Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/149

 which is 1850 feet high, the trap is seen passing from an homogeneous to a porphyritic texture. The latter imparted by lamellar crystals of felspar of a white colour, and of rather considerable size: gradually the accidental ingredient becomes the, essential one; the colour of the felspar becomes greenish, and in that state the rock approaches more nearly perhaps to the character of sienite than to that of trap.

In its genuine state, in the lower part of the mountain, the rock puts on the usual coated appearance which originates from decay.

On the eastern slope of Slieve Birna, one of the Mourne mountains, at one half nearly of its height, I observed a bed of hornblende rock, apparently interposed in the granite.

At Slieve Anisky, a hamlet on the road from Castlewellan to Dromore, I noticed another bed of hornblende rock, but I am doubtful whether it belongs to this class or to that of transition.

This subordinate member of the primitive trap series frequently occurs, forming distinct beds, in the mica slate of Antrim and Londonderry.

In the former county it is found in the valley of Glendun, and along the coast from Cushendon to Tor-point.

In the latter it occurs in Bennady-glen, in Aglish-glen, and in the bed of the Roe river near Dungiven. The bed of hornblende slate in the latter place occupies an extent not less than four hundred yards, ending by the old church, where it runs parallel to a