Yol. 56.] THE SILURIAN SEQUENCE OF RHAYADER. 105 



(b) Confirmatory Sections through Corn Gafallt Hill. 

 (1) Clap-round. 



The accompany in g section (fig. 11, p. 104) is taken through a 

 spur on the south side of Corn Gafallt Hill at the eastern end of 

 Erw Fawr Wood, and is continued up to and beyond the summit. 



The Lower Conglomerate (Ba x ) is admirably exhibited in a 

 series of fine scarps extending for a vertical height of about 300 

 feet up the hill, and dipping at about 20° north-north-westward. 

 The beds show the same rotten felspathic matrix as that described 

 on p. 100, in which are embedded large grit- and quartzite- 

 pebbles, with smaller pebbles of white quartz, felsite, etc. The total 

 thickness, as measured on the ground, is about 700 feet : at Caban 

 Coch it is less than one-third of this amount. So sudden an 

 increase in thickness, in a distance of | mile, is difficult to account 

 for : some proportion of it may be due to strike-faulting ; but to 

 prove this is no easy matter. Certainly no sign of such a dis- 

 location can be discerned east or west of the section. Allowing, 

 however, for the remote possibility that the whole of the beds are 

 repeated, we should still have a thickness of 350 feet for the whole 

 group, or 150 feet more than at Caban Coch. 



A well-marked hollow and scarp at the summit of the lower 

 group seems to indicate the place of the Intermediate Shales 

 (' Ba^), but no exposures are visible. The estimated thickness is about 

 150 feet. The Upper Conglomerate (Ba 3 ), which succeeds the 

 former, has increased in thickness to 150 feet. As a whole, the 

 pebbles are much smaller than in the Lower Conglomerate, and 

 consist mainly of white vein-quartz and grey grit. 



The Gafallt Beds (Bb) are now displayed to their best ad- 

 vantage, and fairly accurate measurements may be taken. The lower 

 division (Bb x ) has a thickness of about 250 feet, of which the first 

 200 consists mainly of regularly alternating black-banded grits, and 

 grey-and-green shales. The grit-surfaces of the beds, as before, are 

 strikingly parallel, and the sinuous curves of many of the black 

 bands, as seen on the joint-planes, suggest false bedding. The 

 average thickness of the individual grits is from 4 to 6 inches, but 

 this is exceeded in places at the base of the group. The indurated 

 red weathering and the plentiful occurrence of crinoid-rings still 

 form characteristic features. The uppermost 50 feet or so consists 

 of a series of dark grey and white-striped grey flaggy shales, very 

 much cleaved and contorted ; and the whole group is capped by a bed 

 of puddingstone, measuring at this point about 30 feet in thickness. 

 This forms a very persistent band along the strike of the hill, and 

 has proved exceedingly useful in the field for following out the run 

 of the beds. The pebbles are chiefly of vein-quartz, and rarely 

 exceed a cricket-ball in size ; fragments of pale-blue slate are not 

 uncommon. The matrix is very felspathic, for detached blocks of 

 the rock weather into characteristically spheroidal boulders, the 

 Tock being recognizable at a glance. The decomposed rock has a 

 honeycombed structure, as a result of the corrosion of the felspar. 



