Vol. 56.] 



SILURIAN SEQUENCR OF RHAYADER. 



75 



(a) Cerig Gwynion Grits (Aa). 



These grits are not exposed in the stream of our typical section, 

 but make their appearance about 100 yards to the east ; and still 

 farther east, where a small footpath crosses the outcrop, a vertical 

 thickness of about 100 feet is laid bare (see fig. 1). The beds run 



Fig. 1. — Section from Gwastaden to Rhayader. 



N.W. Builth Gwastaden S£ ' 



>» Crossing of R 9 dd 



Rhayader £ Birmingham j Plantatiofi^^^ 



Acf 3 Acf t Ad, Ac 3 Ac 2 Ac { Ab s Ab £ 



[Approximate length = 1J miles.] 



C = Rhayader Pale Shales. 

 A = Grwastaclen Group. 

 Ad = Gigrin Mudstones. 



Ad 3 

 Ad 

 Ad' 



Pale Grey Mudstones. 

 Zone of M. convolutus. 

 Calcareous-Nodule Beds. 



Ac = Ddol Shales. 



Ac 3 — Zone of M.fimbriatus. 

 Ac 2 — Zone of M. cyphus. 



Ah = Dyffryn Flags. 



Ab., = J), modestus Flags. 



Ab = Ptottenstone Beds. 



Ab x = Micaceous Flags and Grits. 

 Aa = Cerig Gwynion Grits. 

 = Blue-black Shales. 

 = Fault. 



up to about 6 or 8 feet in thickness, and dip at an angle of about 

 28° north-westward. The rock itself is a very hard, compact, 

 greenish-grey siliceous grit or grauwacke. It is extremely tough 

 under the hammer, and, being almost free from calcareous or fel- 

 spathic matter, fragments are detached from its weathered surfaces 

 with as great difficulty as from beds newly laid open in the quarry. 

 A detailed description of this rock, however, will be given on a 

 subsequent page (p. 95). The total calculated thickness of the 

 group at this point is about 170 feet. 



About 50 yards south of this grit-band, a small outcrop of an 

 underlying group of a very different mineralogical character may 

 be seen. There is no trace of the siliceous matter of the grits, 

 but only the argillaceous material of a set of flaky, blue-black shales. 

 These shales are very highly cleaved, and, indeed, may be described 

 as schistose. They create ab once the impression that they are of 

 far greater geological antiquity than the beds of the overlying 

 grit-group. Isolated rock-patches, extending for several hundred 

 yards southward, still reveal the same general characters. The 

 actual line of junction of the shales and the grits is not exposed, 

 but no transitional phases seem to exist between the rocks of the 

 two groups. It would appear in fact, from the very sudden petro- 



