230 Cotre—On the Geology of Slieve Gallion, in the County of Londonderry. 
The hornblende-granite of Ballybriest was long ago described, under the 
name ‘sienite,” by J. F. Berger,* who appears, however, to have overlooked the 
more prevalent micaceous type. His ‘‘felspar-porphyry”’+ seems also unduly 
extended, though he doubtless appreciated the difference between the eurites and 
the coarsely developed granites. 
Mr. Wattst has referred to the prominent rocks of the Slieve Gallion area as 
‘¢eoarse granites more or less foliated, and passing into quartz-diorites.” Blue 
quartz, hornblende, and biotite, are noted as constituents. 
The characteristic rock, as seen, for instance, on the south side of Carndaisy 
plantation, is a red granite of medium grain, with dark green chloritised biotite. 
The orthoclase is dulled, and the Carlsbad twinning is rarely recognisable in the 
mass. Pyrite occurs, and occasional inclusions of the diabasic rocks are recognis- 
able. These have been noticed on the craggy slope of Mobuy, but not exceeding 
2 em. in length. 
A second felspar is present, as in most granites, but rarely shows its repeated 
twinning, even under the microscope. This considerable alteration of the felspars 
makes it difficult to trace them in the contact-rocks where intermingling has gone 
on; and even their specific determination is impossible. 
As recorded by previous observers, hornblende becomes a constituent in the 
south-west of the area; and there is little wonder that some of the fine-grained 
forms of the granite have been regarded as passage-rocks into the schistose diabases 
and andesites. 
Between Anney’s Well (on the road from Lough Fea) and Ballybriest Bridge, 
the granite exhibits remarkable variations in grain. It is perfectly possible, at any 
rate, that the dark fine-grained type, at the same time rich in quartz and in horn- 
blende and black biotite, results from amalgamation with the basic material ciose 
at hand. Half a mile to the north-east are the abrupt bosses of diorite already 
referred to, in the angle of two roads from Glenarudda Mountain; and into these 
the granite has sent numerous veins. How much of this diorite has been actually 
lost by intimate penetration and diffusion, it is now impossible to say; but the 
hornblende-granite, occurring at such a point, with diorite again to west of it, 
seems logically to carry out what has already been observed in Letteran.§ 
In section, the granite of Ballybriest Bridge shows that the quartz and most of 
the felspar crystallised out simultaneously, producing a rough pegmatitic structure. 
The idiomorphie felspar that occurs can often be recognised as plagioclastic, and 
* Geological Features of N. E. Ireland,” Trans. Geol. Soc. Lond. vol. ii1., p. 145. 
+ Lbid., p. 144. { M‘Henry and Watts, op. cit., p. 78. 
§ Compare the important work of M. Michel Lévy, ‘‘Compte-rendu du 17 septembre,”’ Bull. Soc. geol. 
de France, 3me sér., t. xvili. (1890), pp. 915 and 916; and Ch. Barrois, on similar occurrences in 
Brittany, 7b7d., pp. 917 and 920. 
