Yol. 62.] TARANXON SERIES OF TARANNON. 667 



localities, however, in this northern area this species is common at 

 the same horizon, and therefore its rarity here may be accidental. 

 This highest black band is overlain by 40 feet of typical pale-green 

 mudstones, with occasional thin flags, and the collective fauna of 

 the Lower Green Mudstone Sub-group in this section must be about 

 90 feet. 



Middle Purple Mudstones (Bd 7 ). — The overlying Purple 

 Mudstone Group is well exhibited in the bed of the Afon Iaen, the 

 strata here striking 60° east of north, and dipping at 35° north- 

 westward. It consists of the following divisions, in ascending 

 order: — (1) 40 feet of purple mudstones, with one or two thin 

 green flaggy beds near the top ; (2) 1 foot of green mudstones ; 

 (3) 18 inches of purple beds; (4) 5 to 6 feet of green, (5) 12 feet 

 of purple, (6) 8 to 10 feet of green, and (7) 15 feet of purple 

 mudstones ; making a total thickness of about 85 feet. The 

 thicknesses of the individual bands must be regarded as merely 

 approximate, as the bed of the stream is not accessible at this 

 locality. The highest of the purple bands extends to and slightly 

 beyond the small footbridge, and is overlain by the typical Upper 

 Green Mudstones. 



This Purple Sub-group is fairly well exhibited also along the road- 

 side a few yards to the east of the river, and in the small stream- 

 course above the road at Pandy-bach ; but the thicknesses of the 

 individual purple and green bands are not the same as in the river- 

 section. 



Upper Green Mudstone Sub-group (Bd 3 ). — Eeturning to 

 the section in the Afon Iaen, we find that the Upper Green Mud- 

 stones succeed the Purple Sub-group, and can be followed along 

 their dip for some distance, the angle of inclination varying from 

 35° to 42° north-westward. These strata present the same cha- 

 racters, of compact mudstones with rare flaggy beds, as those which 

 distinguish this sub-group in the Tarannon section. 



These beds are particularly well exhibited along the western side 

 of the railway-bank just above the Iaen, at Pandy-bach. Unlike 

 the corresponding beds in the Tarannon section, the Upper Green 

 Mudstones have yielded fossils. These occur in two black shale- 

 bands on the west side of the railway, which dip at 42° north- 

 north-westward. The upper seam, which is the thicker, contains 

 Retiolites Geinitzianus (C), M. priodon, and M. crenulatus (f. C). 

 Some 30 yards above these black bands the beds are disturbed and 

 penetrated by quartz-veins, black shales being involved in the fold 

 at the foot of the bank ; while a few yards farther north two dark 

 bands, also about 4 feet apart, are well shown high up in the bank. 

 These black graptolitic seams are apparently terminated abruptly 

 on the south by a small fault ; and there can be little doubt that 

 they are identical with the two just described, having been brought 

 up again to the surface by an anticlinal fold and fault combined. 



Compact green mudstones continue to the extreme end of the 



