I. On the Geology of the Coasts of Antrim and Londonderry, and on the Age of the Giant’s Causeway; being Observations made in the North of Ireland in the Autumns of 1865 and 1866

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The distance from Belfast in Antrim to the shores of Lough Foyle in Londonderry is, as the crow flies, about 60 miles, but nearly 100 miles by following the curvatures of the numerous bays along the line of coast; and there are few places so well worthy of a visit from the tourist and geologist as the coast of the north of Ireland, consisting, as it does, of frowning precipices, steep cascades, deep but lovely glens, and narrow ravines cut in the basalt and chalk. The western portion was visited by me in 1865, the eastern in 1866. In this paper I propose, in the first place, to lay before you a general description of the geology of the coasts of Antrim and Londonderry. Generally speaking, the most of the rock formation of Antrim and part of Londonderry is chalk, and this is capped by basalt. The basalt is thickest at Magilligan on the shores of Lough Foyle. Its thickness is there about 900 feet. Dr Berger says that its average thickness over the county of Antrim is about 545 feet, and its extent 800 square miles. This immense thickness and extent of basalt, or cooled lava, shows that the northeastern portion of Ireland must at one period have been the centre of intense volcanic activity as great as that in any portion of the globe in recent times; and it is the province of the geologist to endeavour to ascertain the geological date of that period of volcanic activity.