Cole — On Comjjosite Gneisses in Boylarjii, West Donegal. 215 



In our present area, commencing with the hare wliito liog's-hack 

 of Trusklieve, we find that the granite has included a great eye of 

 schist, a quarter of a mile long, above Kanny Lough, and has caused 

 kyanite to develop throughout it by contact-action. According to the 

 interesting views of Weinschenk,' this mineral implies that the 

 intrusion was accompanied by earth-pressures, sufhcient to determine 

 the formation of the denser aluminium silicate rather than andalusite. 

 The whole rock thus comes to have a specific gravity as high as 2-86. 

 This schist, viewed under the microscope, is a very handsome quartz- 

 mica-garnet rock, rich in brown biotite, which has a small optic 

 axial angle, and with a little plagioclastic felspar. The longer axes 

 of the kyanite crystals lie in the surfaces of foliation, and tufts of 

 sillimanite have developed extensively in the micaceous patches, and 

 spread throughout the granular quartz. The latter mineral thus 

 comes to resemble cordierite when seen in section ; but its granules 

 are, of course, uniaxial.^ 



The main boundary of the granite occurs at Ranny Lough, and the 

 rock becomes darker, showing the usual gneissose bands of biotite as 

 it intrudes among the schists ; but another band of granite comes up 

 along the strike of the schists and epiclorites at Felmurry Lough, a 

 third of a mile to the south-west, while small veins in the townland 

 of Earragan point to the continued proximity of the igneous masses 

 below. The two great dykes, however, at the old stone fort in Cor, 

 still further south against the Gweebarra, belong in all probability 

 to the later and unfoliated pegmatite series. They have given rise to 

 interesting phenomena of admixture and re crystallization in the massive 

 amphibolite which they traverse. 



At the north end of Toome Lough, and the south-east corner of 

 Trusklieve, the marginal granite contains abundant inclusions of the 

 schist, which here dips under the main mass of Trusklieve. A bold 

 banded gneiss has been produced by the intrusion of parallel sheets of 

 pegmatite along the foliation planes of the schist (PI. i., fig. 1). Its 



^ " Memoire sur le dynamometamorphisme et la piezocristallisatioii," Congres 

 geol. internat., Comptes rendus, viii« session, Paris (1901), pp. 329 and 3il. 



- Salomon [op. cit., Zeitschr. d. deutsch. geol. Gesell., Bd. xlii., p. 524) 

 similarly indicates the intimate association in his Italian contact-rocks of sillimanite 

 and biotite, the former mineral appearing to replace the latter. The sillimanite 

 also penetrates the quartz in these rocks, both in needles and dense bundles. The 

 connexion between biotite and sillimanite was observed by Levy, as far back as 

 1879 ("Formation gneissique du Morvan," Bull. soc. geol. de France, 3"«' serie, 

 t. vii., pp. 861 and 869). 



