376 



Proceedings of the Royal Irish Academy. 



equivalents of, and belong to the same period as, the granites and 

 associated earlier basic igneous rocks of Leinster, which are admittedly 

 of Devonian age. 



The relations of the Old Red Sandstone and metamorphic series 

 can be seen in the neighbourhood of "Windy Gap, to the north of 

 Castlebar. Here also can be noticed the effects of the post-Old Eed 

 Sandstone movement, or overthrusting of the later Old Red Sandstone 

 on to the older metamorphosed rocks, and the formation between the 

 two series, at this place, of a pseudo-conglomerate or zone of fault 

 and crush breccia, 40 feet and more in thickness, the breccia being 

 made up of the broken-up materials of both series, re-cemented into a 

 compact rock mass (fig. 5). 



Fig. 5. 





- ^A-^ • - — -^ ,Zf — : - ^ yi^ - — 



i MILE 



Overthriist of Old EedSandstone on to metamorphosed grit or quartzite (Q.). 

 "Windy Gap, N. of Castlebar, Co. Mayo. 



Proceeding north-eastward along the Ox Mountain range, the 

 included sediments become scarcer till they almost entirely disappear 

 and give place to the igneous rocks, as in the region north of Coolaney, 

 south of Lough Gill, and west and north of Manorhamilton. As 

 previously mentioned, the simulation of bedding seen in those rocks to 

 the south of Sligo and elsewhere is not due to sedimentation or 

 deposition, as originally supposed, but, on the contrary, is entirely due 

 to movement and shearing of the rock masses (fig. 6). 



It may be mentioned that a small ridge of similar deformed rocks 

 crops out from beneath the Carboniferous strata a few miles to the 

 north-west of Sligo, in the Rosses peninsula. 



The general characters of the Ox Mountain rocks are again re- 

 peated in south Donegal, in the region to the north of Pettigo, and 



