In a paper dealing with some phases of volcanic action in North Wales, it was urged that contemporaneous eruptive rocks occurred on Cader Idris at lower horizons than the acknowledged base of the Arenig, although the actual vents from which they had arisen were lost and concealed under later accumulations. It was pointed out that the mass of Rhobell Fawr was of composite character, but was probably distinct from the series on Cader Idris. The fragmental character of much of its material having been recognized by the author of the Survey-Memoir, several theories are discussed in that work to account for its stratigraphical position. In spite of the lack of detail, when compared with other areas, with which Rhobell Fawr has been represented on Sheet 75 S.E. of the Map of the Geological Survey, the ashes, “breccias,” and other materials piled here upon the edges of the Lingula Flags are fully dealt with in the text of the Memoir, and we are left finally to select the most probable explanation of those put forward.