Cole — On Composite Gneisses in Boylagh, West Bonerjal. 219 



its present characters to dynamic metamorphism. It is cut, however, 

 by a brown granite, with a specific gravity of 2-60 ; the foliation in 

 this does not agree with that of the older mass, but is clearly due to 

 igneous flow, running as it does parallel to the margins of the veins. 

 This granite does not show metamorphic deformation under the micro- 

 scope ; its foliation is due to the arrangement of plates of light and 

 dark mica as it flowed. Here we have, in the coarser and earlier rock 

 of Galwolie, the type of foliation that has been accepted as that of the 

 Donegal granite as a whole. But hitherto our observations in Boylagh 

 have shown us the phenomena of the fluidal veins occurring on a large 

 scale, and emphasised by inclusions of schist within the granite. 

 It becomes of considerable interest to inquire to which type the 

 foliated structures belong, which are developed on a still broader scale 

 south of the Gweebarra, between Derryloaghan and Finntown. Our 

 conclusions in regard to them may justly affect our views of similar 

 gneissic districts throughout central Donegal. 



On Sheet 15 of the Geological Survey map, the great granite mass 

 west of Finntown has foliation marked on it in some thirty places, 

 and in most cases a dip is assigned to the foliation-planes. South- 

 easterly dips prevail, but north-westerly ones occur near the Gwee- 

 barra, and predominate on the other side of the river. The general 

 structure, then, is that of an anticline, measuring some four miles 

 across. 



In the strike of this foliation, streaks of epidiorite, limestone, and 

 a little quartzite have long ago been noticed, especially on the south- 

 east flank of the mass in the neighbourhood of Finntown. Mr. E. H. 

 Blake^ thus recorded vertical layers of limestone and mica-slate within 

 the granite ; and Mr. E,. H. Scott^ deduced from those in Glenleheen 

 the metamorphic origin of the granite, in the sense that it was derived 

 from alteration of the ancient sediments in place. Like Dr. Haughton, 

 he favoured this view for the greater part of the granite of Donegal, 

 though he recognised that even the gneissose granite occasionally 

 pierced the other rocks in the form of veins. ^ Mr. Scott, in the first 

 paper quoted, provides a section along Glenleheen, in which the bands 

 of sedimentary rock are shown going vertically down in continuous 

 layers, with the granite cleanly interstratified between them. 



1 " On the Primary Eocks of Donegal, "Journ. Geol. See. Dublin, vol. ix (1862), 

 p. 296. 



2 " On the Granitic fiocks of South- West Donegal, " ihid., p. 290. 



3 " On the Granitic Eocks of Donegal, " 2nd notice, ihid., vol. s (186i; paper 

 read 1862), p. 20. 



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