228 Cotr—On the Geology of Slieve Gallion, in the County of Londonderry. 
of a precisely similar character. In this case, the presence of an intrusive micro- 
pegmatite is clearly visible in the mass, the whole rock being cut up into horn- 
blendic patches two or three centimetres across, divided by much whiter veins. 
This rock probably comes, like so many erratics on Slieve Gallion, from the region 
immediately to the south-west. 
In section, the original gabbro, with clouded felspars interlocked with ural- 
itic hornblende, is seen forming the darker areas; while the veins consist of a 
somewhat coarse intergrowth of quartz and a dusky and unrecognisable felspar. 
Biotite occurs, however, in these veins, and strongly suggests that they are offshoots 
of the biotite-granite of Slieve Gallion. The green hornblende that abuts on or is 
included in these veins has a well marked idiomorphie character, and has crystallised 
out under their influence, or after complete fusion in them (PI. xtv., fig. 3). 
The last aphanite that I need mention lies in the small townland of Tatnagilta, 
on the road from Lough Fea to Cookstown, a third of a mile north-west of Unagh 
School; the rock here passes into diorite. In section it is seen to contain 
much magnetite, and the green hornblende is largely chloritised. Doubtless the 
rock was formerly pyroxenic; and its structure is that of a typical dolerite. Veins 
of calcite, and also of epidote, traverse it, and there is some secondary quartz. 
The rock is important, as a link between the diabase series of Slieve Gallion and 
the gabbros and diorites of the Oritor area. 
Still better evidence is given as to a connection between the two series in 
the exposures on the steep south-west flank of Glenarudda Mountain, or “‘ Mobuy 
Top.” Although the granite separates, in a band half a mile wide, the horn- 
blende-andesites on the level of the highest farms from the handsome diorites in 
the angle of the roads below, yet some parts of the bosses formed by the latter 
have a structure essentially intermediate between the lavas and the diorites. Dark- 
green mica occurs, however, in the diorite of Mobuy, in addition to the horn- 
blende; and I have not been able to determine if it is derived from commingling 
with the granite. 
The compact diabases (originally andesites) of the summit of Glenarudda 
Mountain have their counterparts, again, in the somewhat schistose series in contact 
with the granite further south. This series comes in, for example, at the bridge 
half a mile north of Crockanney, 1000 feet below the summit; and the diabase of 
Tatnagilta, and other compact forms, serve to coutinue it into the moorland west 
of Cookstown.* 
On the north-west side of Slieve Gallion, the only diorite that I have met with 
is a very decomposed mass, with reddened felspars, on the right bank of the 
White Water, among the ashy-looking rocks 500 feet below a conspicuous little 
waterfall. Owing to the proximity of eurite veins, I suspect that this rock, if a 
* Compare F. W. Egan, Mem. sheet 27, p. 1. 
