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XI. 



EEPORT ON THE OX MOUNTAIN ROCKS AND THEIR 

 PROBABLE CONTINUATION FROM GALWAY AND 

 MAYO INTO DONEGAL, TYRONE, AND LONDON- 

 DERRY. 



By ALEX. M'HENRY, M.R.I.A. 



Read June 22, 1903. 



The range of hills of which. Slieve Gamph and the Ox Mountains form 

 the main portion, begins a little to the north-west of Castlebar, and 

 continues in a somewhat sinuous north-east course past Foxford, 

 Coolaney, and Lough Gill to Manorhamilton, where the older rocks 

 formiag it sink beneath the Carboniferous strata of the plain, which 

 formation bounds the range on both sides along its whole coui'se of 

 sixty -five miles. 



The several great divisions of the metamorphosed sedimentary 

 rocks of the range are similar in every respect to those lying to the 

 west in Galway and Mayo, and are, in fact, a continuation of them. 

 These rocks were originally considered by the Geological Suiwey to be 

 mainly metamorphosed Lower Silurian strata ;^ and this opinion I 

 believe to be correct ; with the exception that in places, such as south 

 of Clew Bay, in Croagh Patrick Moim.tain, rocks of Upper Silui'ian age 

 must be included in the metamorphosed group, as was proved by my 

 colleague, Mr. Kilroe.^ 



The maia divisions of this metamorphosed sedimentary series that 

 are so persistently and well recognised in other districts in Ireland 

 are well marked at several places in the south-west end of the range, 

 to the north of Castlebar, and to the east and north-east of Foxford, 

 in the vicinity of Loagh Talt. 



^ Explanatory Memoirs of the Geological Survey of Ireland, Sheets 73, 74, 

 S3, and 84 (1876), and Sheets 93 and 94 (1878). 



- Annual Eeport of the Geological Survey for the year 1896, p. 50. 



