Yol. 55.] ARE OF TYRONE AND LONDONDERRY GRA-NITES. 273 



18. On the Age of certain Granites in the Counties of Tyrone 

 and Londonderry.^ By Prof. Gtrenville A. J. Cole, F.G.S. 

 (Read March 22nd, 1899.) 



In a recent paper ^ on the geology of Slieve Gallion, in the County 

 of Londonderry, I had occasion to discuss the age of the granite 

 which forms so large a part of the basal mass of that mountain, 

 llepeated visits to the moorland farther west have convinced me as 

 to the identity of the granite of Eastern Tyrone with that of Slieve 

 Gallion itself; and thus any section elucidating the relations of 

 the former rock to its surroundings is of interest, on account of its 

 bearing on a very considerable area. 



Mr. Joseph Nolan ^ in 1878, during his survey of this obscure 

 and difficult district, found no satisfactory sections in the granite- 

 area north of Beragh. The rock is here surrounded by Lower Old 

 Eed Sandstone, and is exposed sparingly in the floor of a broad 

 valley. The Camowen liiver at present flows in the middle of a 

 strip of cultivated alluvium, with only a few boulders to indicate 

 the granite in its bed. In his paper published in 1879, Mr. Nolan 

 refers to an induration of the sandstone at Drumdulf Bridge as 

 probably due to the action of the granite. He cites, however, the 

 section at Aghnagreggan Bridge, west of Carrickmore, as affording 

 far clearer evidence. 



The best discussion of this section occurs in the Survey memoir 

 of 1878, for it is there pointed out that the granite, regarded as 

 intrusive in the Old Red Sandstone, is still pre-Carboniferous. 

 The Lower Carboniferous beds near Moneymore are thus full of 

 debris from the granite of Slieve Gallion. The evidence in the 

 townland of Aghnagreggan, upon which reliance is placed, is that 

 Devonian grits, in c-ontact with the granite, are ' vitrified and 

 converted into quartzite.' The unaltered sandstones nearer the 

 main road are, of course, duly noted. 



When Mr. Nolan wrote, the views held by Portlock as to the 

 metamorphic origin of the granite of this area were still prevalent 

 in the writings of his successors, and Mr. Nolan was departing 

 from traditional lines in regarding any part of the granite as 

 intrusive. His reference to the 'yellowish quartzite,' "^ however, 

 makes it possible that he regarded a fine-grained yellowish granite 

 near the junction as an altered grit, and was consequently misled 

 in his reading of the section. 



After my paper on Slieve Gallion had been published, Mr. J. 

 St. John Phillips and myself examined the rocks west of Pomeroy ; 

 and Mr. Phillips opened up a junction of granite and sandstone in 

 Aghnagreggan, a little south of the boundary shown upon the 



^ [This paper was read under the title of ' A Critical Junction in the County 

 of Tyrone.'j 



"^ Trans. R. Dublin Soc. vol. vi (1897) p. 243. 



« Geol. Surv. Irel. Expl. Mem. ISbeet, 34 (1878) p. 15, and 'On the Meta- 

 inorpbic & Intrusive Rock.s of Tyrone,' Geol. Mag. 1879, p. 159. 

 ; * Geol. Mag. 1879, p. 159. 



Q.J.G. S. No. 218. T 



