204 Proceedings of the Royal Irish Academy. 



are pre-Devoniaii and post-Silurian ; and the trend of the great granite 

 masses across the country implies that these were intruded about the 

 time that the folding was in progress. If, then, in the present paper, 

 we are led to ascribe the foliation of the granite over a wide area 

 to original conditions of flow and intrusion, we must fairly recognise 

 that such flow took place under the influence of the Caledonian 

 processes of mountain-building.^ At various points the invading 

 granite had doubtless become solid and resisting before the earth- 

 movements came to an end ; and here deformations and new structures 

 were set up, and must be attributed to true dynamic metamoi-phism. 

 In Boylagh, however, it appears that foliated gneisses arose during 

 the ordinaiy course of igneous intrusion, though their structures were 

 emphasised to some extent by subsequent pressure. The more pro- 

 found changes that we observe in the contact-zone in the field were 

 not brought about by mineralising emanations fi'om the gi'anite, nor 

 yet by molecular re-arrangements in the heated sedimentary rocks. 

 They were due, rather, to that bodily intenningling and incorporation 

 of two dissimilar masses, which results in the formation of comriosite 

 gneiss. The gneisses of Eoylagh retain, then, to a considerable degree 

 the structui'es both of an igneous and a stratified material. This, at 

 any rate, is the argument of the present paper. 



II. — The Dojie of Aedaha. 



Our first observations may be made in the area cited by Mr. J. R. 

 Kilroe^ as clearly exhibiting the effects of pressure in producing 

 foliation in the granite. Mr. Kilroe, in his memoii', and in the 

 beautiful maps prepared by him in conjunction with his colleagues,^ 

 shows how the foliation of the schists mns parallel to the curving 

 margin of the granite between Ardara and Clooney, and how similar 

 foliation occurs, also parallel to the margin, at certain points within 



1 Dr. S. Haughton, forty years ago, came to a very similar conclusion regarding 

 the granite of Donegal, when he stated that in the centre it was prohably igneous, 

 " deriving its cleavage -planes and gneissose character from the pressure exercised 

 upon it by the stratified rock, which has been lifted, to the north and south, to a 

 nearly vertical position." (" Experimental Eesearches on the Granites of Ireland," 

 part iii., Quart. Joum. Geol Soc. London, vol. xviii., 1862, p. 406). The remark 

 is of importance, as showing that this author was not a convinced advocate of a 

 metamorphic origin for the granite as a whole. 



* Memoir to maps of south-west Donegal (sheets 22, 23, 30, and 31), Geol. 

 Sui-v. Ireland (1891), pp. 28-30. 



» Geol. Surv. Ireland, sheets 15 and 23. 



