Vol. 56.] SILURIAN SEQUENCE OF RHAYADER. 99 



for following out in the field, as it forms a prominent ridge right 

 across the country. The ground falling away on both sides, it 

 stands out for a length of over 3 miles as the highest ground in the 

 lower parts of the Rhayader Valley. In the gorge south of Cefn- 

 Ceideo Hall there is a complete section of the Calcareous-Nodule 

 Group (Ad^ up to the base of the Monoc/raptus-convolutus 

 Zone (Ad.,). This section shows the division to have a thickness 

 of about 200 feet. 



No further exposures of the upper bands of the Gigrin Mud- 

 stones occur east of Lower Downfield, for as they approach the eastern 

 limit of the district they are gradually overlapped by the Rhayader 

 Pale Shales (C); and at Cwm Barn, only a few feet of the 

 basement-beds of this mudstone-group peep out from beneath the 

 rocks of the overlying series. 



(e) Summary. 



We are now enabled to summarize our investigation of the 

 Gwastaden Group. It is seen to its best advantage in the 

 Wye Valley, where it reaches a thickness of over 1800 feet ; while 

 from 1 to 3 miles to the westward only from 700 to 900 feet 

 is exposed. The various subdivisions of the group, so well shown 

 in the Wye Valley, can be scarcely recognized in the easternmost 

 portion of the district ; but when followed westward they may be 

 separated out, to as far as 4 miles from our first typical section. 

 The grits at the base of the series rapidly thicken out when followed 

 westward, and become greatly attenuated in the opposite direction. 

 The upper divisions, however, maintain their thickness fairly well 

 throughout the district. In the Wye Valley proper, the whole 

 group is overlain irregularly by a series of pale shales — the 

 Rhayader Group (C). 



West of the River Elan, a thick conglomerate-group is found 

 resting unconformably upon the lower divisions of the Gwastaden 

 Group. The next problem is to determine the relations existing 

 between these two overlying groups : which is the older, and which 

 the younger; and how they are stratigraphically related one to 

 the other. 



(B) The Caban Group. 



This most interesting group, which we have as yet barely touched 

 upon, is met with only in the western half of the district. Roughly 

 speaking, it may be said to lie within the curve of the Elan, between 

 Caban Coch and the Elan's confluence with the Wye. It is never 

 found north of this stretch of the river, except in one small area ; 

 nor is the smallest exposure to be seen to the eastward. Irs 

 outcrop extends over a length of only 2 miles, with an average 

 breadth of from J to | mile. Yet, notwithstanding the relatively 

 small area of country occupied by this group, the included rocks, 

 whether considered as a whole, or bed by bed, are by far the 

 most interesting in the district. They are best exhibited at 

 Caban Coch, some 3 miles west of Rhayader. In this gorge, 



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