Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/193

 of the regular articulations and neatness of form which distinguish the basaltic pillars of the Causeway; they form enormous prismatic masses, often quadrilateral, and these latter appear to be formed of a congeries of smaller prisms, aggregated in a manner which brings to the mind the clustered assemblage of shafts forming a Gothic column: the greatest length of these columns is not less than 250 feet; the greenstone is highly crystallized, the concretions being distinct and large, and contains augite.

Slievemish, remarkable mountain, which lies like a colossal landmark in the middle of the county, is from its basis to its summit composed entirely of greenstone, thus forming a mass of nine hundred feet in thickness.

Notwithstanding Slievemish has at a distance the appearance of at cone, yet it is, like all the other mountains in Antrim, much more extended in the direction from north to south than in a transverse section; the ascent is steep and almost impracticable on the west side, where we rise to the top by a succession of short terraces similar to a flight of stairs.

The greenstone is here remarkably beautiful, being of a tender mountain green, interspersed with crystals of augite and granular olivine; the fracture is in flat or scaly concretions: it lies in distinct tabular masses two or three inches thick, perpendicular to the horizon, or sometimes with a slight dip to the westward.

The mountain of Teabuliagh near Newton Glens, has a cap of finely granular greenstone five hundred feet in thickness.

The rock which overlies the chalk at Magheralin, may perhaps with greater propriety be arranged as greenstone than basalt.