232 Cotr—On the Geology of Slieve Gallion, in the County of Londonderry. 
out that they form some of the more broken scenery of the mountain, particularly 
in the craggy and picturesque descent from Glenarudda Mountain to Ballybriest. 
The eurites of Sleve Gallion occur frequently in the form of veins, often 
tinged with epidote, but usually pink, traversing the basic lavas. Sometimes the 
intrusion has the character of a delicate network, as occurs in the farm-road west 
of Tirgan Rock, and on the edges of many larger veins on Tintagh Mountain.* 
Even the coarser veins at Tirgan Rock consist of a finely granular type, in 
which the quartz and felspar are barely separable with the lens. But micro- 
pegmatitic structure is determinable, and the rock is clearly a reproduction, on a 
smaller scale, of the fine-grained aplitic granites in Carndaisy Glen. A section of 
the adjacent augite-andesite shows signs of admixture, as stated in the second 
division of this paper; traces of quartziferous vems occur in it, and a certain 
amount of biotite, which has possibly arisen as a contact-product. 
Passing up over the broken andesitic ledges of Craigmore, we come upon a 
broad area under the Chalk, where small boulders of eurite le in a pale sand 
derived from them. An actual exposure of coarse red eurite, approaching 
granite, is seen in a digging on the south side of the road; and Mr. Egan has, 
with characteristic accuracy, carried the eurite (elvanite) on his map from this 
point over to the White Water Pass. A similar pavement of small boulders, 
consisting in this case of mixed granite and andesite, seems to point to a boss of 
these rocks within the region mapped as Cainozoic basalt. It occurs in the 
middle of the southern prolongation of the basaltic tableland, and possibly 
represents an old eminence which the Chalk itself did not overtop. 
The eurite on which the chalk was deposited in Brackagh-Slieve-Gallion is a 
compact grey type, with quartz granules and a fair amount of porphyritic plagio- 
clase, indicating a step towards the quartz-aphanites. But it is probably only an 
offshoot of the granite series, rich in soda. The section examined shows the 
characteristic green biotite ; but the rock is injured by brecciation, like several of 
the andesites in the lower part of the same townland. In its present condition, 
this eurite has a specific gravity of 2°72, clearly representing the basic end of 
the granite series. 
More typical grey and pink-brown eurites occur on the top of the granite of the 
White Water Pass, and in contact with the diabases of the overlying Tintagh 
plateau. They have been exposed in the cuttings along the upper part of the road 
from Tintagh post-office. The rock has a very compact and almost flmty ground, 
with granules of porphyritic quartz, and dull and inconspicuous idiomorphic 
felspars. This eurite is traceable to the north, until it becomes lost in the bog 
around the upper branches of the White Water. 
* This intrusive character was of course not overlooked by Mr. Egan (Mem. sheet 27, p.13). See also 
Portlock, ‘Geol. of Londonderry,” p. 542. 
