In the mica slate district of Antrim, several beds of felspar porphyry are found; they may be traced along the coast from Torpoint to Cushendon, and thence inland along the old road to Ballycastle.
In Londonderry, the fundamental rock on the east side of Slieve Gallion is a variety of felspar porphyry strongly resembling sienite, with which rock it probably alternates in this mountain.
In the same neighbourhood felspar porphyry may be traced without interruption from the top of the glen of Latterane to the bed of the Knockadoo river on the road from Lissane to Moneymore.
F. Sienite.
I have traced this rock in the low country adjoining the town of Newry for three or four miles on the road to Dundalk, for the same space on that to Armagh, and towards Bainbridge as far nearly as the four mile house. It is perhaps more extensive.
The sienite is unstratified, cropping out in independent masses, but rent into pieces mostly of a rhombic figure, the natural joints of which are generally rusty.
It is finely granular, composed of felspar of a blood red, flesh red, or greyish colour, with some quartz, hornblende, and black mica, besides two ingredients, accidental but not uncommon, namely, iron pyrites and small garnets.
The sienite that crowns the summit of Foy mountain near Carlingford, contains only a few broad plates of black mica, with greenish felspar and hornblende in large concretions.
