The country I propose to describe, comprises the northern districts of Mayo and Sligo. In constructing the map*, I have used for the former county, the excellent map of Mr. Bald, some fishery charts by Nimmo, and some local surveys; and for Sligo, a corrected copy of Larkin’s Map. In connecting the two counties, I have used triangles resulting from the operations of the Ordnance Survey, most liberally communicated by Captain Portlock, with the permission of Colonel Colby; and where doubts and deficiencies in the details occurred, observations were made with a three-inch sextant, or Kater’s Compass. The drawings (pp. 158. 162. 165. 167.) are from sketches taken on the spot, with every determination to sacrifice picturesque effect to accuracy. The only notice which has been published respecting this part of Con-naught, is a short but very able and accurate sketch in Mr. Griffith’s paper on its Bogs, attached to the 4th Report of the Commissioners. The Ox mountain ridge extends through the district from south-west to north-east. It is chiefly formed of mica slate, hornblende slate, gneiss, and quartz rock, the strata dipping rapidly to the south. At the base occurs a conglomerate, alternating with sandstone and slate clay; and overlaid by the carboniferous limestone, which extends westward from Sligo to the Nefin group. This group is the commencement of the primary tract, reaching northward and westward to the ocean. The section from Lough Talt to Downpatrick Head, displays the succession of strata†. The Ox Mountains have a mean