Vol. 62.] 



TAKANXOX SERIES OF TAKANNON. 



(iii) Gelli-dywyll Stream-Section. (See fig. 3, p. 676.) 



Leaving the southern part of this western area and going north- 

 ward towards its centre, we rind that of all the small tributary 

 streams of the Twymyn which drain this part of the Tarannon 

 tableland, the only one that affords a good section is that near the 

 farm of GeHi-dywyU. The heights of Mynydd-esgair above the 

 source of the stream are occupied everywhere by the Talerddig Grits, 

 while the stream itself reveals a complete section of the underlying 

 Gelli Group down into the Brynmair Beds. The Gelli Beds are, on 

 the whole, less sandy in this central area than in the southern part 

 of the country, and contain more mudstones and shales. Two 

 grits, about 2 feet and 1 foot thick respectively, similar to those 

 observed in the Tarannon section low down in the Gelli Group, 

 occur close to the eastern gate of the farmyard, and are repeated 

 on the east by an anticlinal fold. Some papery shales overlying 

 these two grits yield Monograptus crispus (f. C), M. priodon (C), 

 and M. discus (K) [(1) in fig. 3j. 



Descending the stream, we find the Gelli Beds continued to a 

 point some 300 yards west of the farm. The strata are much 

 folded, and have not been examined for fossils in detail ; though 

 M. crispus was found in abundance on the northern bank, some 70 

 to 80 yards below the mill-wheel. At a point (2) in fig. 3, however, 

 where a small tributary stream enters on the northern bank, a band 

 of dark-blue to black mudstone, dipping at an angle of 53° north- 

 westward, yields : — 



Monograptus runcinatus, rout. (C). 

 Monograptus cf. proteas (0). 

 Monograptus nudus (f. C). 



Rastrites Linncei (young) (f. C). 

 Monograptus turriculatus (B). 

 Eetiolites cf. obesus, Lapw. (R). 



This assemblage of species has only been met with at one other 

 locality in the district : namely, in the Twymyn, about halfway 

 between Gelli-dywyll Mill and this Gelli-dywyll stream ; and it is 

 difficult to identify some of the forms with known species. The 

 fauna, however, appears to indicate that these beds must be referred 

 to a horizon fairly high up in the Brynmair or M. -turriculatus Group. 

 I have provisionally drawn the boundary-line between the Gelli and 

 Brynmair Beds above this point along a line of disturbance. The 

 Gelli Group in this section must be at least 800 feet thick. 



The Brynmair mudstones and shales are continued in gentle 

 folds down to the junction of the stream with the Twymyn, where 

 the beds are vertical in position ; no graptolites were obtained from 

 them. 



(iv) Section in the Gelli Stream. (See fig. 4, p. 678.) 



The Gelli stream north of Llanbrynmair village affords a very 

 similar section to that just described, but here practically the 

 complete thickness of the Brynmair Beds is shown. * This 

 small stream rises high up on the western flanks of JNewydd 

 Fynyddog, and unites with the Twymyn south of Bont-y-Green. 



