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X. 



THE INTKUSIVE GNEISS OF TIllEERILL AND 

 DEUMAHAIR. 



By GRENVILLE A. J. COLE, M.R.I.A., F.G.S., Professor of 

 Geology in the Eoyal College of Science for Ireland. 



Read June 8, 1903. 



The beautifully contrasted scenery between Ballysadare and Manor- 

 hamilton, in tbe counties of Sligo and Leitrim respectively, is due to 

 the ridge of ancient gneiss, with its irregular and rounded summits, 

 which here appears through Lower Carboniferous strata. On the 

 south-east rises a broad upland, that culminates in the coal-field of 

 Lough Allen; on the north-west, the Carboniferous Limestone 

 weathers out in huge scars and terraces, from the cliffs of Glenade 

 and Benbulben, to the massive outlier of Knocknarea,^ The con- 

 spicuous gneissic axis, running north-east and south-west, in continua- 

 tion of the line of the Ox Mountains, has been the subject of various 

 investigations. 



By its general character and trend it is to be classed with the 

 Caledonian folds of M. Bertrand, as a mass which was brought into 

 its present position by earth-movements in earliest Devonian times. In 

 this it agrees with the main axes of folding throughout the county 

 of Donegal ; but it is well recognised that the rocks thus brought 

 into prominence and re-arranged may be much older than the Cale- 

 donian epoch of earth-movement. Prof. Hull- included the gneiss 

 now under consideration "provisionally" in his Laurentian group; 

 and I cannot bring forward any conclusive proofs that it is of later 

 age than the close of the Archsean era. The Hercynian movements 

 sent earth- waves against it, which uptilted the Carboniferous strata on 

 its flanks, while preserving its north-east and south-west trend. As 



1 Compare A. B. Wynne, " On the Geology of Parts of Sligo, &c." Joiirn. 

 Geol. Soc, Dublin, vol. x. (1863), p. 34. 



- " On the Laurentian Rocks of Donegal and of other parts of Ireland." Trans. 

 R. Dublin See, vol. i. (1882), p. 252. 



