674 MISS E. II. R. WOOD ON THE [Nov. I906, 



This band undoubtedly marks the same horizon as that at the 

 summit of the Dolgau Group described in the road-section (p. 668). 

 Above it the Dolgau Beds pass up into the Nant-ysgollon Shales, 

 and these in their turn into the overlying Fynyddog Grits. 



(v) Stream east of Lledcwm. 



The passage from the Dolgau Group into the Nant-ysgollon 

 Shales and from those into the Fynyddog Grits may be examined in 

 a small stream, on the south side of the hill east of Lledcwm. The 

 uppermost band of the Dolgau Group here yields Monograptus crenu- 

 latus^ M. priodon, M. Linnarssoni, and Metiolites Geinitzianus, and 

 is overlain by typical Nant-ysgollon Shales with their characteristic 

 fossils. In some of the higher beds of this Wenlock Group a few 

 small brachiopods were obtained, associated with Monograptus 

 riccartonensis, which appeared to be the characteristic fossil. 



(vi) Afon Cwm-Calch Section. 



The Afon Cwm-Calch is the main tributary of the Iaen. It rises 

 on the Tarannon tableland near Sarn-ddu, and flows in a general 

 north-easterly direction to join the Afon Iaen near Talerddig. 



For the lower half-mile of its course its valley is occupied by 

 various members of the Talerddig Group, which are bent into gentle 

 undulations. The highest grit-band is shown just below the ford 

 and small footbridge south-west of Fron ; and some few yards 

 above a thin dark shale-band, only a fraction of an inch thick and 

 weathering to a brilliant orange tint, yields the following Talerddig 

 species, with the local addition of the form here provisionally 

 referred to M. dextrorsus, which occurs in great abundance : — 



Monograptus cf. dextrorsus (C). 

 Monograptus griestonensis (f. C). 

 Monograptus subconicus (C). 

 Monograptus priodon (C). 



Monograptus discus (f. C). 

 Monograptus nudus (f. C). 

 Monograptus Marri (R). 



No further exposures of the Talerddig Grits are seen in the 

 valley, but they occupy the whole of the high ground of Ffridd-yr- 

 Tstrad on the south-east. 



The Dolgau Group is exposed in the valley between this hill and 

 that of Sarn Bigog, but its junction with the underlying Talerddig 

 Grits is not seen. The passage between them and the overlying 

 Nant-ysgollon Shales is, however, well exhibited in the steep and 

 narrow gorge of Nant-ysgollon, which affords the finest and most 

 picturesque section of these beds in the district. 



These confirmatory sections prove clearly that, throughout the 

 whole of this northern area, the lithology, the fossils, and the 

 sequence of the strata are similar to those in the Tarannon section 

 on the south. 



