A glance at a geological map of the British Isles will show that the metamorphic rocks of the south-western Highlands of Scotland are prolonged into the north of Ireland, where they spread over a region many hundred square miles in extent. They retain there the same general character and present the same difficult problems as to their true stratigraphical relations. Quite recently, however, a new light seems to have arisen upon these Irish rocks. My colleagues on the Irish Branch of the Geological Survey have detected several detached areas of coarse gneisses, which in many respects resemble parts of the Lewisian gneiss of north-west Scotland. In some cases these areas lie amidst or close to "Dalradian" rocks, but with that obstinacy, which so tries the patience of the field-geologist, they have persistently refused to disclose their true original position with regard to these. Some fault, thrust-plane, tract of boulder-clay or stretch of bog is sure to intervene along the very junction-line where the desired sections might have been looked for. There can be little doubt that a strong unconformability exists between them. A close examination of the ridge of old gneiss in Tyrone and Fermanagh showed me that though the actual basement-beds of this Dalradian series could not be seen resting on the coarse gneiss, the lithological character, and tectonic arrangement of this series are only explicable on the supposition of a complete discordance between it and the gneiss. As these two groups of rock have never been found in close proximity in Scotland, and as the determination of the true age of the Dalradian series is a question of such great stratigraphical importance in the general mapping of the United Kingdom, I requested Mr. A. McHenry, of the Geological Survey of Ireland, to continue the tracing of the mutual boundaries of the old gneiss of the Ox Mountains and the Dalradian series in County Mayo. He informs me that he has found in that series a conglomerate full of blocks of the old gneiss, and resting in one locality apparently unconformably upon it. If this observation is confirmed it will finally set at rest the relative position of the coarse massive gneiss and some portion, at least, of the Dalradian series. Of course there is no absolute proof that the coarse gneisses of Ireland are really the equivalents of the Lewisian masses which they so closely resemble. But there is a strong presumption in favor of their identity. In England and Wales many detached areas of rock have been claimed as pre-Cambrian, and successive formations have been classified among them. I have already dealt in part with this question, and without attempting here to review the voluminous literature of the subject, I will content myself with stating briefly what seems to me to have been established on good evidence.