Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/143

 slope gradually into the plain, the same rock reaches Rathfriland, a table-land of inconsiderable elevation.

Within the geographical boundaries just assigned, the granite is spread over a surface that measures 324 square English miles, comprehending the highest ground in the north of Ireland.

A few masses of other unstratified rocks, of which notice shall be taken elsewhere, occur in this formation.

Some primitive but stratified rocks also rest upon it in many places, generally arriving at their greatest elevations on their south-western boundaries.

The texture of this granite is either porphyritic or finely granular. The felspar always appears the prevailing ingredient, usually grey, more rarely milk white and earthy; the quartz has a smoky tinge, and in the granular variety it generally occurs crystallized in double six-sided pyramids; the mica is of a brown-black colour, and bears but a very small proportion to the other two constituents.

Amongst the accidental ingredients I remarked but two, viz. crystallized hornblende, chiefly abundant in the porphyritic variety, and small redish garnets in the granular. The two varieties are, I believe, mingled both together, so at least they occur on the top of Slieve Donard. I more particularly noticed the granular on Slieve Muck, Slieve Birna, Ravensdale, and Slieve Gullen; the porphyritic on Newry and Fathom mountains. The imbedded crystals of hornblende cause this rock to act on the magnet.

Water-worn pebbles of porphyritic syenite, occasionally containing flesh red crystals of felspar and iron pyrites, are very frequent at the base of the Mourne mountains on the road from Ross Trevor to New Castle. It is probable that these have been derived from the disintegration of neighbouring masses of that rock, occurring