Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/194



The top of Squires hill and of Cave hill, near Belfast, are both composed of this rock.

The little table-land forming the summit of Divis mountain consists of a beautiful clinkstone porphyry of a reddish brown colour, containing elongated lamellar crystals of glassy felspar, and concretions of bluish white chalcedony: the rock is very sonorous. A variety of clinkstone porphyry also occurs in the neighbourhood of the old red sandstone, near Newton Glens, so situated that it is difficult to pronounce what is its position or geognostic relations with regard to that rock: it will be more particularly described in the explanatory notes on the section.

The porphyries of Sandy-brae, &c. in the interior of the basaltic aræ, of which the geognostic situation is likewise uncertain, are referred to a separate article at the end of these extracts.

Occurs in beds of various thickness, alternating with and underlying the basaltic strata in the cliffs near the Giant's Causeway; in Cave hill, on Macgilligan, along the east side of Ushet lough in the isle of Rathlin, and in several other places. Although the specific gravity of the bole is low it contains a great proportion of iron, (acting powerfully on the magnet) and might with propriety be classed among the earthy and ochrey iron stones.