122 Proceedings of the Royal Irish Academy. 



would be lower if the pyrite could be eliminated. By di:fferences of 

 colour in successive growths and zones, the nucleus of older hornblende, 

 and the later additions to it, can be sometimes traced. The last stage 

 consists in the formation of brown mica, which spreads in some places 

 along the cleavage planes of the hornblende, and in other places, 

 perhaps in the same crystal, along irregular cracks that have developed 

 (fig. 2). Had the process continued, irregular nests of mica would 

 have arisen in an environment of granite. Mr. Harker^ has recently 

 suggested that the addition of alkali from a granite magma promotes 

 in such cases the growth of biotite in place of hornblende. 



In 1904, I was able to observe similar features, on a still bolder 

 scale, on the north face of the crest of Cashel Hill, south of the 

 Gweebarra, and five miles west of Cor.'^ The hornblende - rock 

 attacked by the granite is again practically devoid of felspar, and has 

 a specific gravity of 3*06. The biotite that appears in it increases in 

 prominence near the granite, and is, I think, entirely secondary. At 

 the same time, the hornblende, which is allowed a certain freedom in 

 its new environment, becomes clearly idiomorphic, and ultimately, 

 in the stimulating contact-zone, forms prismatic crystals 20 mm. and 

 even 25 mm., long. Here a lime-soda felspar, at least as basic as 

 andesine, appears side by side with orthoclase. Pyrite is abundantly 

 developed in this zone, again illustrating the association of mineral- 

 veins with surfaces of metamorphic interaction. 



The composite rock itself has remained, however, wonderfully 

 fresh ; and the deposition of iron sulphide was doubtless contempo- 

 raneous with its formation. 



Two feet from the visible junction, which here is obviously not a 

 sharp one, the granite of Cashel Hill remains highly charged with 

 material from the amphibolites ; and its specific gravity is as high as 

 2'75. Biotite and epidote are abundant in it ; and, eastward along 

 the hill-top, it is still to some extent darkened by absorption of 

 material from the Dalradian series. 



A mile and a quarter south, on the north-west flank of the granite 

 dome of Ardara, the granite has, over a wide area near Summy 

 Lough, the speckled character of a composite and modified rock.^ 

 Patches of biotite, hornblende, and epidote lie in a coarser ground of 



1 '* The Tertiary Igneous Rocks of Skye," Mem. Geol. Survey, p. 171. 

 " For this granite, see G. Cole, " Composite Gneisses in Boylagh," Proc. ll.I.A., 

 vol. xxiv., sect. B. (1902), p. 208. 

 Cf. Barker, op. cit., p. 182. 



