XVIII. The Geology of Portrush
J. S. M'Lennan
Transactions of the Geological Society of Glasgow
Portrush, make several trips by the electric railway, to inspect the many wonderful geological phenomena to be found in the district. In July last (1900) I carried out this programme, and now propose to give you a few of my observations in the following paper. Portrush is built on a rocky ridge which runs in a northerly direction for nearly a mile, and ends in Ramorne Head, with its rugged, mural precipices. West of this is the harbour, and beyond it a level shore flanked by sand-dunes. Then
... abrupt and rocky coast-line again becomes prominent, stretching as far as Port Stewart, where there is a small opening, and the trap rocks assert themselves as far as the mouth of the River Bann. At Downhill there is an exposure of the white rocks of the Chalk, succeeded by cliffs of trap along the shore for some distance, and through a projecting part of which two tunnels have been cut for the Northern Counties Railway. Beyond this the coast-line, to the eastern shore of Lough Foyle, is very flat. The rocks which rise inland belong to the Cambrian or Silurian formation, as do also most of those of the extreme north-west. Along the eastern shore of the Portrush ridge the rocks lie a few feet above sea-level, and here occur the Liassic beds which have made the district geologically classical. Further east they sink from view, and the 44 Long Strand" succeeds. It stretches for over two miles along the shore, and is flanked inland by an extensive background of sandy heights and hollows, now used for the well-known golf-course. The "White Rocks" follow this, rising to about 100 feet, and extending along the shore for nearly half-a-mile. These are March 6, 2015 at Queen's University on http://trngl.lyellcollection.org/ Downloaded from M'LBNKAN-THE GEOLOGY OF PORTRUSH. 181 capped by a trap overflow, forming, from its dark colour, a striking contrast to the white masses of the underlying Chalk, which disappear further east, when the trap alone is visible. A small isolated mass, rising abruptly to about 120 feet, has its whole surface covered by the ruins of Dunluce Castle, said to be amongst the most picturesque in Ireland, and from it very fine views of the wide coast scenery can be obtained, including "the White Strand," Portrush, *' The Skerries " (a group of islands), and the high cliffs which overhang the " Giant's Causeway." Then comes White Bay, from which the " White Rocks" extend for a considerable distance till they are succeeded by the trap. The well-known sea-stack, " Carrick-a-rede," famous for its connecting rope-bridge, is near this, and to the north may be seen the Island of Rathlin, with the white limestone appearing on its south-western shore, under the dolerite which covers it to the north. One of the most interesting geological phenomena of the North of Ireland is the great basalt overflow, varying in thickness from 10 to more than 1000 feet, which covers nearly the whole of Antrim, half of Londonderry, and portions of Down and Armagh, or an area of from 1000 to 1200 square miles. It has been poured out over the underlying sedimentary rocks, not from one centre of emission, but from various fissures, afterwards filled up with the ejected material, so that their exact position cannot now be definitely made out. This overflow did not all take place at one time, but in suc cessive eruptions differing in nature and extent, as may still be distinctly seen in all the expo'sed beds, which show great diversities of thickness in their various layers. It must have continued at intervals through a long series of years, as in some cases the period of quiescence has been sufficient to allow the surface of the trap layer to be disintegrated by weather action, and so prepared, before the next eruption, for the growth of vegetation, of which the fossilised remains can yet be seen. These remains of plantlife, buried by the next outflow, have been changed by the com bined action of pressure and heat into lignite. A bed of this mineral at Ballintoy, east of the "Causeway," from 1 foot to 4 or 5 feet thick, and very solid in its upper portion, lies close under the trap, resting on a dark carbonaceous clay-bed. In it plan*- March 6, 2015 at Queen's University on http://trngl.lyellcollection.org/ Downloaded from Of clustering columns wedged in dense array; With skill so like, yet so surpassing art, With such design, so just in every part, That reason pauses, doubtful if it stand The work of mortal, or immortal hand."