218 Scientific Proceedings, Royal Dublin Society. 



the adjoining parts of Londonderry are the great sheets of intru- 

 sive rocks of Eocene age, which have their granitic roots, or elvans, 

 near Portrush, at Tardree, and other places. The elvans near 

 Portrush are associated with metamorphosed Lias. 



Territorial Description. 



In North- West and North Ireland, including portions of Gal- 

 way, Mayo, Sligo, Roscommon, Leitrim, Fermanagh, Donegal, 

 Tyrone, Londonderry, and Antrim, there was at one time, as 

 already mentioned, one large tract occupied by metamorphic and 

 granitic rocks ; the rocks now altered being the equivalent of the 

 Llandovery (?), Ordovician, Arenig, and Cambrian, the meta- 

 morphism having taken place in or towards the end of the Llan- 

 dovery age. All these strata having been contorted, upheaved, 

 metamorphosed, and denuded before the overlying Silurian rocks 

 were deposited on them, the patches of the later rocks that now 

 partly separate the different tracks having had no special influence 

 on the extent of the original area. 



The areas of exposure of these metamorphic rocks may be con- 

 veniently grouped, for the purpose of description, into four districts, 

 as follows : — Gahcay and West Mayo ; East Mayo, Sligo, Roscommon 

 and Leitrim; Donegal, Tyrone, Londonderry, and Fermanagh; and 

 Antrim. 



s 



GALWAY AND WEST MAYO. 



Galway is the foremost county of Ireland as to the variety in 

 composition and colour of its granitic rocks, these varying from 

 fine and nearly compact to largely porphyritic, and being of many 

 colours, white, green, shades of red, greenish, yellowish, blackish, 

 &c. ; some being mottled and others clouded ; while associated with 

 them are variously- coloured elvans, porphyries, and felstones. The 

 extent of the tract is about equal to that of Donegal, and about a 

 third of the size of the Leinster granite area. To the southward 

 the marginal granites have a bedded appearance ; while to the 

 north and north-east the foliated porphyritic granite {granitic gneiss) 

 graduates through gneiss into schist. Coming up through the por- 

 phyritic oligoclasic granite, but more frequently in the adjoining 

 gneiss and schist, are dykes and masses of the ordinary grey ortho- 



