112 ME. H. LAPWORTH ON THE [Feb. I9OO, 



Fossils may be extracted from certain carbonaceous seams within 

 the grits. From a small exposure at the roadside about 50 yards west 

 of the tunnel-mouth, the following have been obtained : — Mono- 

 grapius Sedgvnckii, M. spinigerus, M. spiralis, M. Proteus, Diplo- 

 graptus Hughesii, Metiolites perlatus, Mch., Dictyonema venustum(T) 

 Lapw., and Climacograptus normal is ; and from beds about 800 feet 

 inward from the end of the Foel Tunnel (Birmingham Aqueduct) : — 

 Monograptus spinigerus, M. lobiferus, M. sp. with a tenuis-like 

 curve, Dictyonema delicatidum, and Retiolites perlatus. 



This M.-Sedgivickii Group does not cross the line of section, but 

 it is convenient to note the exposure at this stage. The actual 

 traverse passes 100 yards or so east of the point where the upper- 

 most beds of the grit-group meet the river. The puddingstone-bed 

 has not been detected at this locality : it is possible that it is merely 

 of local occurrence, and thins out to the northward. 



Returning to the section, we find ourselves in the Gafallt Shales 

 (Bh 2 ) immediately above the new road. The beds dip at variable 

 angles southward and south-eastward below the crest ; above the 

 crest they seem to be very greatly folded, and even inverted. I 

 attempted to subdivide the Gafallt Shales and to work out the 

 structure of the hill in detail ; but so folded are the rocks, and so 

 similar throughout, that the idea was at last abandoned as im- 

 possible. Two facts, however, are certain : the general dip of the 

 beds to the south and south-east, and the great sag of the Caban 

 Group in the river-valley. The reappearance of a small patch of 

 the M.-Sedgwichii Grit at Careg Bica Cairn, on the north side of the 

 syncline, completes the proof of our general results. Only a few thin 

 black -banded grits are seen in situ at the cairn itself; but they are 

 distinctive and easily recognizable as belonging to the lower grit- 

 division. The small exposure of rock shows intense folding and 

 inversion. 



The whole of the high ground of Cefn Llanfadog Hill is, from 

 Abernant stream to Llanfadog-Uchaf, occupied by these southward- 

 dipping Gafallt Shales, and at Llanfadog-Uchaf a well-marked fault 

 brings down the Rhayader Pale Shales (O) against them. This 

 fault is traceable from Llyn-clap, through the stream-hollow at 

 Llanfadog-Uchaf, on to Pen-y-rhiw Farm, where it appears to die 

 out. The abrupt termination of the puddingstone-bed south of 

 Fron-dorddu is probably due to a branch of the same fault. 



(d) Discussion and Summary. 



Before discussing the possible explanations of the various pheno- 

 mena which have been presented to us during our examination of 

 this Caban Group, it may be as well to recapitulate the general 

 results that have been obtained, The Caban Group consists of 

 two main members — a lower group of two massive conglomerates 

 separated by a series of shales : and an upper group, with grits at its 

 base, passing up into pale grey flags and shales. The whole group 

 occupies the huge syncline, the axis of which runs from north-north- 



