118 



Proceedings of the Royal Irish Academy. 



earlier masses, sedimentary or igneous, as it flows. Mr. Harker,^ 

 however, lias recently shown that he fully realises the composite 

 origin of many rocks that become thrust up towards the surface ; and 

 the complexity of subterranean masses becomes more clearly forced 

 upon us as we leave volcanic districts for those in which some 

 ''batholite" lies exposed. 



In 1893 Professor SoUas^ also described the modification of an 

 amphibolite near Glendalough, in County Wicklow, by veins of 

 quartz, which contain potash-felspar, and which traverse the adjacent 

 schist. I have had the advantage of collecting from this spot on two 

 occasions under Professor Sollas's guidance, and fully agree that the 

 highly siliceous veins are the cause of the alterations and added 

 materials in the basic rock, which, in its original state, consists 

 almost entirely of hornblende." In view, however, of the nature of 

 the changes, and their parallelism with those occurring in basic rocks 

 enveloped by granite, I cannot help thinking that the amphibolite of 

 Glendalough penetrated the schists before the upwelling of the last 

 granite of the district, and that the quartz-mica-diorite is a composite 

 rock due to direct interaction and admixture. Professor Sollas's 

 views are given more fully in a second paper,^ in which he goes, as 

 I venture to think, somewhat out of his way to suggest that the 

 varied types of igneous rock in the Leinster chain have arisen from 

 one primordial magma-basin. 



In the cases about to be described, it is not necessary to make any 

 such assumption ; nor would it, if put forward, bear upon the principal 

 question of the production of new rocks by admixture. As in the 

 case dealt with by Professor Sollas at Glendalough, and in others 

 recorded from the county of Donegal,* massive ampliibole usually 

 degenerates, under contact-alteration, into actinolitic fibres, and 

 ultimately into biotite. Much of the biotite-gneiss of north-west 

 Ireland thus arises from the absorption of hornblende-schist and 

 garnet-amphibolite by granite. But occasionally a new stimulus, as 



1 " The Tertiary Igneous Rocks of Skye," Mem. Geol. Survey (1904), pp. 177, 

 186, 219, &c. 



2 "On the Transformation of Amphibolite into Quartz-Mica-Diorite," Eep. 

 Brit. Assoc., 1893 (1894), p. 765. 



3 ''On the Geology of Dublin and its Neighbourhood," Proc. Geol. Assoc., 

 vol. xiii. (1893-4), p. 111. 



* G. Cole, *' Metamorphic Rocks in East Tyrone, &c.," Trans. R. I.Acad., 

 vol. xxxi., p. 456 ; also Proc. R. I. Acad., vol. xxiv., sect. B, p. 366. 



