1862.] HAUGHTON GEANITES OF IRELAND. 403 



After this paper had been sent in to the Society, I learned, from the 

 * Proceedings of the Koyal Society,' that Mr. Prestwich attributed 

 the deepening of the valleys of the Somme and Seine and other rivers 

 of France and England, below the level of the high freshwater 

 gravels, to the same subaerial action that I had appealed to for the 

 formation of the valleys of the South of Ireland. — J. B. J., Oct. 2nd, 

 1862. 



2. Experimental Researches 0% i^^e Granites o/Ieeland. Part III. 

 On the Granites of Donegal*. By the Rev. Samuel Haughton, 

 M.A., F.R.S., F.G.S., FeUow of Trinity CoUege, and Professor of 

 Geology in the University of Dublin. 



Contents. 



I. Geographical position of the 

 G-ranite of Donegal. 

 11. Physical Structure of the Gra- 

 nite. 



III. Its Greological Relations. 



IV. Its Chemical Composition. 



V. Minerals of the Granite. 



A. Constituent Minerals. 



B. Accidental Minerals. 

 VI. Calculation of the percentage 



of the Constituent Minerals 

 in the Granite of Donegal. 



I. Geographical Position of the Granite of Donegal. — The granite 

 axis of Donegal extends for sixty miles from Malin Head to the 

 neighbourhood of Ardara, in a direction 43° south of west (true), 

 well marked by the two great, though not continuous, valleys of 

 Glenveagh and Gweebarra, which occupy a nearly central position in 

 the granite band, nine miles wide, that traverses the County from 

 N.E. to S.W., from Glen to Doocharry Bridge, opposite which latter 

 place the Gweebarra valley ceases to be central, as the granite 

 expands out to a breadth of eighteen miles, of which three miles lie 

 to the south-east, and fifteen miles to the north-west in the direc- 

 tion of Dunglow. At the south-western extremity of the granite 

 axis, it is separated from the granite of Ardara by the intervention 

 of metamorphic slates ; and at its north-eastern extremity, it is 

 separated by quartz-rock and the sea from the granite of Dunaff Head 

 and Malin Head, with which it is evidently continuous. 



To the south-east of the granite axis, there is an isolated patch of 

 granite, divided into two portions by the Barnesmore, or Great Gap, 

 through which the road from Donegal to Stranorlar and Strabane 

 passes. The bearing of this important pass is 42° 15' south of west; 

 and the granite-mass which it divides has its greatest diameter in 

 a direction 40° south of east, and its least diameter in a direction 

 coinciding with that of the Barnesmore Gap. 



Still farther to the S.E., at Beleek and Castlecaldwell, in the Co. 

 Fermanagh, on the borders of the Co. Donegal, the metamorphic 

 slate becomes gneissose, and is traversed by numerous veins of the 

 granite, which evidently lies beneath it. 



n. Physical Structure of the Granite of Donegal. — The granite of 

 Donegal possesses a stratified structure, in beds which are nearly 



* For Parts I. and II., see Quart. Journ. Geol. Soc, Lond. vol. xii. p. 171 ; 

 and vol. xiv. p. 300. 



VOL. xviii. — part I. 2 E 



