242 GRANITES OF THE NORTH-WEST OF IRELAND. 



hardening effect is sometimes communicated to a distance of two or 

 three feet into the neighbouring rock, but generally not to more than 

 a few inches. The hardened parts sometimes stand up in narrow 

 crests. Where dykes cross, it has been found that one of the planes 

 of intersection of the basalt dykes has been marked by the occurrence 

 of a very narrow band of black pitchstone. The base of the pitchstone 

 pillars of the interposed bed in Corygills is decomposed to a kind 

 cf kaolin where it touches the sandstone below. 



It is impossible to say what was the geological epoch of the later 

 eruptions of Arran, further than that they were posterior to the red 

 sandstone. They may be as modern as the basaltic eruptions of the 

 north of Ireland. 



Fig. 55. Granitic Ridges, Glen Sannox (Arrau). 



Granites of Ireland. 



Donegal. The granite axis of Donegal extends from Malin Head for 

 60 miles S. "W. to near Ardara. It is marked by the two great valleys 

 of Glenveagh and Gweebarra, which are nearly in the centre of the 

 granite band which traverses Donegal from Glen in the KE. to 

 Doocharry Bridge in the S.W. The granite is at first nine miles 

 wide, and afterwards spreads out to a breadth of 18 miles. It is 

 separated by the sea and by quartz rock from the granite of Dunaff 

 Head and Malin Head. There is an isolated patch to the S.E. of the 

 granite axis, divided into two portions by the Barnes More or Great 

 Gap. Still further to the S.E. granitic veins are numerous in the 

 gneissose rocks and metamorphic slates, where Donegal borders Fer- 

 managh, at Beleck and Castle Caldwell. This granite has a stratified 

 structure, the beds are nearly vertical, and run parallel to the great 

 valleys mentioned. The joints are nearly at right angles to the 



