650 MISS E. M. E. WOOD ON THE [Nov. I906, 



Silurian Sequence of Rhayader, he suggested that I should take up 

 this special Tarannon work. 



For the last four years I have spent my available leisure in 

 working out the stratigraphy of this Llanbrynmair-Tarannon area,, 

 and in collecting and identifying the fossils of its recognizable 

 zones. The present paper embodies the results that I have obtained 

 in the district. 



(B) The Tarannon District. 

 (1) General Topography. 



The Llanbrynmair-Tarannon district, as described in the present 

 paper, includes an area which covers some 22 square miles. It is. 

 well defined on two sides by physical features. Its western margin 

 is constituted by the valley of the Afon Twymyn, a river which 

 flows almost due north for a distance of nearly 6 miles from Dyliffe 

 to Llanbrynmair Station, at which place it turns abruptly westward 

 to join the Dyn at Cemmaes Road. Its north-eastern margin is 

 formed by the transverse valley of the Afon Iaen, a tributary of the 

 Twymyn, and along this valley run the main road and the railway 

 from Llanbrynmair to Carno. The south-eastern margin, on the 

 other hand, is a purely-arbitrary one as regards physical features, 

 and may be roughly defined by a line drawn from the New Inn, 

 Stay-a-little, to Carno across the hills of Esgair-drain-llwyn. 



The central part of the district is occupied by an elevated table- 

 land, which is cut off from the main tableland of the Denbighshire 

 Grits on the north by the transverse valley already mentioned. It is 

 intersected by a second transverse valley which partially separates 

 the heights of Newydd Fynyddog on the north from the moors of 

 Tarannon proper on the south. This tableland attains in a few 

 places an elevation of 1600 feet, and forms a rolling upland covered 

 for the most part with coarse grass, the monotony being broken here 

 and there by rocky exposures, dry stretches of bracken and heather, 

 peat-stacks, or rushy hollows where the spongy moss stores up the 

 rainfall in treacherous bogs. Flocks of sheep find pasturage on the 

 moorland, and the loneliness of the scene is relieved by an occa- 

 sional farmhouse nestling on the sheltered sides of the valley. 



Everywhere along its western side the Tarannon moorland 

 plateau descends abruptly into the well-wooded and cultivated 

 valley of the Twymyn the slope being sometimes roiyided and 

 grass-covered, sometimes precipitous and craggy. But westward 

 beyond the valley of the Twymyn rise the bare crags of the 

 ' Pennant Metalliferous Slates,' gloomy and forbidding, with long 

 slopes of barren scree stretching down to the river, and forming a 

 marked contrast with the green restful undulations of the Tarannon 

 Hills to the eastward. 



North of the Tarannon moorland rises Newydd Fynyddog. 

 This is a fine heather-clad height falling abruptly on all sides, 

 especially on its northern face, where it descends through bracken- 



