50 STEATIGKAPHICAL GEOLOGY 



This series has been extensively invaded by igneous rocks, both 

 basic and acid, the dykes and veins of the latter cutting through 

 the former which are consequently the older, though all are 

 compressed and sheared by still later movements of the crust. 



The " Boulder-bed " has been identified in varying thickness 

 all over Galway, Mayo, and Donegal. The boulders in it are 

 almost all of an unfoliated granite, unlike anything now visible at 

 the surface. They are mostly angular or sub-angular, only a few 

 being rounded, and are sparely distributed in a fine-grained matrix. 

 They vary in size up to 3 feet across, and Mr. M'Henry has 

 suggested that the bed is a genuine boulder-clay of glacial origin, 

 but its constant superposition on a limestone over such a large 

 region makes this rather unlikely. 



Besides the dykes and veins of igneous rock there are large 

 tracts of more or less foliated granite which have often the 

 appearance of being interbedded with the schists, but they sometimes 

 pass into banded gneiss, which is thus shown to be a composite 

 rock formed by the intrusion of tongues of granite into the schists, 

 the banding being due partly to the incorporation of the latter 

 and partly to a flow structure developed in the granite itself, while 

 the whole mass has been sharply flexured, compressed, and sheared 

 by the subsequent pressures. 



Describing the granitic dome of Ardara in West Donegal, 

 Professor Cole 16 points out that the pure granite of the central 

 mass has a border zone of banded gneiss, which is of composite 

 origin and not an original gneiss nor a my Ionised rock. " The 

 phenomena," he says, " are those of intrusion along the pre-existing 

 foliation -planes of a schist, and a good gneissose rock results, set 

 with inclusion -flecks, in which these foliation-planes are still 

 apparent." 



Whatever may be the age of these intrusive gneissic granites, 

 that of the schistose series seems now to be practically settled by the 

 discovery of Arenig rocks in Mayo and their superposition on the 

 schists south of Killary Harbour. This disposes of the old view 

 that the schists were only metamorphosed Ordovician sediments. 



3. Wales 



Archaean rocks come to the surface both in North and South 

 Wales ; they occupy large parts of Anglesey and smaller areas in 

 Carnarvonshire, while in Pembroke round St. David's they are 

 known as the Pebidian Series (see map, Fig. 16). 



Anglesey. Much has been written about the Archaean rocks 

 of Anglesey, and many conflicting views have been published. A 

 good description of the rocks themselves was given by Ramsay in 



