Vol. 62.] TARAUNON SERIES OF TARANNON. 685 



the typical Upper Brynmair mudstones, as exhibited near Bont- 

 y-Green, occupy about three quarters of a mile of the Twymyn 

 Valley to beyond Gelli-dywyll Mill. Near Bout Dolgadfan (10 in 

 fig. 5, p. 680) these beds dip 37° almost due north, and contain 

 abundant examples of Monograptus cf. clensus. 



(ii) Afon Fachdre, Pennant. (See fig. 6, p. 686.) 



Farther up the Twymyn Valley other important exposures occur, 

 showing the relationship of the Brynmair Group to the older 

 Llandovery Beds. These are exhibited in the Twymyn and its 

 tributary the Afon Fachdre, near the hamlet of Pennant, about 

 2 miles south of Dolgadfan. 



Brynmair Group (Ba). — The bed of the Twymyn, from Pont- 

 bren Llam to the Pennant factory, is occupied by the typical mud- 

 stones of the Upper Brynmair Group, which are arranged in gentle 

 undulations, the dip varying in amount from 8° to 10°, and the 

 strike swinging round through an angle of 90°. A few yards to 

 the north-east of the factory itself, a small thrust-fault with a low 

 hade is seen in the left bank, the crush-band being 1*6 to 2 feet in 

 width. The throw of this particular fault is slight, for the beds on 

 both sides are similar in lithological character ; but it is significant, 

 as indicating the type of movement met with in this part of the 

 district. These Brynmair strata (1 in fig. 6, p. 686) afford grapto- 

 lites in the thin shale-partings between the mudstone-beds, including 

 Monograptus cf. e.viguus (v. C), M. cf. densus (C), M. turriculatus (R), 

 M. nudus (f. C), and Petalograptus palmeus, var. tenuis. 



Dolgadfan Group (Ab). (Zone of Monograptus fimbriatus, 

 Ab 1# ) — Leaving the main river, and ascending the tributary of the 

 Pachdre, we find these Upper Brynmair mudstones,, exposed in the 

 stream for some 50 or 60 yards. On the northern bank, however, 

 at the point where the brook turns abruptly to the south, and quite 

 close to the old Mill-wheel (2 in fig. 6, p. 686), we come suddenly 

 upon older rocks of a different lithological type, much contorted and 

 disturbed. They consist of finely-banded mudstones and shales, 

 weathering deeply to an ashen-grey tint, and stained orange ; they 

 yield an abundance of graptolites in some of the darker seams. 

 The following species were identified : — 



Monograptus fimbriatus, Nich. (v. C). 

 Monograptus gr eg arius (f. C). 

 Climacograptus Hughcsi (C). 



Diplograptus magnus (C). 

 Diplograptus tamariscus (f. C). 

 Diplograptus parvulus, H.Lapw. ? (C). 

 Petalograptus minor (f. C). 



Climacograptus rectangukcris (f. C). 

 Climacograptus scalaris (C). 



This fauna possesses so many features in common with that of 

 the Dolgadfan Beds (zone of M. convolutus), that there can be no 

 hesitation in including these deposits with the Dolgadfan Group ; 

 but the rarity of species of Monograptus, and the great prepon- 

 derance of Climacograptids and Diplograptids, point clearly to the 

 probability of these beds belonging to a lower horizon in that 

 group. The abundance of M. fimbriatus naturally suggests that 



