba x ), some 50 feet thick, occupying a 

 position which corresponds apparently with that of the Eot ten- 

 stone Beds. This conglomerate runs rapidly downhill, and ends 

 abruptly at about 100 yards north of the section-line. Before 

 dealing with the cause of the incoming of this new conglomerate- 

 group, it will be expedient, first of all, to examine and describe 

 the remaining sections. 



