The strata lying above and below the Lingula Flags have already been well described and illustrated: the Menevian Gronp below, by Messrs. Salter and Hicks, and the Tremadoc Group above, by Messrs. Homfray, Ash, and Salter. The great mass of strata lying between has not fared so well, through several notice of it, to which I shall refer, have appeared. In the present paper I propose to describe these strata in detail; and the remarks I have to offer embody the results of three years' researches, during part of which I have had the advantage of the company and able co-operation of Messrs. Ezekiel Williamson and J.C. Barlow, whose discoveries I shall have to mention in my description of the rocks and their fossil contents. To facilitate the study of the district around Dolgelly, which is exceedingly faulted and complicated, I have carefully mapped out nearly the whole of the rock exposures. This may seem to have been unnecessary, seeing that we have already the Geological Survey maps of the district. But since the officers of the Survey examined and mapped out the rocks of Merionethshire, from fifteen to eighteen years have elapsed, and the maps which then added so much to our knowledge are now far behind our requirements.