684 MISS E. M. E. WOOD ON THE [Nov. I906, 



Passing now to the other and west side of the anticline, we find 

 that, as the river bends to the north, the beds gradually pass from 

 a vertical position to a steady dip of 40° or 50° north-westward. 

 Finely-banded grey and black shales and mudstones are exposed in 

 the bed and left bank of the river, but they are accessible only 

 when the water is at a very low level. At a point between 80 and 

 90 yards due north of 5 in fig. 5, p. 680, and immediately to the 

 south of a small tributary stream (6 in fig. 5) on the right bank, a 

 fossiliferous band yields characteristic Lower Brynmair species : — 



Monograptus regularis, Tornq. (R). 

 Monograptus Haiti (R). 

 Petalograpius minor (R). 

 Climacograptus scalaris (f. C). 

 Diplograptus tamariscus ? (f. C). 



Monograptus turriculatus (v. C). 

 Monograptus runcinatus (v. C). 

 Monograptus jacidum (f. O). 

 Monograptus nicdus (f. C). 

 Monograptus Becki (?). 

 Monograptus involutes, Lapw. (?) (f. C). 



Thus, although the Twymyn Beds have not yet been actually 

 detected in this section, it is practical^ certain that on this 

 western limb of the anticline there is, as in the Pontbren-dibyn, 

 a continuous and undisturbed sequence from the Dolgadfan Group 

 up to the Brynmair Group. 



Returning to the eastern limb of the anticline, we find that the 

 1 SedgiuicJcii '-Beds are cut off abruptly on the east, by a line of great 

 disturbance which is visible all along the eastern bank of the river, 

 the beds being twisted up, faulted, and penetrated by quartz-veins 

 of considerable size. The right bank at this point (7 in fig. 5, 

 p. 680) is high, and is occupied by Brynmair mudstones and shales 

 dipping at a steep angle north-westward. They yield : — 



Monograptus cf . densus (v. C). 

 Monograptus proteus (B>). 

 Monograptus nudus (R). 



Monograptus gemmatus (1). 

 Eastrites Linnm (!). 



The fauna, although not very distinctive, appears to show that 

 these beds occupy a horizon fairly high up in the Brynmair Group. 



Thus, in the valley of the Twymyn at this locality, the Llandovery 

 Beds are folded into a sharp and broken anticline. The Dolgadfan 

 Beds (zone of M. convolutus), represented by two bands, occupy the 

 centre, and are followed on the west side in perfectly conformable 

 succession by the Twymyn Beds (zone of M. SedgwicJcii), and these 

 in their turn by the Lower Brynmair Beds (zone of M. turriculatus). 

 On the east side the ' Sedgivickii '-Beds are seen in part, but they 

 are faulted against the Upper Brynmair Beds, the whole of the 

 lower portion of the group being cut out. 



Additional sections are exhibited in the bed of the Twymyn 

 between the point last described and Bont Dolgadfan, but they 

 have not been examined in detail, owing to the difficulty of access 

 to the exposures. The relations of the beds are therefore only 

 indicated generally on the sketch-map (fig. 5, p. 680). Somewhat 

 older Brynmair Beds, containing Rastrites Linncei, R. hybridus (?), 

 Monograptus runcinatus, M. gemmatus, M. Becki (?), and Diplo- 

 graptus tamariscus, occur some few yards below the weir (8 in 

 fig. 5), and Rastrites I/innwi was found in fair abundance on the 

 north side of the road to the east of the bridge (9 in fig. 5) ; while 



