Vol. 56.] SILURIAN SEQUENCE OF RHAYADER. 103 



The shales are well exposed under the Upper Conglomerate ; but 

 they are highly cleaved, and the extraction of fossils is a difficult 

 matter. I have, however, identified Monograptus lobtferus and 

 Climacograptus normalis from these rocks. 



The Upper Conglomerate is exhibited from top to bottom, and 

 from the road it is seen to be crumpled into numerous folds. 

 Fragments of E>ic?*inites, Favosites, and Lindstroemia occur in certain 

 bands, but they are difficult of removal, and uncertain of identi- 

 fication. The total thickness is about 90 feet. 



Only 60 feet or so of the black-banded grits of the succeeding 

 group is exposed along the line of section ; and no sign of the 

 puddingstone-bed has been detected at the top of the group. It 

 appears to be cut out by a longitudinal strike-fault, which tilts the 

 beds immediately above the cliffs into a vertical position. No fossils 

 have been found in this neighbourhood ; but, from the abundance 

 of the species Monograptus SedgwicHi, Portl. in the same zone in 

 other localities, I have ventured to give the name Monograptus- 

 Sedgwickii Grits to this division (Bb^. 



From the fault to the Abernant stream, the sandstone-banded 

 shales (Bb 2 ) are found to be greatly folded and no important 

 thickness is exposed. At the stream itself the Abernant Fault 

 brings down the Gwastaden Group (A) on the opposite side of 

 the stream ; and if the latter be followed down to its junction with 

 the Elan it will be found that all the underlying divisions of the 

 Caban Group terminate at the brook, or thereabouts. The grey- 

 and-green shales are poorly shown in this area, being best exhibited 

 on Corn Gafallt Hill, from which they derive their title. 



The rocks on the north side of the Elan at Caban Coch all appear 

 to have been subjected to much squeezing, for in hand-specimens 

 and rock-slides alike the effect of movement is easily traceable. The 

 proximity of the Abernant Fault possibly has something to do with 

 this, for the nearer the beds approach to the dislocation the greater 

 appears to have been the amount of crushing. 



The Caban Group as developed at Caban Coch, we have now 

 seen, consists of the following members : — 



(1) A conglomerate-group at the base, made up of three divisions : 



{a) a lower conglomerate about 200 feet thick ; (b) a mass 

 of shales from 250 to 300 feet thick; and (c) an upper 

 conglomerate about 90 feet thick. 



(2) An upper group of two main divisions : (a) a series of alter- 



nating black-banded grits and pale grey shales, and (b) grit- 

 banded and sandstone-banded grey shales. 



The succession may be tabulated as follows : — 

 f Gafallt Beds (Bb). 



(M 2 ) Gafallt Shales. 

 (Bbi) Monograptus-Sedgwic/iii Grits. 

 Caban J 

 Group. \ Caban Conglomerates (Ba). 



(Ba 3 ) Upper Conglomerate. 

 (Ba 2 ) Intermediate Shales. 

 {Ba x ) Lower Conglomerate. 



