Vol. 56.] SILURIAN SEQUENCE OF RHAYADER. 119 



(C) The Bhayader Pale Shales. 



This great group of pale shales and mudstone is by far the most 

 complicated and perplexing of the entire succession. 80 monotonous 

 in lithological characters, so devoid of fossils, and in addition so 

 highly cleaved are the rocks, that they offer little inducement to 

 the geologist to unravel the stratigraphy of the great area over 

 which they are spread. They closely resemble the underlying 

 Gafallt Shales, and it was some considerable time before I could 

 lind any means of distinguishing between them. If the line of 

 strike of the base of the Rhayader Group be carried west of the 

 Elan it will be found to follow the small rill immediately east 

 of Fron-dorddu Farm. In the steep slope on the west side of the 

 brook, patches of the Rhayader Group occur at the bottom of Coed 

 Bwla, and on the east side of the brook the Gafallt Shales make 

 their appearance in the neighbourhood of Fron-dorddu. It was 

 in this particular locality that I found a clue which proved of use 

 to me in separating out the two groups. The only means of 

 detection lies in the fact that the Gafallt Shales are always striped 

 with thin arenaceous bands, while the shales of the Rhayader 

 Group show little or no sign of such banding. This may seem a 

 slender basis on which to work. It seems but reasonable, however, 

 to suppose that if this distinction exists at the point where the two 

 groups are seen together, it will exist for a mile to the westward, 

 and it is only within this mile that the difficulty of separation 

 occurs ; for at the end of that distance we are at the western 

 limit of the area under examination. By the above method the 

 fault from Llyn Clap to Pen-y-rhiw was detected. This has already 

 been mentioned as bringing down the Rhayader Shales against 

 the Gafallt Group. It is doubtful whether this fault has much 

 downthrow : this can be proved only by extended investigations. 

 It may be that there is a gradual passage up from the one series to 

 the other ; but, so far as I have been able to make out, the line 

 of separation between the two lies along this fault, or there- 

 abouts. 



The shales in the low-lying parts of the valley between Coed-y- 

 mynach and Llanfadog-Uchaf all dip in a south-easterly direction 

 in correspondence with the syncline produced from Caban Coch. 

 On reaching the higher ground to the northward, however, great 

 folding and inversion apparently begin to take place. 



Fig. 18 (p. 116) is a section from Gyrn Hill to Allt Goch, in a 

 north-to-south line. This shows the three main groups, namely, 

 the Gwastaden, Caban, and Rhayader. In order that their rela- 

 tionship may be thoroughly understood, the old ground need not 

 be described again. The Rhayader Pale Shales are well exhibited 

 in jSTant-yr-haidd and in the scarp of Gyrn Hill. They present 

 throughout the same uniform type of rather soft, pale grey, blue, 

 and green flags and mudstones. Some of the beds are striped with 

 white and blue seams, a few of which yield fairly well-preserved 



