Vol. 56.] THE SILURIAN SEQUENCE OF RHAYADER. 129 



separated at the base of the Monograptus-exiguus zone. This fossil 

 is one of the first to be fou ad in the Gala Beds, as at Glenkiln 

 Barn and elsewhere, and has never been detected in the underlying 

 black shales. 



In Central Wales we have the equivalents of the Caban Group in 

 the Aberystwyth Grits and a portion of the so-called ' Metalliferous ' 

 Slates ; it is not unlikely, however, that only the Gafallt Beds are 

 the true representatives of these western rocks. The Monograptus- 

 SedgwicJcii Grits strongly resemble the beds of the Aberystwyth 

 Group, in the regular succession of evenly-bedded, fine-grained grits 

 and shales. Moreover, the occurrence of the dendroid graptolites, 

 so characteristic of the Gafallt Beds, in addition to Monograptus 

 Clingani, points almost conclusively to their occupancy of the same 

 stratigraphial parallel. M . tumculatus, which is quoted from these 

 West Caerdiganshire rocks, is a higher form, but it was detected 

 at a distance of over 1| miles west of Aberystwyth, in the cliffs of 

 which town the grits are best developed. Probably, therefore, it has 

 been extracted from the top of the group. 1 



Referring once more to the Rhayader Pale Shales, we see that 

 they bear a rigid comparison with the Tarannon Shales of Conwav ■ 

 for, of the 19 graptolites quoted from the former, 9 appear in the 

 latter; and if we add the Rastrites-maximus zone to the upper 

 group, the number of common species is increased to 13. The 



Rhayader Shales, moreover, are exactly of the Tarannon type 



pale grey, green, and purple mudstones and shales. 



Exception being made of the work of Prof. Hughes and Mr. Marr, 

 the Lower Llandovery formation of North Wales has not hitherto 

 been worked out in any detail. Erom the researches of those 

 authors, however, it becomes evident that the Corwen 2 and Cerrig- 

 3'-druidion Grits 3 fall naturally into line with the Cerig Gwvnion 

 Grits — the base of the Silurian system. 



Though no zonal work has yet been completed on the Valentian 

 rocks of South Wales, the boundaries of the main divisions, as they 

 occur along the south-eastern fringe of the Central Wales complex, 

 are fully indicated on the maps of the Geological Survey. Along 

 this fringe the broad grouping of the various formations beais 

 satisfactory comparison with the arrangement of their equivalents in 

 the Rhayader district. In the vicinity of Llandovery, the Tarannon 

 Shales rest with apparent conformity on Upper Llandovery rocks. 

 Near Newbridge, where they emerge from beneath the Wenlock 

 Shales, they are again seen to overlie the same beds; but as the 

 village is approached from the westward, they rapidly pass over the 

 latter on to the Lower Llandovery, and eventually come to rest on 

 rocks of Ordovician age. The district that has been examined in 

 the neighbourhood of Rhayader is, unfortunately, not of sufficient 



1 Quart. Journ. Geol. Soc vol. xxxvii (1881) p. 167. 



2 Ibid. vol. xxxiii (1877) p. 20?. 



3 Ibid. vol. xxxvi (1880) y. 278. 



Q. J, G. S. No. 221. k 



