Vol. 62.] IGNEOUS AND SEDIMENTARY ROCKS OP LLANGYNOG. 231 



the south side of Glog-ddu hill, where it apparently cuts out the 

 green beds and brings the red marls against the Tetragraptus- 

 conglomerates, although the former are buried under drift. 



At Pen-gelli-uchaf the green beds reappear, and are well-exposed 

 in the fields between that place, Waun-das, and Pen-y-coed. They 

 contain numerous beds of hard rocky marl full of pebbles (ranging 

 up to an inch or so in length) of rhyolite derived probably from the 

 igneous masses of the district. Instead of dipping south-eastward 

 towards the succeeding red marls, they dip about north- north-west, 

 and apparently overlie the red marls. They must, therefore, be 

 either inverted or faulted-off from the red marls on the south. 



They are faulted on the north and west against Tetragraptus- 

 Beds ; and, for some distance in the direction of Coomb, appear to be 

 inverted, as they dip north-westward towards the Tetragraptus-Beds. 



North of Coomb the base of these green beds leaves the fault by 

 which they are separated from the Tetragraptus-~Beds on the north, 

 and can be traced with complete precision along the eastern side of 

 the Coomb dingle, crossing the successive outcrops of the andesites 

 and rhyolites of the Coomb complex ; while, on the western side of 

 the dingle, the lowest bed, a rhyolite -conglomerate, 10 feet thick, 

 can be seen dipping at 60° from the rhyolites along the southern 

 side of Castell Cogan. 



At Pentre-newydd the green beds pass from the igneous rocks 

 onto the Didymograptus-bifidus Beds, and the junction may be 

 examined in the road west of the Cross Roads. 



(b) Detached Areas. 



Between Waun-das and Moelfre a small area of the basal green 

 beds has been thrown-in to the north of the main outcrop ; it is 

 faulted on all sides, except on the north. The beds consist of green 

 and buff marls and highly-micaceous sandstones, with occasional 

 bands of rhyolite-conglomerate. The green marls and sandstones 

 are best seen in Pen-y-coed Brook 300 yards north-north-west of 

 Waun-das, and dip south-south-eastward at 25°; from beneath 

 them rise lower beds, which contain conglomerates visible in the 

 fields to the north. The conglomerates are, however, still better 

 exposed at Ffynnon-wen, and also at Moelfre, where they strike 

 nearly due east and west with a high dip, and contain large pebbles 

 of a felsitic rock. 



One other, though somewhat doubtful, patch of these basal, beds 

 remains to be described. On the southern edge of a low gorse- 

 covered ridge, a small roadside-quarry 80 yards west of Capel 

 Bethesda exposes some 8 feet of a very coarse conglomerate, made 

 up of well-rounded pebbles and boulders (ranging up to about a foot 

 in diameter) of white rhyolite, among which a few small pebbles of 

 vein-quartz and fine white quartzite are present. The upper part 

 of the section is much weathered, and there is very little matrix ; in 

 the floor of the quarry the rock is more coherent. 



We are uncertain whether to refer this conglomerate to the green 



