The area under consideration is a portion of the mountain massif bounded on the east by the Conway River, on the north by the narrow coastal plain between Conway and Bangor, and on the west and south by the popular tourist route through Nant.-Francon pass by Llyn Ogwen and down the Afon Llugwy to Bettws y coed. The investigation includes the horseshoe-shaped ring of mountains, of which the highest is Y Foel Frâs (3,091 feet), surrounding the upper course of the Aber River (Cwm yr Afon Goch). To the north-west this igneous complex is bounded by the precipitatious crags over which the above stream descends by leaps and bounds, forming the well-known Aber Falls. Upon Sheet 78, south-east of the old Geological Survey map the area is coloured as “Massive intrusive felspathic rocks”, surrounding two oval patches indicated as “Greenstones”, whilst a further elongated patch of the latter forms the south-west boundary. The north-east boundary is limited by the Aber-Llanbedr fault. Besides the hog-backed summit of Y Foel Frâs, the area includes the similarly outlined summits of Y Drosgl West (2,483 feet), Yr Arryg (2,875 feet), Llwydmor Bâch (2,256 feet), and the more rugged summits of Bera Mawr (2,587 feet), Bera Bâch and Llwydmor (2,749 feet).