I. I ntroduction . The area described in this paper is situated in North-West Radnorshire. The village of Abbey-Cwmhir, which is about 5 miles north-east of Rhayader, is in the centre of the district. The southern boundary is about a mile and a half south of Nantmel, and runs approximately parallel to the main road from Rhayader to Pen-y-bont. The northern and eastern margins are defined by a conspicuous range of hills, which are made up of Wenlock shales and grits; namely, Yr Allt, Cefn Du, Ysgwd Ffordd, and Beddugre Hill, etc. The adjacent ground on the west was described by Dr. Herbert Lap worth in 1900. About 30 square miles have been investigated, and mapped on the scale of 6 inches to the mile. The area is included in the 1-inch Ordnance Survey Maps, Sheets 70 & 80 (coloured edition), and in the Geological Survey Maps, Old Series, Sheets 56 N.W. & N.E. The district forms part of a dissected plateau, which attains an altitude of about 1500 feet, but Camlo Hill (1670 feet) stands up above the general level of the plateau, and is the highest point in the area. In this plateau, the streams are deeply trenched, and some of the valley-slopes are steep, particularly in the northern part of the region. The rivers belong to the drainage-system of the Ithon, which joins the Wye about a mile and a quarter south of Newbridge. The principal stream in this district is the Clywedog Brook, with its tributaries (Pandy Brook,