Ooi-E — On Compofiite Gneisses in Boylagh, West Donegal. 217 



cult to say, in the ease of such inclusions, what features may he due 

 to the suhsequent contact-metamorphism. Sphcne and Liotite have 

 often heen noted as products of contact-alteration and of marginal 

 absorption of amphibolites or hornblende-schists^ ; and thus the 

 epidiorites of Cleengort doubtless yield some material to the adjacent 

 biotite-gneiss: At other times we may expect derived liornblende 

 to remain, and even to revive in the granite magma, as Mr. Kilroe'"' 

 recognised in an example south-west of Ardara. Such instances 

 probably ai'ise where the proportion of derived material is large in 

 comparison with that of the granite magma. 



An inclusion of epidiorite from the Cleengort gneiss has a specific 

 gravity of 3"06; iron ores are not prominent, and the amphibole is 

 clearly of high density. It displays dark green and yellower green 

 face-colours, strong axis-pleochroism, and extinction-angles of as 

 much as 20° away from the vertical axis e. The study of a mass 

 such as this has an important bearing on the hornblende-granites 

 and hornblende-gneisses farther to the east. 



As we cross the bog of Derkmore, and climb to the geat dyke of 

 Cainozoic dolerite on the pass leading over towards Glenties, Ave see 

 the complex Dalradian series invaded by sheets of graphic granite 

 along the foliation-planes, and also by granite dykes cutting across 

 them. Dr. Hyland^ has recorded sheared granite from near Meena- 

 largan Hill in this locality, and evidences of the later earth-movements 

 are doubtless traceable here, as at Carbane. But, on rounding the 

 pure granite masses that form the south side of Derkbeg Hill, we 

 come upon patches of foliated granite which have all the characters of 

 fluidal composite gneiss. Included layers of schist are clearly seen, 

 striking north-east, and these are, as usual, surrounded by biotite 

 gneiss, its foliation striking in the same direction. 



In places, again, we have a delicately injected schist, which passes 

 at its margins into gneissoid granite. A sample of this granite, in 

 which both light and dark mica occur in strings and layers, shows a 

 few " strain-shadows " under the microscope, but no sign of shearing 

 or mylonitic flow. It is a white aplite, with a few micaceous additions 

 from the schist. In contrast, one of the much darkened portions of the 



1 See references in " Metam. EoeksinE. Tyrone, etc.," Trans. E. Irish Acad., 

 vol. xxxi., p. 454, and plate xxvi., fig. 5. 



2 Mem. S.W. Donegal, p. 53. Also Mem. Sheets, 3, 4, 15, etc., p. 78. For 

 numerous references to quartz-hornblende-diorites, formed by invasion of granite 

 into basic rocks, see paper in Trans. R.I. A., above referred to, pp. 438 and 439. 



3 Mem. Sheets, 3, 4, 15, etc., p. 134. 



