206 Proceedings of the Royal Iri^h Academy. 



streaking out of tlie consolidated igneous rock, Eut it cannot fail to 

 be ol)serredtliat tke cleaiir foliated masses are formed of dark granite, 

 ricli in. elongated flakes of biotite. Dr. J. S. Hyland^ regarded tbi& 

 excess, of biotite in. tbe gneissic granite of Donegal as a secondary 

 feature, "as its frequency increases Tvith. the intensity of tbe meta- 

 morphism." Mr. Kilroe,- hoTvever, from bis field-observations, bolds 

 tbat sbearing Tvas facilitated wbere tbe biotite originally -was abun- 

 dant. He elsewhere remarks,^ "-n^here tbe mica abounded, as in tbe 

 more basic portions, tbe rock bas yielded most freely " to tbe earth- 

 pressures. This presumes a previous process of differentiation in the 

 mass, Tvhereby more basic knots bad gathered here and there. In tbe 

 field, however, inclusions of schist, drawn out along the foliation- 

 planes of the granite, again and again accompany the "metamorphosed"' 

 granite. I doubt if they are absent from any of the points where the 

 foliation has been thought of sufficient importance to be recorded on 

 the maps of the Geological Survey, !^^ear Kilgole, immediately north 

 of Ardara, these inclusions are obviously connected with and emphasise 

 the foliation. In Grarvegort Grlebe, again, nearer to Grlenties, a few 

 rugged exposures behind tbe school-house show a well-developedbiotite- 

 gneiss, including similarly elongated fragments of mica-schist, some of 

 which contain tourmaline. Tbe occurrence of this mineral completes 

 the resemblance of the inclusions near Ardara to those caught up in 

 the Caledonian granite near Ballycorus, in County Dublin. 



In both tbe above-mentioned cases of foliated granite between 

 Ardara and Glenties, the undoubted Dalradian series lies only a 

 himdred yards or so away upon the south. At Kilgole tbe contact- 



1 Mem. to sheets 3, 4, \b, etc. (1891), p. 135. 



» lUd., p. 78. 



3 Mem. S.W. Donegal (1891), p. 29. Dr. Haughton notices that the gneissos& 

 granite of Ardara contains black mica "in large quantity" (Quart. Journ. Geol. 

 Soc. London, vol. xviii., 1862, p. 408). Mr. E. H. Scott, discussing the analysis 

 of one of these Ardara masses, which contains only 55 -20 per cent, of silica, says 

 that it is, " properly speaking, not a granite at all. " (Journ. Geol. Soc. Dublin, 

 Tol. ix., 18G2, p. 287 ; see also Dr. J. S. Hyland, Survey Mem. S. "W. Donegal, 

 p. 64). The analysis, which was also published in Haughton's paper, shows us a 

 rock allied to Eosenbusch's ilinette-Kersantite series, verging on kersantite, but 

 with 4*63 per cent, of soda and only 3-17 per cent, of potash. Since soda is not 

 more abundant than potash in ordinary phyllites and mica-schists, it seems Lighly 

 probable that this particular darkened and gneissose granite of Ardara became 

 modified by the absorption of an epidiorite or hornblende-schist, as is the case 

 with the gneiss of Cam and Lough Derg in southern Donegal, and the gneissic 

 granite of Derkbeg to be described later. 



