Vol. 65.] HARTFELL-VALENTIAN SUCCESSION AROUND PLYNLIMON. 471 



them show, when cleaved, a well-defined parallel structure. I 

 have seen similar structures in other fine-grained sediments which 

 have been subjected to strong lateral pressure, accompanied probably 

 by shearing forces. It is the prevalent structure in the marls in 

 the Old Red Sandstone on the south side of the Pembrokeshire 

 coalfield, where they have come under the influence of the 

 Armorican movement. Also, in the same region, the beds of coal 

 often tend to break up into phacoidal pieces, with extremely sharp 

 edges and highly polished curved surfaces. 



A fault of large downthrow towards the north-west brings the 

 mudstone group to an end. It is visible in section near some old 

 heaps of mine-refuse about 300 yards north-east of Camddwr-mawr 

 Farmhouse ; the fault-plane is nearly vertical, and the brecciated 

 material carries a certain amount of galena. The course of the 

 fault along the north-western flank of Drosgol Hill is indicated 

 either by a deep notch, or else by a sharp fall of tho ground towards- 

 the downthrow side. Its position was accurately indicated on the 

 Geological Survey Map (old series, 59 S.E.) as a metalliferous lode, 

 which at Camddwr is marked as containing lead, while about half- 

 a-mile to the north-east it is labelled as a ' Manganese Vein.' 



On tracing the grits and conglomerates which underlie the mud- 

 stone group along the summit of Drosgol, they are found to end 

 abruptly against the Camddwr Fault, as it may be called. In this 

 region it forms the northern limit of the ground which I have 

 examined in detail. 



Immediately on the north side of the fault, where it is exposed 

 in section, some dark-blue massive mudstones, with well-marked 

 parallel cleavage, dip westwards at an angle of 56° ; their 

 weathered surfaces are stained a light orange colour. Not far below 

 them is an exposure of phacoidal ' black-leaded ' mudstones, 

 resembling those forming the western slope of Drosgol on the other 

 side of the fault. The strata immediately succeeding the orange- 

 stained massive mudstones are concealed under the superficial 

 deposits of the valley of Nant-y-Barracks (name on the 6-inch 

 Ordnance map ; on the 1-inch map it is Nant-y-Benglog), which 

 runs in a northerly direction following the strike. On the west 

 side of that valley blue flags and shales, weathering on the sur- 

 faces to a bright rusty red, are exposed along an old water-leat. 

 As will be shown below, strata of this character occur everywhere 

 immediately above the base of the succeeding Pont Erwyd Stage, 

 while the orange-stained massive mudstones form the natural 

 upward continuation of the underlying ' black-leaded ' mudstones ; 

 these will be referred to collectively as the Bryn-glas Mudstones 

 (Ac) from the name of a prominent hill north of the Aber- 

 ystwyth-Llanidloes road near Dyffryn Castell. They form the 

 whole of that hill, and in the immediate neighbourhood their 

 relations to the underlying grit group and to the overlying Pont- 

 Erwyd Beds can be clearly made out. 



I have found no fossils, either in the Drosgol Grits or in the 

 Bryn-glas Mudstones. 



