The region with which this paper deals lies in the north-western corner of Breconshire, and is situated some 5 miles north of Llanwrtyd Wells, the nearest place of note. The area surveyed is one bordered on the south-west by the rivers G-wesyn and Culant, both tributaries of the Irfon, and stretches roughly 4 miles to the north of this line; its western boundary is marked by the Irfon—Towy watershed. Abergwesyn, a small village, stands at the junction of the Irfon and the G-wesyn. The district comprises part of the plateau of Central Wales, and is drained by the Towy and tributaries of the Wye, of which the Irfon, roughly bisecting the area, is the most important. The Irfon and its two main tributaries, the Gwesyn and the Cammarch, cut through the rocks generally at right angles to the strike, so that the streams (at least on the south side, where the valleys are deeply incised) afford good sections through the greater part of the succession. The country on the north, however, closes in to form a desolate moorland at a height of about 1600 feet, rising to 2115 feet in Drygarn, a prominent mountain-top which can be seen from most parts of Central Wales. This northern region is thickly covered with peat, so that the rock-exposures are much fewer and of by no means so satisfactory a character as on the southern side of the area. The district has, apparently, never been described in detail, and there are no difinite