I. Introduction A study of the existing geolagical maps of North Wales reveals at the south-eastern ‘corner’ of the Harlech Dome, inside the main Ordovician outcrop, a group of hills which, although composed of igneous rocks, do not appear to form an integral part of the ‘ring of fire’, from which they are separated by a narrow belt of Cambrian slates. The study of the geology of this group of hills, centred about Rhobell Fawr, and involving the investigation of the character of the rocks, their age, and relationship to those of the main Ordovician escarpment, has attracted several geologists before the present research was under- taken at the suggestion of Prof. A. H. Cox. The position of Rhobell Fawr in relation to other districts recently in North Wales is shown in the appended sketch-map (fig. 1). The district examined lies within the parish of Llanfachreth, and has been mapped on the 6-inch Scale. The quarter-sheets covered by the geological map illustrating this paper are Merioneth xx S.E.; xxvii S.E. ; xxviii N.W. ; xxviii S.W. ; xxiii S.E. ; xxxiii N.E. ; xxxiv N.W. ; and xxxiv S.W. The stratigraphical succession ranges from low down in the Ffestiniog Beds to the Bala Mudstones. The former occur in the west, while the regional dip brings higher beds in towards the east. The central physiographic feature is the Rhobell mountain-mass, composed chiefly of volcanic material of early Arenig age. The mountain-mass comprises the central peak of Rhobell Fawr (240S feet) and