Vol. 56.] SILURIAN SEQUENCE OE RHAYADER. /3 



given of the rocks of Gwastaden and Craig-y-foel. These rocks are 

 assumed to be the same, and are assigned, to the Upper Cambrian 

 horizon. A geological section of the country is also given in the 

 accompanying plates. 



In 1847 Sedgwick, in his paper on the ' Classification of the 

 Fossiliferous Slates of North Wales, etc.,' ] described the slates of 

 this district and introduced the term Rhayader Pale Slates. 

 At the same time, however, he clung to Murchison's opinion that 

 they belong to the Cambrian System, and placed them above his 

 Plynlimmon and Aberystwyth Groups. 



About this time the officers of the Geological Survey were 

 engaged on an examination of the rocks of the district. In 

 1850, Sheet 56 N.W. of the Survey maps, which includes the 

 town of Rhayader, was published. This map embodies the work 

 of Ramsay and Aveline; and for the first time boundary-lines 

 are shown separating arenaceous from argillaceous rocks. In 

 company with the maps, Sheet 5 of the Horizontal Sections was 

 issued. This includes a section through the Caban Coch area, which 

 brings out clearly the arrangements of the various grits and con- 

 glomerates in that locality. In this section, however, as in the 

 map, the various beds were considered as belonging to one and the 

 same formation. But a striking change of opinion had been brought 

 about within the previous three years : for, instead of being assigned 

 to the Cambrian, the rocks were referred to the Lower Llan- 

 dovery, on the grounds already stated (p. 69). 



About twenty years later (in 1872), the Rev. W. S. Symonds 

 published his well-known ' Records of the Rocks/ On pp. 135-36 

 of that volume will be found a short description of the rocks 

 and scenery of the district. Comment is made on the difficulty of 

 working out the geology, and it is stated that up to that time the 

 stratigraphy was by no means cleared up. The Gwastaden and 

 Caban rocks are briefly described, and are both assigned to the 

 Lower Llandovery., 



In 1881 the late Walter Keeping, in his most valuable paper on 

 the 'Geology of Central Wales,' 2 advanced a new suggestion as to the 

 arrangement of the various rock-groups between Aberystwyth and 

 Rhayader. He showed that the Plynlimmon Grits overlie the 

 Aberystwyth Group, and he proved the latter to be of Upper 

 Llandovery age. In the Plynlimmon Grits he found no fossils ; 

 but because of their superior position he assigned them to the 

 Tarannon horizon. At the same time he united in one single 

 group the Gwastaden Grits, the Caban Coch Conglomerates, and the 

 Rhayader Pale Slates ; and he suggested that it occupied the same 

 stratigraphical parallel as the Plynlimmon Grits. 3 



It has been pleasant to find, from the results of my own researches, 

 that many of the suggestions of the later investigators were, in part, 

 correct. In the present paper I shall endeavour to show that, within 



1 Quart. Journ. Geol. Soc. vol. iii (1847) p. 153. 



2 Ibid. vol. xxxvii (1881) p. 141. 



3 Ibid. p. 161. 



