Vol. 62. ,] THE TAEANNOX SERIES OF TAEAKN T ON. 689 



A complete sequence through the whole of the Twymyn Beds is 

 shown in this section, and they must have a thickness of about 

 150 feet. Another fossiliferous band is found at the base of the 

 Lower Twymyn Beds, about 200 yards north of the tributary 

 stream, and south of a small group of trees on the right bank ; here 

 a black shale-baud rich in iron-pyrites yields : — 

 Bastrites p&regrinus, Barr, (C). 



Monogra/ptus regularis (C). 

 Monograptus crenularis, Lapw. (C). 

 Monograptus liarpago (v. C). 

 Monograptus limatulus, Tornq. (C). 



Monograptus decipiens (?) Tornq. 



(f. 0). 



Climacograptus Hughesi (C). 

 Climacograptus scalaris. 

 JHplograptus tamariscus. 



This assemblage of species corresponds to that of the zone of Oephalo- 

 grcvptus cometa, which elsewhere in Britain, and in Scania, underlies 

 the zone of Monograptus SedgwicTcii. 



The axis of the main Twymyn anticlinal fold is seen still higher 

 up the river, while the junction of the Twymyn Beds with the Bryn- 

 mair Group on the eastern limb of the fold is well exhibited about 

 one-third of the way up the gorge of the tributary stream. Here 

 the ' Sedgiuickii '-Beds, containing the characteristic graptolites, are 

 folded and faulted against Brynmair Shales which have yielded : — 

 Monograptus Halli, M. nudus, Petalograptus palmeus var. tenuis, 

 JRastrites Linncei, Retiolites cf. obesus, and other typical forms. The 

 Brynmair Beds occupy the whole of this gorge up to the top of Poel, 

 and must have a thickness of at least 700 to 900 feet. They are 

 arranged in a synclinal form, which may account for the hill of Foel 

 intervening between the valleys of the Twymyn and the Crygnant. 



Summarizing the results arrived at by the examination of the 

 sections in the western part of the district, we find that, as we pass 

 from the heights of the Tarannon tableland on the east to the 

 valley of the Twymyn on the west, there is a complete descending 

 sequence of strata from the Wenlock Series at the summit, through 

 the Tarannon Series along the flanks, down to the Llandovery 

 Series at the base. Unlike the higher beds at the top of the 

 tableland, which are arranged in a synclinal form, the lower beds 

 are irregularly disposed on both sides of a broken anticline running 

 more or less parallel to the course of the river-valley of the Twymyn. 

 The general arrangement of the strata is shown in the accompanying- 

 generalized sections (fig. 7, p. 688 & PL XL7III, fig. 2). 



Note on the Eastern Area. — We have now completed our 

 survey of the various sections exposed in the Tarannon district, 

 with the exception of those on the eastern flanks of the Tarannon 

 tableland in the direction of Carno. In this area, however, the 

 rocks are so folded as to result in the repetition of the Talerddig 

 group of strata for great distances. ^lost of the streams, too, are 

 entirely occupied by Drift, and exhibit no rock-exposures — it has 

 therefore been necessary to map the boundaries between the forma- 

 tions almost entirely by the form of the ground. None of the 

 sections throw additional light, either on the sequence of the strata 

 or on the structure of the conn try, and they need not therefore be 

 described. 



