Vol. 62.] TAKAXNON SEEIES OF TAEANNOX. 673 



remember that the folds pitch at a considerable angle to the 

 southward, thus exposing higher beds in a southerly direction. 



(iii) Braichodnant Stream-Section. 



We have already seen that all the grit-beds of the Wenlock 

 Series have been removed by denudation in the valley of the Iaen. 

 To the south of that river, however, rise abruptly the steep slopes 

 of Xewydd Eynyddog, attaining to an elevation of 1400 feet. 

 The top of this hill is capped by the massive grits belonging to 

 the Denbighshire Series, and some of the streamlets which flow 

 down its northern slopes present us with a complete sequence 

 through the Xant-ysgollon Shales down to the Dolgau Beds of the 

 main transverse section. 



The westernmost of these streams is the Braichodnant, which 

 flows almost due north into the Iaen east of Braichodnant Farm. 

 The lower 600 yards of its course are occupied by the Dolgau Group ; 

 but the upper beds are not exposed, and the junction with the 

 Nant-ysgollon Shales is not visible. About 300 yards from the 

 point where the stream is joined by a small tributary we find an 

 exposure of bluish mudstones and shales, which belong undoubt- 

 edly to the lower part of the Nant-ysgollon Shales. These have 

 yielded at one or two horizons typical Lower Wenlock graptolites — 

 Monograptus vomerinus, M. priodon, and Retiolites Geinitzianus. 



Proceeding up the stream, the slope of which becomes increasingly 

 abrupt, we find a continuous exposure of the j^ant-ysgollon Shales. 

 The upper beds are less shaly than the lower, the mudstones 

 becoming more compact and weathering into large spheroids. 

 These upper beds have not yielded fossils as yet. 



Immediately above come the Fynyddog Grits, the junction 

 between them and the JS"ant-ysgollon Shales being well seen at the 

 bend of the stream below the point where the 1000-foot contour- 

 line cuts it. The grits are massive, attaining a thickness of 2 to 

 4 feet, and, with alternations of flags and shales, they continue up to 

 the summit of the hill. The total thickness of the JNant-ysgollon 

 Shales in this section must be at least 500 feet, which agrees fairly 

 well with that (450 feet) estimated in the Tarannon section. 



(iv) Plas-bach Stream-Section. 



The junction of the Nant-ysgollon Shales with the underlying 

 Dolgau Beds is well seen in the stream which flows down into the 

 Afon Iaen, near Plas-bach. 



The high, steep banks of the lower part of the stream are occupied 

 by the Upper Green Mudstones of the Dolgau Group. At a paint 

 where the 700-foot contour-line cuts the bed of the stream, a black 

 shale-band has yielded abundant graptolites in a good state of 

 preservation. The following species were recognized : — 



Monograptus cremdatus (C). 

 Monograptus spiralis, Tullb. (f. C). 

 Monograptus cf. nodifer (f. C). 

 Monograptus speciosus (C). 



Monograptus Linnarssoni (f. C). 

 Monograptus priodoll (f. C). 

 Monograptus nudus (R). 

 Retiolites Geinitzianus (f. C). 



