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IV. 



ON COMPOSITE GNEISSES IN BOYLAGH, WEST DONEGAL. 



By GRENVILLE A. J. COLE, F.G.S., Professor of Geology 



and Mineralogy in the Royal College of Science for Ireland. 



(Plates I. to Y.) 



[Read May 26, 1902.] 



I . — Inxeodfction . 



The observations on which the present paper is based are in direct 

 continuation of those recorded in 1900,^ and have been carried on with 

 the assistance of a grant made by the Koyal Irish Academy for the 

 study of metamorphic and other rocks in the north-west of Ireland. 

 The barony of Boylagh (Baeighellach) includes the country between 

 the Owentocker and the Gweedore River, with the long sea-inlet of 

 the Gweebarra in the midst of it. New roads have been cut of late 

 years across some of the wilder portions, and the bridge made by the 

 Congested Districts Board over the Gweebarra River has greatly facili- 

 tated communications. 



The earlier discussions^ as to the nature of the granite masses 

 which play so important a part in Donegal may now be regarded as set 

 at rest. Dr. Callaway^ showed, in 1885, that the granitoid gneiss of 

 northern Donegal was an intrusive rock, and his observations were again 

 and again verified throughout the whole county during its detailed 

 examination by the Geological Survey, which resulted in a series 

 of maps and memoirs published between 1887 and 1891. The move- 

 ments that have undoubtedly left their impress on the granite in many 

 parts of the county of Donegal, as, for example, in the Barnesbeg 

 area, are in all probability of Caledonian age ;* that is to say, they 



1 "On Metamorphic Eocks in Eastern Tyrone and Southern Donegal." Trans. 

 Royal Irish Acad., vol. xxxi. (part xi.), pp. 431-470. 



^ See references in above paper, pp. 449-450, and the good Bibliography in 

 Geol. Survey of Ireland, Mem. to sheets 3, 4, &c., p. 23. 



^ " On the Granitic and Schistose Rocks of Northern Donegal." Quart. Journ. 

 Geol. Soc. Lond., vol. xli., p. 221. 



* The recent aitempt of some English writers to use the terms "Caledonian," 

 "Hercynian, " etc., so clearly defined by Bertrand and Suess to express the trend 

 of folds in general, rather than folds of a particular epoch of mountain-building 

 tends to deprive European geology of a very valuable piece of nomenclature. 



R.I. A. PROC, VOL. XXIV., SEC. B.] S 



