344 Proceedings of the Royal Irish Academy. 



Cambro-Silurian age of the Pomeroy fossiliferous rocks is conceded, it 

 simplifies the matter; as the rocks in the hills to the north and north- 

 west are much older, and must, therefore, belong either to the 

 Cambrian or an older formation. To the northward and north- 

 westward sides of the altered eruptive rocks of the hills there is a 

 considerable thickness of apparently overlying schists, generally sup- 

 posed to be metamorphosed Cambro-Silurian. 



The conclusions I have drawn fi'om the apparent stratigraphical 

 position of the rocks now under consideration seem to be strongly 

 confirmed by the petrological characters of the eruptive rocks associ- 

 ated with them. 



In Craigballyharky and its neighbourhood the rocks are princi- 

 pally varieties of hornblende rock, that is, basic eruptive rocks and 

 tufiPs, which have been changed by metamorphic action. These, as is 

 usually the case, are very variable in composition ; as, however, they 

 are more or less similar to the different varieties of hornblende rock in 

 TVest Galway, elsewhere described,^ it is unnecessary to re-describe 

 them here. Traversing and breaking up through these rocks, especi- 

 ally in Craigballyharky, are veins of Galway granite of the " Omey 

 type," and a black granite very like one found near Furbogh and 

 in other places in Vest Galway, except that in the Co. Galway 

 amphibole and titanite are not uncommon in it, while in. the Co. 

 Tyrone they seem to be rare. 



In the neighbouring hills (Craigardhessiagh and Garragrim) the 

 rocks, for the most part, belong to an intrusion of the " Omey type" 

 granite. They are, however, more or less cut up and traversed by 

 dykes and courses of a very siliceous compact granite, common as a 

 vein rock in Galway, and similar to the rock called granitite by Eose. 

 The mass of the granite in this hill has been called " Syenitic 

 granite," by which, I suppose, hornblendic granite is intended. The 

 rock, however, seems scarcely to come under this classification, as its 

 essential constituents are quartz, two, and in places three, feldspars, 

 black and white micas, the first predominating ; pyrite, and in places 

 amphibole : the last, however, seems to be more an accessory mineral 

 than an essential. In every respect this rock agrees with that 

 described in the Memoirs of the Geological Survey as the "Omey type" 

 of the Galway gi-anite. 



In the western part of the district now described, the rocks are 

 principally varieties of hornblende rock very similar to those found in 

 Errisbeg, Errismore, and other places in West Galway ; while here, as 

 there, some of the rocks are changed by methylotic action into 

 ophite, steatite, and allied rocks. These rocks in "West Galway are 

 found in the uppermost portion of the " Great Micalite series," which. 



1 " Granitic and other Ingenite Rocks of Tar Connaught and the Lower Owle," 

 Proceedings R. I. A., 2nd ser., vol. ii., p. 102, et seq. 



