I. Introduction. The area to be described is limited on the north-east by the Pass of Llanberis from near the village of Nant Peris to Pen-y-gwrhyd; on the south-east by the Vale of Gwynant down to Beddgelert; on the south-west by the Colwyn and Gwyrfai valleys to near the village of Salem; and on the north-west by the supposed line of junction between the Cambrian and the Ordovician rocks. The approximately quadrilateral area of Ordovician rocks thus defined has an extent of about 30 square miles. It is represented on portions of the 1-inch Ordnance Survey maps 106 & 119, of the 6-inch sheet, Caernarvonshire 22, and of the Quarter-sheets 16 S.E., 17 S.W., 21 N.E. & S.E., 27 N.E., & 28 N.W. It includes parts of the Quarter-sheets 75 N.E. & 78 S.E. of the 1-inch Geological Survey map (old series). The general configuration of Snowdon is represented on the map, PL. XXIX. Prof. W. M. Davis's paper on ‘Glacial Erosion in North Wales’ [42] affords a useful guide to the topography, and is considered adequate for the present purpose. (a) Historical Review. On his first visit to North Wales in 1831, Sedgwick drew a section through Snowdon, in which he depicted the synclinal structure of the summit and the descending sequence of beds towards the north-west. From this section and from others across Y Glyder Fawr and Moel Hebog, he ascertained the general parallelism of the fold-axes in this part of Caernarvonshire. Subsequently, he determined the general succession