M'Henry — Report on the Ox Mountain lioch 



.377 



ubout Lough Derg. The intrusive rocks of this area have been 

 described as Archsean on the late editions of the Geological Survey- 

 Maps of that district. At one time I believed them to belong to this 

 group ; but I now consider them to be contemporaneous with the 

 rocks of the Ox Mountain chain, i.e. early Devonian. 



Continuing north and north-east into Donegal and Tyrone, and on 

 to Londonderry, we have repetitions of the Ox Mountain series, both 

 in the character and the conditions of the rocks, i. e. quartzite, 

 "Boulder Bed," limestone zone, black schist or slate, and the 

 lowest pebbly grit, and with the basic and acid intrusions in the same 

 order of sequence. 



Contorted and crumpled shear structure in complex of basic and acid igneous 



rocks, simulating bedding. 



Four miles "W. of Coolaney, Co. Sligo. 



In Fanad area, Donegal, to the north of Knockalla Mountain, we 

 find the metamorphosed sediments (quartzite) overthrust on to the 

 Old Eed Sandstone, with a zone of crust or overthrust breccia along 

 the line of movement, the direction of the overthrust being to the 

 north-west, here again proving the occurrence of great earth-stress 

 in post-Old Eed Sandstone times. 



As in the case north of Castlebar, the intrusions of igneous rocks 

 were prior to the folding and shearing, and both sediments and 



