78 me. k. LArwoETH on the [Feb. 1900, 



fossils are to be obtained from a few black bands about 10 yards 

 below a small footpath over the, stream, about 130 yards above the 

 road. Prom these beds at this point I have extracted Diptograptus 

 modestus, D. longissimus, Kiirck., Climacograptus rectangular is, 

 M'Coy, and CI. medius (?) Tornq. In a thin nodular band at the 

 top of the plantation occur, in addition to Climacograptus rect- 

 angularis, examples of the genus Lindstrcemia, and badly preserved 

 specimens of Orthis sp. 



The rocks of the foregoing series, including the Micaceous Flags 

 and Grits (AbJ, the Rottenstone Beds (Ab 2 ), and the Diplograptus- 

 modestus Flags (Ab s ), have a common lithological character, which 

 distinguishes them collectively from the strata above and below, 

 namely, the preponderance of flags. They form together a very 

 natural group, which I have named the Dyffryn Flags, after a 

 small cottage called The Dyffryn (pron. ' Duffrin ') on the Builth 

 road, about 14 miles from Rhayader. This lies full in the centre of 

 the outcrop of the group. 



The group has been described at some length in the preceding 

 paragraphs, in order to show that within its main divisions, on the 

 line of the typical section, certain subdivisions also can be made 

 out. These subdivisions have been used in mapping as far as they 

 were applicable, but as the examination of the rocks was gradually 

 extended farther east or west of the typical section it was found 

 that these minor subdivisions could not be readily made out. The 

 three divisions of the Dyffryn Flags, however, enumerated above, 

 maintain their general lithological characters more or less throughout 

 the district, and at the same time are quite sufficient to prove the 

 general sequence. 



(c) Ddol Shales. 



Zone of Monograptus tenuis (Ac^), — Crossing the Builth road 

 and ascending to the Wye our section-line meets the river at the 

 bend. Lining the river-banks on the south side of a small island 

 in the crook there is an exposure of rocks strikingly dissimilar to 

 any with which we have hitherto met. In place of the pale 

 weathered grey flags and flaggy shales of the preceding group a 

 series of blue-banded, blue-hearted shales with a few thin flags 

 come in here. The shales weather to brilliant red-brown and 

 mottled green, and are characterized by their curious hummocky or 

 mammillated surfaces. The dip at this point is about 35° north- 

 westward. 



No further exposures of the rocks of this division occur in the 

 river-course ; but on the east side of the Builth road, about 50 

 yards north of a small stream which crosses the road, a few patches 

 of these shales have been laid open. They exhibit the same general 

 peculiarities of weathering coloration, but here a brilliant scarlet 

 predominates. The shales are more shivery, and show a curious 

 ashy whiteness on their weathered surfaces. The flags are sandy 

 and dark grey, and the hummocky surfaces form a characteristic 

 feature. 



