Yol. 56.] SILURIAN SEQUENCE OE RHAYADER. 77 



Kottenstone Beds (Ab 2 ). — Continuing the section up to and 

 beyond the second cliff below the fence previously mentioned, one 

 next passes over a thickness of 50 feet of rock (Ab 2 ), possessing 

 features so distinctive that it is clearly entitled to rank as a separate 

 division. The brilliant weathering colour of the underlying beds 

 extends for a short distance into this group, but through the greater 

 part of it tones down into a pale red- orange. The black-banded 

 grits are still met with in this group, but are rarely more than 

 l inch thick. The blue flags become thicker as the grit-bands 

 thin out, and at the summit of the division are as much as 2 feet 

 in thickness. They are, however, less micaceous than in the under- 

 lying rock-group. At the commencement of this group inter- 

 calated shales make an appearance for the first time. These are 

 distinctly platy in the lower half, but as they pass up they become 

 softer, and finally are seen to be almost shivery. Readily yielding to 

 the influence of the weather, and splitting off in numerous fragments, 

 they strew the banks of the stream with a litter of shaly screes. At 

 the summit of the group these shales are highly nodular in structure,. 

 and in this form pass into the lower subdivision of the succeeding 

 group. 



The chief characteristic of this division is the presence of a large 

 quantity of calcareous matter in the beds. This is especially 

 evident in the occurrence of bands ofrottenstone among the flags 

 and shales. These rottenstone-seams reach a thickness of 3 or 

 4 inches. The calcareous matter has often been almost removed, 

 and has left only a very soft earthy deposit of iron-oxide. The 

 rottenstone-bands thin out and disappear at the top and bottom of 

 the group, but in its centre they form so distinctive a feature that 

 wherever this division is exposed it is recognizable at a glance, and 

 has, in consequence, constituted a zone of the greatest utility in 

 unravelling the rock-sequence of the district. The dip is about 33° 

 north-westward. 



Fossils are very rare in this zone, but a few of the carbonaceous 

 bands yield Diplograplus modestus, Lapw. and Climacograptus 

 normalis. 



Diplog rapt us -modestus Flags (Ab 3 ). — The Eottenstone 

 Group passes upward into a series of hard grey-and-blue flags with 

 flaggy shales (Ab 3 ). Some of the blue flags of the previous group 

 have here given place to grey flags, and the grits are represented 

 by only a few |-inch bands, dying out at the summit of this division 

 into thin white siliceous seams, scarcely exceeding y 1 ^ inch in thick- 

 ness. The flags average 9 inches at the base of the division ; but 

 at the top they are only about 6 inches thick ; in the upper half 

 they are usually interbanded with fine brown and green seams. 

 They weather to a pale orange or yellow colour. The dip of the 

 strata slightly increases as we ascend in the group, and at the point 

 where the Prysg stream crosses the Builth road, practically at the 

 summit of this division, the rocks dip at an angle of 35°. The 

 total thickness of the group is about 300 feet. Characteristic 



