﻿416 MISS DREW AND MISS SLATER ON THE [Aug. I9IO, 



mainly the tough grey quartzite named after that hill. This is 

 followed by blue Llathige Mudstones, and underlain by Beili 

 Tew Shales, On the south-west side the dome has been broken 

 up by radial faults into a series of wedge-shaped masses, the Beili 

 Tew Shales being exposed in the centre. One of these faults is 

 marked by a massive quartz-vein. To the south-east the hill falls 

 gently, and the grit is followed by blue Llathige Mudstones ; but 

 on the north-west there is a sudden change of slope, marking the 

 position of a great fault, which ranges for at least a mile to the 

 south-west across Banc Beili Tew and is probably continuous with 

 the Bwlch Cefn Sarth Fault. This latter hill is mainly composed 

 of rocks belonging to the Beili Tew Group ; but in the north- 

 eastern corner Pen-y-ddinas Grits occur, introduced by a series of 

 three parallel step-faults at right angles to the direction of the 

 hill. (See fig. 4, p. 415.) 



VII. General Summary. 



On the south there is a great development of grits and shales 

 belonging to the Beili Tew Group, which we believe to be of 

 Hartfell age. 



The rocks of our Caio Group cover a wide area, and at many 

 scattered localities we have obtained a fauna from which the genus 

 Monograptus is entirely absent, while Climacograptus and Diplo- 

 graptus are fairly frequent. We probably have here the repre- 

 sentatives of the Lower Birkhill zones up to that of Monograptus 

 cyphus. We have not, however, obtained sufficiently detailed 

 palaBontological evidence to elucidate the stratigraphical relation- 

 ships of the different beds. 



In our Llansawel Group we have the zone of Monograptus cyphus 

 represented at the base, followed by the zone of M. communis. 

 Above this are 1200 feet of rocks from which we have been unable 

 to obtain any fossils, but they are presumably the equivalents of 

 the highest zones of the Birkhill and the lowest zones of the Gala 

 Shales. These barren beds are followed by 900 feet of rock 

 yielding Lower Gala forms, and above this again is an enormous 

 extent of rolling beds, which yield at distant intervals graptolites 

 of much the same horizon. There is consequently no evidence 

 that the overlap which occurs at the base of the Gala Beds in 

 the Rhayader district extends to the Llansawel area. 



VIII. Comparison with the Deposits oe other British Areas. 



The only areas with which we have thought it necessary to 

 compare that of Llansawel are the neighbourhood of Rhayader 

 described by Dr. H. Lapworth, 1 and that of Pont Erwyd dealt 

 with in the recent paper of Prof. 0. T. Jones. 2 



The general character of the deposits in these three districts 

 appears to be closely similar, with a marked absence of calcareous 



1 Quart. Journ. Geol. Soe. vol. lvi (1900) p. 67. 



2 Ibid. vol. lxv (1909) p. 463. 



