512 MESSRS. P. LAKE AND S. H. REYNOLDS ON THE [Aug. 1 896,. 



II. Description of the Area. 



The area with which this paper deals lies south and west of 

 Dolgelly, between the Arthog road and the hill called Mynydd 

 Gader, which stands in front of the precipices of Cader Idris. 



The town of Dolgelly stands in the valley of the Wnion, a short 

 distance above the point where that river enters the broad and 

 picturesque estuary of the Mawddach. South of the river the 

 ground rises generally, but not uniformly, to the west of the Cader 

 Idris ridge. From the eastern end of the town a deep and rather 

 narrow valley, cut by the Afon Aran, runs in a southerly direction 

 into the rising ground ; and from Pandy'r-odyn, a short distance 

 west of Dolgelly, another stream has carved a second valley which 

 stretches away to the S. W. along a line of fault, which we shall call 

 the Dolgelly fault. 



Between these two valleys, immediately south of the town, the 

 ground rises steeply to a broad and rocky plateau occupied by a mass 

 of diabase ; and then sinks slightly to a lower and more level plain, 

 which stretches across the map from east to west, and on which is 

 built the house called Ehydwen. In the eastern part of this plain 

 the Nant Ceunant cuts for some distance a deep and narrow gorge 

 along a line of fault ; and from this fault the ground again slopes 

 up, with some inequalities, to the foot of the diabase crags of 

 Mynydd Gader. 



"West of the Dolgelly fault, the contour of the ground, so far as- 

 our map extends, is nearly the same as it is on the east. South of 

 the Arthog road there is a steep rise which gradually becomes 

 gentler as we approach the top of a great rounded mass of diabase ; 

 and on the southern side of the diabase the surface falls to a tract 

 of marshy land — the continuation of the plain of Rhydwen. 



In the N.W. corner of the map, the diabase is narrow and makes 

 only a low and unimportant ridge, while the highest ground is 

 formed by the Lingula-Ylags north of it. 



III. Geological Structure. 



The area examined is divided into four parts by three nearly 

 parallel faults running from S.W. to N.E., the most important of 

 which lies in the valley of Llyn Gwernan and Pandy'r-odyn. 



Another fault, south of Dolgelly, follows the general direction of 

 the lower part of Nant Ceunant, and brings the diabase on tho 

 north against the Middle Lingula-be&s on the south. Along the 

 line of this fault there is a zone of rock crushed into a kind of stiff 

 green clay. It is visible in the bed of the stream itself, but is still 

 better shown in the cliff which forms the right bank of the Afon- 

 Aran just above its junction with the Nant Ceunant. 



The third fault coincides for a short distance with the stream that 

 flows east of the farm called Derwas, in the north-western corner 

 of the map. It brings the black slates and the lava-band of 

 Bryn-y-gwin woods into contact with the Middle Z/in#wZa-Slates of 

 Derwas and Gwern-y-barcud, the beds on both sides striking nearly: 

 east and west. This we may call the Derwas fault. 



