Vol. 65.] SUCCESSION AROUND PLTNLIMON AND PONT ERWYD. 473 



disturbed by a fault wbich traverses the scarp in an east-and- 

 west direction. At one place it was tried for lead. 



Starting from the grit at the cairns as a datum-level, the 

 descending section to the fault shows an alternation of grey 

 felspathic grits with blue-black phacoidal mudstones. North of the 

 fault tbe ground falls gently northwards for about 150 yards, 

 forming a grassy platform ; at the end of this space there is an 

 abrupt plunge down to the lake, although the slope is not so 

 steep as to make the rocks inaccessible. In the upper part they 

 consist mainly of gritty mudstones showing gnarled and knotted 

 surfaces on weathering ; but in the lower part, just above the 

 lake, large numbers of rounded vein-quartz pebbles, together with 

 big cubes of pyrite, are scattered through the mudstones. 



All the rocks in this section bear so striking a resemblance to 

 those of Drosgol that, even if no evidence of their stratigraphical 

 position were forthcoming, one would unhesitatingly assign them 

 to the same general group. In the Plynlimon section, however, 

 the underlying fossiliferous beds of Nant-y-M6ch are not exposed ; 

 but it will be shown below that the grit-group is succeeded by a 

 great mass of mudstones agreeing in every respect with those that 

 succeed the Drosgol Grits on the west side of that hill. 



Returning to the summit of the mountain, the beds of grit which 

 crop out there can be readily traced ; and, as their course brings out 

 the structure of this part of the district in a clear and convincing 

 manner, it may be of interest to describe il in some detail. 



After a few gentle undulations near the cairns, a steady dip to 

 the east sets in which is maintained for nearly 2 miles to the 

 south. The alternations of beds of massive grit and soft mudstones 

 give rise to a series of parallel ridges and hollows on the high 

 ground, which, for a space, divides the drainage of the Rheidol 

 tributaries on the west from that of the Wye tributaries on the 

 east. The strike-features are prominent also down the long slope 

 north of the Peithnant stream. After reaching the bottom of the 

 valley, the grits suddenly swing round to a westerly strike and 

 follow the valley for about 200 yards, then as suddenly turn 

 northwards and range up the hillside with a westerly dip. Some 

 200 or 300 yards to the west is another outcrop of grits, parallel to 

 the last, but having an easterly dip. These two outcrops probably 

 join on the northern face of Drum Peithnant, somewhere to the 

 east of the cairn, but this cannot be observed. An easterly dip 

 is maintained until the Hirnant valley is reached, where a westerly 

 dip sets in once more. 



Such an outcrop is easily explained by the nature of the 

 folding to which the district has been subjected ; the rocks are 

 thrown into a series of anticlines and synclines, with a fairly- 

 steady pitch of 12° to 15° in a southerly direction. The axes of 

 the folds range in a direction about south 10° west. 



With such a structure, it is obvious that, by starting from the 

 grits at the nose of one of the anticlines or synclines, and proceeding 



