Vol. 56.] SILURIAN SEQUENCE OE RHAYADER. 113 



east to south-south-west. The two conglomerates are thickest in 

 the centre of the area occupied by their outcrops, and thin out when 

 followed both eastward and westward ; while the Intermediate Shales 

 (Ba 2 ) become greatly attenuated when traced eastward. Moreover, 

 not only is the conglomerate-group lenticular as a whole, but we 

 have seen, in the Caban Quarries, that its individual beds also are 

 lenticular in form. Again, the conglomerates at the eastern limit 

 disappear under the overlying Monograptus-SedgwicJeii Grits, which, 

 in turn, with the Gafallt Shales, vanish beneath the Rhayader Pale 

 Shales east of the Elan, for having once crossed the river we find 

 no trace of these divisions, the Caban Group having wholly dis- 

 appeared under the Ehayader Group. The remarkable appearance 

 of this huge wedge of the Caban Group between the Rhayader 

 and Gwastaden Groups surely demands an explanation. Three 

 interpretations may be suggested : — (1) faulting, (2) overlap, and 

 (3) faulting and overlap in combination. Let us examine each of 

 these in turn. 



(1) Faulting. — This would require in its simplest form a 

 thrust-fault at the base of the Lower Conglomerate, from Caban 

 Coch to Pen-y-rhiw, passing thence gradually up the line of sepa- 

 ration between the Caban and the Gwastaden Groups, until the 

 base of the Rhayader Pale Shales is met with somewhere about 

 Glan Elan. As, however, the line of strike of the Rhayader Pale 

 Shales is uninterrupted on either side of the river, another fault 

 at the base of the Rhayader Group would be necessary. Now, a 

 thrust-fault in its primary form is a level or inclined plane, more 

 or less flat and undeformed. But if the plane at the base of the 

 Caban Series were developed, it would be found to be extremely 

 irregular, pitching and rolling in all directions. It may be argued 

 that, assuming the existence of a thrust-plane, it has been folded 

 and crumpled since the original thrusting had taken place. Such 

 a suggestion, however, is inadmissible ; for one would expect a 

 corresponding series of folds in the outcrop of the Cerig Gwynion 

 Grits lying immediately to the south, and no such folds are to 

 be found in these rocks : their outcrop is even and regular along 

 the line of strike. Again, with the exception of the Caban area 

 at the extreme western limit of the outcrop, no signs of great 

 movements are exhibited in the Caban rocks themselves at all 

 compatible with the results to be expected from so great an over- 

 thrust. The conglomerates on Corn Gafallt Hill run regularly and 

 smoothly, with no signs of folding and thrusting ; and, except in 

 the neighbourhood of small faults, the beds appear undisturbed. 

 Finally, we have seen that in the basement-beds at Ty'n-y-graig 

 the conglomerates contain fragments of the shales that underlie 

 them in the immediate vicinity. Even admitting, then, the ex- 

 istence of a thrust-fault, it is evident that the Caban Series could 

 not have been pushed forward the distance necessary for so huge 

 a lenticle to shove itself between the Rhayader and the Gwastaden 

 Groups. 



Q. J. G. S. No. 221. i 



