LLANDEILO FLAGS ON THE EAST FLANK OF THE BERWYNS. 307 



approach to the south-eastern face of the Berwyns, the section across them exhibiting 

 a number of anticlinal and synclinal lines. 



The structure of the tract between the Ffyrnwy, and the Taunat, as explained above, 

 and in the section (PL 32. f. 9.) was pointed out to me by Professor Sedgwick. To 

 determine the boundary line between the Cambrian and Silurian Systems, he led me to 

 the flanks of the Berwyns, and on the banks of the lurch, a tributary of the Taunat, 

 we observed that the rocky gorge called Craig-y-glyn and the ridge 1 of Mynidd-maw r r. 

 were composed of Lower Silurian Rocks. These strata dip to the S.E., beneath the 

 Caradoc Sandstone, and repose upon the more ancient rocks of the Berwyn chain. (PL 

 33. f. 9.) They consist of blueish and dark grey calcareous flagstones, slightly mica- 

 ceous, passing into earthy limestone and also into sandstone. Joints of encrinites 

 abound in the limestone, and casts of them in the sandstone. The most interesting fossil, 

 however, is the Asaphus Buchii. It has before been stated, that this crustacean charac- 

 terizes the Llandeilo flags ; a formation well defined by its mineral and zoological cha- 

 racters. The clear evidence of the relative antiquity of these beds was therefore a subject 

 of great satisfaction ; particularly as it proved, that however great and frequent might 

 be the lines of disturbance in this region, occasioning many breaks and powerful un- 

 dulations ; still the strata had progressively increased in thickness from south-east to 

 north-west, exhibiting a fulness and clearness of succession, which it was very import- 

 ant to point out, in the history of such ancient deposits. 



These fossil beds are affected by slaty cleavage like those of the Moel-y-garth, but 

 to a much greater degree. They dip from 15° to 25° to the south-east, the inclina- 

 tion decreasing towards the valley of the Tanat. The lines of true bedding could 

 with difficulty be traced if the rock were not fossiliferous, but the trilobites, encrinites, 

 and other organic remains, including the Orthis protensa, (PL 22. f. 8. 9.) mark the 

 laminse of deposition. The other lines which obscure the true stratification are two-fold. 

 1st. Innumerable lines of cleavage, which cross the beds at high angles, dipping 70° 

 north-west : these are all precisely parallel to each other, the cleavage passing directly 

 through the bodies of organic remains, pebbles, and concretions. 2ndly. Joints, nearly 

 vertical, more or less irregular, which are several feet apart. 



Veins containing lead ore, penetrate the flagstones, and are more or less abundant 

 along the eastern flanks of the Berwyn chain. As persons unaccustomed to work in 

 countries where the strata have the slaty impress, would have difficulty in distinguishing 

 the true dip from joints and planes of cleavage, I refer them to the writings of Professor. 

 Sedgwick, who first clearly drew the distinction, and to the diagrams pp. 359, 360, 393, 

 and 400, explanatory of the structure of similar rocks in Caermarthenshire and Pem- 

 brokeshire 2 . 



1 Cader Ferwyn is 2715 feet above the level of the sea. These lower ridges are about three miles north of 

 Llanrhaiadr, and rather more distant from the celebrated and striking waterfall of Pistill Rhaidyr. (See Map.) 



- In this example the position of the organic remains removes all doubt in distinguishing the lamina? ox 

 deposit from the planes of cleavage. In those rocks where organic remains are absent, Professor Sedgwick 

 has taught us, that we may generally conclude that the great lines of cleavage which affect a whole mountain, 



