208 Proceedings of the Eoyal Irish Academy. 



Here and there an argillaeeons layer occxu's, converted almost wholly 

 into mica at its contact with, the granite vein.^ 



Opposite Swan !\[onnt, ahove the Portnoo Hotel, a granite appears 

 amid crumpled mica-schists, and is darkened, toughened, and altogether 

 modiAed by numerous inclusions of mica-schist and aphanite. There 

 is not the slightest doubt as to the nature and origin of these inclu- 

 sions, and the rock resembles the similar instance at Castle wellan, 

 Co. Down. A large specimen, with inclusions in rarious stages of 

 absorption, has the characteristic specific gravity of 2-77. 



Higher up, on Cashel Hill, veins of granite, with gi-een mica, 

 penetrate a mica-aphanite ; and biotite-granite appears in force near 

 the summit of the hill. All this serves to coiTelate the Portnoo granite 

 with that of the main mass further east ; but it must be borne in mind 

 that pegmatitic veins cut all the rocks of this area, including the 

 foliated granite of the Ardara dome, and that the Portnoo granite may 

 possibly belong to this later series of inti'usions. Some faulting has 

 occurred since the intrusion of the granite veins into limestone near 

 the road on the north side of Xarin Hill,- and these veins may belong 

 to the older granite ; but the typical pegmatites occur, cutting across 

 the foliation of dark schists, as near at hand as Clooney, and also 

 freely throughout Ballyiriston. From the point of view of general 

 principles, however, the masses of pure and modified gi'anite at Portnoo 

 are, of course, available, whatever their age, as links in the argument 

 concerning the composite gneisses in the dome of Ardara. 



"Wlien we come east of Clooney, we are dealing with exposures on 

 the true north flank of the dome. The junction of the Ai'dara granite 

 and the schists is well displayed in a little quarry by the main road, 

 just east of Cashelgolan Hill. The mica-schist is here delicately 

 penetrated by sheets of muscovite- granite, which have been forced 

 along the almost vertical planes of foliation. This foliation is parallel 

 to that noticeable in the granite itself on the south side of the road. 

 The junction shows very -various features. In one place the granite, 

 in which muscovite is the common mica, intrudes in delicate sheets 



' Compare Lacroix on the alterations of calcareous strata in the Pyrenees, " Le 

 granite des Pyrenees, etc.," 1" mem. (1898), Bull. Carte geol. de la France, No. 

 64, p. 15. 



2 This is probably the locality near Xarin where Mr. E. H. Blate noted the 

 difficulty of saying '* where the slate ends and the granite commences" ('• On the 

 primary rocks of Donegal, " Joum. Geol. Soc. Dublin, vol. ix. (1862;, p. 296). 

 The apparent passage from the one rock into the other was recognised early in 

 Donegal, as elsewhere in Europe. 



