Vol. 62.] TARANNON SERIES OF TARANBTOIf. 669 



Of these forms the first four occur also in the lowest part of 

 the Dolgau Beds, but three new species here make their appear- 

 ance in addition. The fauna, however, on the whole is of the 

 Tarannon type, as distinct from that of the C-Miirchisoni zone, 

 and this band is therefore included at the top of the Dolgau Group. 



Nant-ysgollon Shales (Ca). — The beds immediately over- 

 lying this black band are at first similar in lithological characters 

 to those below ; but as one ascends the sequence along the road 

 they are seen gradually to pass into the bluish-brown concretionary 

 shales and mudstones which are so typical of the Nant-ysgollon 

 Shales of the Tarannon section, and indeed of the Wenlock Series 

 throughout Wales. At one locality along the road, immediately to 

 the east of the point where the bank is walled in, the characteristic 

 Nant-ysgollon (Wenlock) graptolites were found in abundance and 

 in a good state of preservation, including: — Monograptus vomerinus 

 {v. C), M. crcmdatus, M. priodon, Retiolites Geinitzianus, and 

 Cyrtograptus Lapworthi (R) ( = C. Murchisoni ?). A thickness of 

 at least 2!00 feet of the Nant-ysgollon Shales is exposed along this 

 section, but the highest beds have been removed by denudation. 



We have here reached the centre of the main synclinal axis 

 of the country, and now begin to descend the sequence. The Xant 

 ysgollon Shales continue for some distance in minor undulations, 

 which are clearly visible in the bed of the Afon Iaen. 



Dolgau Mudstones (Bd). — At apoint, however, at the western 

 end of a small coppice, due south of Cae-twpa Farm, the Dolgau Beds 

 rise out from below the Nant-ysgollon Shales and the black band at 

 their summit is exposed in the southern bank. For the next 300 

 yards the bed of the Iaen is occupied by the various members of 

 this Dolgau Group, and they present the lithological characters 

 that are typical of these beds elsewhere. The Middle Purple 

 division, however, is poorly represented, and consists of a single 

 band of purple mudstone, 8 feet thick, which is seen near Yaenol. 

 Some of the black graptolitic bands at the base of the Dolgau Group 

 are exposed 50 or 60 yards below Yaenol, but no fossils were 

 obtained from them at this locality. 



A small thickness of the highest shale-band of the Talerddig 

 Group is brought to the surface between this point and Pont 

 Dolfach by an anticlinal fold, the eastern limb of which appears to 

 be partly replaced by a fault. Partner west, however, the Dolgau 

 Beds once more make their appearance, owing to the presence 

 of a shallow syncline, and occupy some 200 yards of the river-bed 

 between Dolfach and the bridge below Pen-y-graig. In the centre 

 of this trough a few feet of purple beds arc shown, and on both the 

 eastern and western sides the basement graptolitic beds of the 

 Dolgau Group are seen resting upon the Upper Talerddig Shales. 



On the west side the Lower Dolgau Mudstones reach their best 

 -development in this part of the district, and show at least ten well- 



