240 Cote—On the Geology of Slieve Gallion, in the County of Londonderry. 
In Beaghbeg, the agglomerates have suffered so greatly from crushing, that I 
am prepared to find them transferred in certain quarters to the growing group of 
crush-conglomerates.* But, like the tuffs of Windy Castle, they contain blocks of 
more scoriaceous character than the rocks immediately round them, and in section 
show abundant fragments of andesite-glass, such as ordinarily occur only in tuffs. 
The variety of rocks present, including some ‘‘felsites,” is also in favour of a 
volcanic origin. I would, indeed, go so far as to urge that the crushing, and the 
permeation by veins of quartz, have actually obscured the original fragmental 
structure. The mass has, in fact, been compressed, and to some extent foliated, 
until it resembles many of the rolled-out tuffs of Wales. 
Opposite the farm of Keragh, in the east end of Beaghbeg, great masses of the 
typical hornblende-diorite (altered coarse-grained gabbro) jut out on the surface, 
seamed by veins of granite and of eurite. This fact willserve to complete the 
evidence of similarity on both sides of the gneissic axis, and of the essential 
relationships between the rocks west of Cookstown and those of the massif of Sheve 
Gallion (PI. xtv., fig. 6). 
V.—COoNSIDERATIONS ARISING FROM A SURVEY OF THE PLuronic Rocks ON AND NEAR 
SuievE GALLION. 
In the moorland west of Oritor, we seem to get down to the roots of the volcano, 
the surface-manifestations of which are seen on the plateaux of Sleve Gallion. 
On the latter mountain itself, we have glimpses of the underlying coarsely erystal- 
line rocks, in the diorite bosses of Letteran and Mobuy. But these are merely 
residual, and the great mass of the Slieve Gallion granite has either concealed or 
absorbed its basic precursors. In the western moorland, on the contrary, the 
gabbros survive, interestingly associated with lavas, and even tufts, belonging to 
the same basic series. The granite is down below them, sending off abundant 
veins; but the struggle between the two types of rock can be examined here across 
miles of country, and the mastery of the granite has been by no means absolute. 
A consideration of the aplitic granite of Carndaisy Glen, of the general poverty 
of the eurites in ferromagnesian constituents, and of the veins of pure soda-ortho- 
clase that occur in the neighbourhood of Oritor, make me venture on a somewhat bold 
suggestion. The invading granite magma may originally have agreed in com- 
position with the eurites, and would have crystallised as a virtual aplite; and it 
may owe even its green micas, as well as its occasional richness in hornblende, to 
the pre-existing gabbros that it absorbed.t The variations of a granite mass, in 
* Compare M‘Henry, letter in Nature, vol. liii. (1895-6), p. 414; also Sir A. Geikie, ‘‘ On some Crush- 
conglomerates in Anglesey,” Geol. Mag., 1896, p. 481; and letter by Prof. Blake, ibid., p. 569. 
Similar questions relating to the granite of Aydat, in Auvergne, were dealt with by M. Michel Lévy, 
