Slieve Croob in the lower Iveagh, the most conspicuous hill of a small group that lies nine miles in advance to the north of the Mourne mountains, seems formed on its north-east and south-east sides of different varieties of sienite, some of which are porphyritic and very beautiful: the crystals of hornblende are extremely well defined, and the compact felspar constituting the basis has a brownish or smoky colour.
This sienite crops out at intervals from Bakaderry town to the top of Slieve Croob, occupying an elevation of about 900 feet.
Slieve Gallion, in Derry, exhibits sienite in connection with porphyry, and either of a porphyritic texture or in large concretions; I met with an elegant variety on the road from Lissane to Lough Finca; the felspar is either slightly green or flesh-red with hornblende, quartz, and some pyrites. This rock acts strongly on the magnet.
In the bed of the Black-water near the valley of the Mayowla, on the north-west side of Slieve Gallion, the sienite is mostly composed of crystallized hornblende with some felspar and iron pyrites, verging therefore into greenstone.
