Vol. 62.] 



THE TARANNON SERIES OE TAEANNON. 



679 



as developed in the western part of the Afon-Iaen section, are 

 continued down the stream for a distance of a quarter of a mile 

 from the road. These lower Gelli Beds are much disturbed: at 

 intervals they assume a vertical position, and become penetrated by 

 quartz-veins. On the whole, however, they have a general steep 

 north-westerly dip, and strike 20° to 50° east of north. 



At one or two localities typical Gelli graptolites were obtained. 

 At one of these (2 in fig. 4, p. 678), 350 yards below the road 

 above mentioned, Monograptus nudus (C), M. crispus (f. C), M. exiguus 

 {f. C), and M. Marri (C) were collected ; while at another (3 in 

 fig. 4), 80 yards below, the beds yielded M. crispus (f. C), M. nudus 

 (t C), and M. discus (R). This is an undoubted Gelli fauna, and 

 the rarity of M. discus may possibly indicate that the horizon is 

 somewhere near the base of the group. 



Brynmair Group (Ba). — Another main line of disturbance is 

 visible some 50 yards below the last-mentioned graptolitic locality, 

 at a point where the stream makes a sharp bend ; and a few feet 

 lower down a thin band (4 in fig. 4) of shale weathering to a light 

 orange-brown colour yields a large number of small fragments of 

 M. runcinatus together with M . nudus (B), and one undeterminable 

 species of Petalograptus. The occurrence of M. runcinatus in such 

 abundance indicates that these beds belong to the Brynmair Group ; 

 and the abrupt change in fauna is highly suggestive of a fracture 

 of some importance along the line of disturbance just mentioned. 

 The rocks are unfortunately concealed for a distance of some 200 

 yards, but beyond that point to the Twymyn Biver there is an 

 almost continuous exposure of pale-grey mudstones and shales, 

 dipping at low angles of 20° to 30° north-westward. No fossils 

 were obtained from these in the Gelli stream ; but in the right bank 

 of the Twymyn (5 in fig. 4) a soft, dark, shaly band, splitting with 

 a curious cuboidal fracture, yields the following Brynmair graptolites 

 in a poor state of preservation : — 



Petalograptus palmeits, var. tenuis. 

 Eastrites Linncei (R). 



Monograptus turriculatus (f. O). 

 Monograptus Becki, Barr. (O). 

 Monograptus runcinatus (J) (R). 



Another black-shale band, 140 yards farther down the river, is 

 almost made up of a tangled mass of badly-preserved graptolites, 

 including 



Monograptus cf. densus (v. C). 

 Monograptus cf. runcinatus (v. C). 



Monograptus turriculatus (E). 

 BendrograpAus sp. 



Immediately to the east of Bont-y-Green (6 in fig. 4) good 

 specimens of M. turriculatus, M. cf. densus, and M. cf. exiguus 1 were 

 obtained : the occurrence of Monograptus exiguus indicating that 

 the fauna is approaching to that of the overlying Gelli Group. 

 The beds which occupy the course of the river for the next quarter 



1 The form here referred to Monograptus exiguus has the typical shape of 

 the species, but more distant thecce than are considered characteristic. 

 Q. J. G. S. No. 248. 3 a 



