Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/428

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Dr. Mac Culloch on the Geology of

It is bounded on the north-west by perpendicular cliffs, 200 or 300 feet in height, but on the other sides it declines by a rapid grassy slope to the sea, intermixed however with rocky faces, and covered with heaps of fragments, which are perpetually falling from the bare rock. The square ruined tower which remains at about a third part of its elevation, offers nothing interesting to the antiquary, but the botanist will be delighted with the profusion of lychnis dioica and silene amæna which covers it with a dense coat of flowers; to the exclusion of even the grass. It contains springs at about 200 feet below its summit, which unite in a small marshy plain, covered with enormous plants of hydrocotyle vulgaris.

The rock which forms this insular mountain is in general amorphous, and breaks into large irregular masses, sometimes approaching to a rectangular shape, sometimes without that tendency, and resembling the fragments of quartz rock. In many places it approaches to an obscure columnar structure, and this occasionally acquires great regularity. It is on the north-west side that the columns are most perfect. They are here well defined in their angles, yet adhere together, so as to appear to form one continuous mass, their true structure being only detected by the occasional falling of the huge fragments which strew the narrow beach on this quarter of the island. They vary in the number of their sides, but like basaltic columns, the most general forms are the pentagonal and hexagonal. I could not any where perceive that they were jointed, but they break at right angles to their axes, forming those flat summits which are tenanted by clouds of gannets. Their dimensions are universally large, as they are from six to eight feet in diameter, and extend in height as far as the eye can judge, to a continuous altitude of 100 feet and upwards. Nothing can exceed the magnificence of the columnar wall on this side of the rock; even the