Vol. 55.] GRANITES IN TYRONE AND LONDONDERRY. 275 



mass, giving off its detritus on its irregularly wcathereil sur- 

 faces ; 500 foefc away, the fine-grained decomposing granite is seen 

 definitely in situ at the tuck-mill. 



Sindstone containing fragments of granite, Aghnagreggan( Co. Tyrone). 



1 



[About I nat. size.] 



In the paper on Slieve GrMllion already quoted/ I compared the 

 granite with that of Crossdoney, near Cavan, which is post-Silurian 

 but pre-Uevonian, like so many masses throughout Ireland. There 

 is now no difficulty in regarding the intrusive rocks of Beragh, 

 Pomeroy, and Lissan as connected with the great ' Caledonian^ 

 epoch of mountain-building. The axis along which they appear 

 has, moreover, the characteristic north-easterly and south-WTsterly 

 trend. Except for its association with somewhat dubious cherts,'"^ 

 which may be of Arenig age, the volcanic series penetrated at various 

 points by the granite lends no stratigraphical assistance. Some of 

 the strikingly fossiliferous beds, referred to the Bala Series, near 

 Pomeroy, contain much secondary mica, developed over the surfaces 

 of the fossils. But they are not in any sense metamorptiosed, 

 and are mapped as merely faulted down against the hornblende- 

 granite of JBardahessiagh. The determination of the age of the 

 volcanic series of Slieve G allien and Eastern Tyrone would limit the^ 

 possible antiquity of the granite ; the critical section in Aghna- 

 greggan, above described, tells us only one end of the story. 



1 Trang. R. Dublin Soc. vol. vi (1897) pp. 24.'^245. 



'^ Ibid. p. 239, & Sir A. Geikie, ' Ancient Volcanoes of Great Britain,' vol. i 

 (1897) pp. 239-241. 



t2 



