216 Scientific Proceedings, Royal Dublin Society. 



dation. It is possible that these newer rocks may have extended, 

 at one time, over the whole area; hut of this there is no satis- 

 factory evidence, except in relation to the Carboniferous, outliers 

 of which are found everywhere throughout the area, the most 

 important being small patches of the basal conglomerate on some 

 of the high hills in Galway, Mayo, and Donegal. 



The exotic and allied rocks to be described range from Cam- 

 brian to Eocene (?) in age, and vary in composition from basalt to 

 granitic, and schistose rocks. 



Cambrian and Arenig Granites, Elvans, Whinstones, Eurytes, 

 Felstones, Quartz-Bocks, and Tuffs. — All these, in Connaught, are 

 more or less altered into different varieties of hornblende-rock, 

 ophyte, schist, gneiss, and granite; but in some places in Ulster 

 they are very little, if at all, changed. 



Ordovician and Llandovery Granites, Elvans, Whinstones, 

 Eurites, Felstones, Quartz-Rocks (?) and Tuffs. — In Connaught 

 most of these rocks are altered similarly to those of Cambrian 

 age ; but in Ulster many are unaltered ; the whinstones are gene- 

 rally diabase and dioryte or allied rocks. Some of the quartz- 

 rocks may possibly belong to this age. 



Towards the end of the Ordovician, or during the Llandovery 

 age, there occurred the most active metamorphism of which we 

 have evidence in Ireland. In this territory it altered, in Galway, 

 Mayo, Roscommon, and Sligo all the Cambrian, Arenig, and 

 nearly all the Ordovician rocks. It also altered the similar rocks 

 in Donegal, Londonderry, Tyrone, and Antrim; but here the 

 action was not as universal, the Ordovicians and Arenigs (?) 

 partly escaping. 



Connected with this period of metamorphism was the produc- 

 tion of the metamorphic granites of Galway, Mayo, Roscommon, 

 Sligo, and Donegal, and the older intrusive granites of the same 

 counties, with their accompanying elvans and other intrusive rocks. 



[Some of the intrusive granite of Mayo, namely, that at Termon, Blacksod Bay, may 

 have been pre -Ordovician, and therefore older than the others. The foliated granite 

 of the Castlebar and Lough Conn district was foliated at the same time as the associated 

 metamorphic rocks ; it therefore must also be older, except that the foliation belonged to 

 a second and more recent period of metamorphism, to be mentioned presently.] 



Silurian and Devonian Granites, Elvans, Whinstones, Eu- 

 rytes, Felstones, Quartz-Rocks, and Tuffs. — In Mayo some of the 



