230 Scientific Proceedings, Royal Dublin Society. 



DONEGAL. 



In the north-west of this county, in the baronies of Kilma- 

 crenan and Boylagh, there is a tract of gneiss and foliated granite, 

 associated with eruptive granites [gneiss series), margined by areas 

 of metamorphic rocks (schist series) ; the one passing gradually into 

 the other. The metamorphic granite, gneiss, and schist seem to bo 

 due to one and the same action, while the older eruptive granite is 

 allied to them like as is the granite in other metamorphic areas, 

 such as those of Galway and Mayo. In Donegal a newer granite 

 was intruded at a later time. 



The older intrusive granite occurs principally to the westward, 

 in a tract near the sea, to the northward and southward of Dun- 

 glow, and also further southward, in the group of hills at Barnes- 

 more (Bluestacks, Croaghnagar, &c), to the north-eastward of 

 Donegal town. 



The greater part of the younger granite occurs in the north 

 headlands of the baronies of Kilmacrenan and Inishowen, it being 

 in places much entangled with the associated schists, while extending 

 from the granite tracts are massive courses of el van and porphyr}-, 

 which appear to graduate into basic felstones and whinstones. 



These dykes and courses occur not only in the older rocks 

 {Metamorphosed Arenig (?) and Cambrian (?) ), but also in the later 

 rocks (Orclovician (?) and Llandovery (?) ) ; and it seems possible, if 

 not probable, that the masses of granite may represent the lacco- 

 lites, or roots of the sheets of Donegalyte, so conspicuous in the 

 Killygarvan series of rocks (Llandovery (?) ), occupying the county 

 N. W., N. and N. E. of Eathmullen.. ' 



In the older rocks (Cambrian (?) and Arenig (?) ) there were many 

 intrudes and interbedded (?) masses of whinstone, which were 

 variously altered, with the associated rocks, in accordance with 

 the amount of metamorphism to which they were subjected ; the 

 whinstones being now represented by granitic rocks and different 

 varieties of hornblende-rock. As some of the older sedimentary 

 rocks were altered into gneiss, and others into schists, the latter 

 being of all varieties down to nearly unaltered rock, so similarly 

 the alteration in the associated intrusive rocks is of varied amount, 

 some being greatly altered, and others only very partially so. 



