368 



Proceedings of the Royal Irish Academy. 



Leitrim gneiss points to a corresponding variety in the rocks forming 

 the more ancient series trayersed by it. 



The characteristic blocks of homblendic rock in. the gneiss are 

 ■R-ell seen again at the north-east termination of the chain, in the 

 townland of Pollboy, lA- miles west of llanorhamilton (fig 4). The 

 basic inclusions, which do not seem to hare attracted attention in this 

 area, are as striking as those of Glennagoolagh, near Eallydawley 

 Lough. They similarly weather away more rapidly than the 

 surrounding gneiss, leaving in places mere lenticular cavities. In 

 section they resemble dull and altered diorites, rich in hornblende. 



Tig. ^. 



Glaciated surface of banded gneiss, showing included blocks of 

 amphibolite (aphanite and diorite), \rhich. are often drawn out 

 parallel to the general flow. A dyke of later granite cuts the 

 M'hole. Pollboy, near ilanorhamilton. From a photograph by 

 the author. 



The gneiss has here become much darkened by biotite, and is 

 traversed by later veins of coarse white granite, corresponding with 

 those so frequently seen in Donegal. 



Eetuming now to the instances of amphibolite originally observed 

 by !Mr. Hardman, we have no difficulty in recognising them as 

 inclusions in the gneiss. 



The typical gneiss in Glennagoolagh is banded and rich in biotite ; 

 and garnet and green pyroxene occur in the micaceous bands. The 

 rock is obviously darkened in the neighbourhood of the inclusions, 



