668 MISS E. M. R. WOOD ON THE [Nov. I906, 



railway-section above Pont Pant-glas, where the axis of a synclinal 

 fold is exhibited both on the eastern and western banks, the beds 

 dipping at angles of 15° to 20° north-eastward. A thickness of 

 about 2 feet of black shales is exposed in the centre of this fold 

 on the west side, and this band is probably identical with that 

 which, as we shall see later, marks the summit of the Dolgau Group. 

 If this be so, then the thickness of these Upper Green Mudstones 

 must average between 250 and 300 feet, while that of the whole 

 Dolgau Group is 420 to 470 feet, figures which agree closely with 

 those recorded in the Tarannon-Eiver section. 



The bed of the laen, from the railway-bridge to beyond Llawr-y- 

 coed — a distance of one-third of a mile — , is occupied by various 

 members of the Upper and Middle Dolgau Beds, their repetition 

 being due to sharp folds of no great depth. The section has been 

 worked on the 25-inch scale, but it need not be described here in 

 detail. 



Between Pont Bell and Plas-bach the Middle Purple Sub-group is 

 brought up to the surface by an anticlinal fold, the purple bands 

 being here much in excess of the green ones. On the west side of 

 the fold the beds are snapped by a few small faults of very slight 

 throw, but the eastern limb near Pont Bell has suffered to a greater 

 extent. Careful examination shows that some of the upper beds 

 are cut out altogether, and the junction between the Purple Sub- 

 group (Bd 2 ) and the Upper Green Sub-group (Bd 3 ) is probably 

 marked by a reversed fault. 



A syncline brings up the Upper Green Mudstones immediately 

 to the west of Plas-bach, while the Purple Beds are well shown at 

 the Ford and opposite Glandwr. Near Factory Llawr-y-coed some 

 compact green mudstones, the dip of which is obscure, are seen in 

 the bed and left bank of the river ; but along the footpath above 

 they dip at an angle of 10° southward, and the axis of a fold, 

 marked by the presence of quartz-veins, is indicated. The Purple 

 Beds are again fairly well-developed in the bed of the river just 

 above Llawr-y-coed, and pass up gradually into the Upper Green 

 Mudstones, while these in their turn are immediately overlain 

 by the Nant-ysgollon Shales. The transition between the Dolgau 

 and Nant-ysgollon Beds is best exhibited along the north side 

 of the road, some 300 yards to the west of Llawr-y-coed, in a 

 high bank, which shows good exposures for nearly 200 yards. 

 There is here, however, no trace of the grit-bands which form so 

 convenient a division-line in the Tarannon-Biver section ; but at 

 about the same horizon there occurs a purplish-black band some 

 2 feet thick, which is particularly carbonaceous at its base. In 

 this lower part graptolites are abundant, although they are in a 

 poor state of preservation ; it is nevertheless possible to identify 

 the following species : — 



Monograptus crenulatus (O). 

 Monograptus priodon (C). 

 Monograptus speciosus, Tullb. (f . C). 

 Monograptus (Ci/rtograptus) spira- 

 lis, Tullb. (f. O). 



Monograptus nudus CR). 

 Monograptus Linnarssoni, Tullb. 

 Betiolites Geinitzianus (f. C). 



