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XI. — Notices on the Geology of the North Coast of the Counties of 

 Mayo and Sligo in Ireland. 



By archdeacon VERSCHOYLE, M.A. 



Communicated by Roderick Impey Murchison, Esq., P.G.S. 



[Read November 7 & 21, 1832.] 



1 HE country I propose to describe, comprises the northern districts of Mayo 

 and Slig-o. 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- 

 tlie 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 

 Rater'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 strataf . 



The Ox Mountains have a mean height of 1300 feet. On the north-west, 

 the summits present abrupt rocky peaks ; but the ravines are not of great 



* Plate XI. t Plate XI. fig. 3. 



