Cotr— On the Geology of Slieve Gallion, in the County of Londonderry. 233 
The fragments of eurite on the pass supply the red colouring to the taluses, the 
granite pebbles associated with them decomposing in far paler tones. But this is 
due rather to the development of iron-ores in fissures in the higher levels of the 
mountain, and the consequent staining of the eurite, than to any difference of 
constitution between the rocks themselves. 
The granite at the highest farm in Tintagh makes an interesting junction with 
the vesicular andesites, as will be presently described, sending off veins of eurite 
into them. The whole contact-phenomena can be easily studied in these little 
protrusions between the farm-house and the road. On the east of Tintagh Mountain 
itself, eurite veins are common in the andesites, but have been extensively attacked 
by decomposition. In outward appearance they resemble the very fine-grained 
type, the so-called ‘‘ compact felspar” of older authors; but they are disappointing 
in microscopic section, owing to the prevalence of epidote and caicite. In fact, 
they have become much more calcareous than the vesicular andesites which they 
traverse. 
An almost continuous section in eurite, extending over half a mile, is seen in 
the ravine of the stream descending from Slieve Gallion South into the White Water. 
This is probably typical of much that is hidden by the boggy slopes on this side 
of the mountain. The eurite is here traceable, from a pinkish and greyish form 
at about 1500 feet above the sea, to red porphyritic forms at 1200 feet. In the 
middle of the section, hematitic gravel obscures the banks, and the eurite becomes 
much redder in its neighbourhood. I think we shall be right in regarding the grey 
and grey-brown types as representing the original condition of the eurite. 
On the same general level, but against the main stream of the White Water, 
pink compact veins traverse the diabases. ‘The specific gravity of these veins is 2°71. 
An unexpected band of dark grey-eurite, with porphyritic quartz and colourless 
felspar, crosses the White Water north-west of the bridge on the Desertmartin road. 
Its relations to the schistose and probably ashy diabases are unseen, but it is no 
doubt a northerly protrusion of the great granite series of Sheve Gallion. Already, 
owing to signs of secondary flow, due to earth-pressures, it has assumed a meta- 
morphic aspect; and it is easy to see how further metamorphism would convert it 
into a fine-grained gneiss, closely resembling, for example, those of Torr Head in 
the county of Antrim. 
The finest exposures of eurite, in a little modified state, lie at 1250 feet on the 
shoulder of Letteran, between the Letteran and Derryganard Hollows. In the 
stream-cuts north-east of Slieve Moyle, the heematitic granite passes here and there 
towards eurite; but further east, between Tory’s Hole and the boss of diorite, a 
somewhat coarse porphyritic eurite, grey and tresh-looking, appears in a series of 
small outcrops. A red form, which is clearly the same rock stained with iron oxide, 
is closely associated with it, and is exposed again higher up the slope. Ordinary 
