124 MR. H. LAPW0RTH ON THE [Feb. I9OO, 



probably so slight that it would be scarcely visible on a natural-scale 

 section. 



No attempt has been made to lay down more than the southern 

 edge of this series of Pale Shales. They cover an immense area of 

 ground ; for they may be traced nearly to the Severn Arms, some 

 5 miles north-west of the town of Rhayader, and 10 miles north- 

 eastward to Bwlch-sarnau, where they join the Tarannon Shales, 

 indicated on the Geological Survey map. 



Two other localities for fossils may be mentioned. The first, an 

 old quarry on the road to St. Harmons about 300 yards north of 

 Pen-y-bwlch, yields Monograptus exiguus, M. nodifer, Tornq., 

 M. nudus, M. jacidum, M. discus, Tornq. and M. spiralis, Gein. 

 The second, in the lane-side at the ford across Nant Serth, 300 yards 

 south-east of Middle Nant Serth Farm, has yielded Monograptus 

 eaiguus, M. crispus (?) Lapw., 31. nuclus, M. discus, M. priodon, 

 and M. galaensis(l) Lapw. 



In concluding this part of our survey it will be of interest to exa- 

 mine the palaeontological results, and to observe how they bear out 

 those which have been deduced from stratigraphical evidence alone. 

 If we refer to the list of fossils recorded from the topmost beds of 

 the Gwastaden Group, we find that out of the species enumerated, 

 only one — Monograptus lobiferus — passes up into the Rhayader 

 Group. What has become, then, of the remaining forms ? The 

 list of the Caban fossils contains at least four of these, namely : — 

 Monograptus convolutus, M. crenularis, Diplograptus sinuatus, and 

 Climacograptus normalis, besides M. lobiferus, which is common to 

 all three groups. Again, these Caban rocks yield M. Sedgivickii 

 and M. involutus — species occurring in the Rhayader Pale Shales 

 but not in the Gwastaden Group. The Caban beds, therefore, 

 occupy a position intermediate between the Gwastaden and Ehay- 

 ader Groups. This is in perfect accord with the inferences already 

 drawn from stratigraphical results alone. 



III. General Summary. 



(a) The Rhayader Sequence. 



We are now in a position to summarize the collective results 

 which have been obtained from our examination of the Rhayader 

 district as a whole. We have seen that no one section taken in 

 any direction across the country shows a complete succession from 

 bottom to top. In any area or sub-area that we may choose to 

 consider, one group of rocks is always better developed than another, 

 and some portion at least of one group is certain to be missing. 

 Little is it to be wondered, then, that the geology of the district 

 from a stratigraphical aspect has presented problems of some com- 

 plexity. Prom the evidence that has now been secured, however, the 

 true sequence of the various groups is placed beyond question. The 

 detailed stratigraphy alone would be sufficient; but, in addition, 



