Yol. 62.] IGNEOUS AND SEDIMBNTAEY ROCKS OF LLANGYNOG. 229 



as sandy-shale debris appears at the foot of the slope, the grits 

 evidently extend no farther eastward. The shales themselves, 

 however, are immediately cut off by a north-and-south fault, 

 which introduces the green basement-conglomerates of the Old Red 

 Sandstone of Moelfre Farm. 



(c) Shales south of the Grits. 



Reverting now to the eastern part of the district, we find olive- 

 green shales cropping out in a lane at the western end of the grit- 

 ridge of Glog-ddu ; but west of this point they are concealed by 

 boulder-clay, although they doubtless occupy the low ground north 

 of Pen-gelli-uchaf. They have been quarried on the east side of 

 Pen-y-coed Brook, halfway between Waun-das and Craig. Thence 

 they extend over the low ground south of Ffald, and can be seen at 

 Ffynnon-wen immediately north of the edge of the Old Red 

 basement-beds. South of this Old Red tract, they are poorly 

 exposed in Pen-y-coed Brook, 300 yards west of Waun-das, and 

 consist of highly-micaceous soft shales, with thin bands of hard grit. 

 Grey micaceous shales are to be seen at the junction of three roads 

 half-a-mile east-north-east of Llangynog Church. 



In the road by Llangynog Church, and thence southward to 

 Llangynog Farm, there is a development of thin grits in the shales ; 

 and shales with thin grit-bands are exposed occasionally in the 

 road between the Church and the Plough-&-Harrow Inn. 



West and south of this, the Tetragraptus-Beds are faulted off from 

 the Didymograptus-bifidus Beds of Gardd-erfm and the andesites of 

 the Coomb complex. 



(2) The Didymograptus-bifidus Beds. 



These occupy the ground to the north, west, and south-west of 

 the Tetragraptus-Beds described above, and are everywhere faulted 

 off from them. In the neighbourhood of Llangynog they exist as 

 an inverted series, excellently exposed in the middle part of Cwm 

 Crymlyn, where, although the Tetragraptus-Beds appear to pass 

 normally under them, there is in reality a fault of considerable 

 magnitude which throws the higher beds of the Didymograptus- 

 bifidus shales against the Tetragraptus-Beds. 1 In the dingle, the 

 typical characters of the shales may be observed, and specimens 

 of graptolites of the D.-bifidus type collected. 



A thick band of grit crosses the dingle, and can be followed 

 south-westward past Uchel-ole to a large old quarry on the north 

 of a ravine at Eithin-duon, where it is cut off by a north-westerly 

 fault which runs down the dingle. The grit reappears one-third 

 of a mile farther south, on the western side of the Lambstone 

 porphyry-hill. The grit is followed on the north-west by shales 

 which extend down to the Cywyn Valley. These are to be seen in 



1 ' Summary of Progress of the Geological Survey for 1904 ' Mem. Greol. 

 Surv. 1905, p. 47. 



