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into ridges and escarpments of considerable elevation; and amongst these may be specially named the ridge on which is situated the Glasgow Necropolis, and the bold mural escarpment of Salisbury Crags. 1 3. Tabular Sheets. In the North-East of Ireland, extending from the northern shore of Belfast Lough on the south, to the precipitous cliffs of Fair Head and the Giant’s Causeway, we have one of the grandest exhibitions of sheets of basaltic and doleritic rocks in the British Islands. Here the different lava- flows of Lower Miocene age are consolidated into several successive beds of hard, generally columnar trap, separated by irregular beds of bole, ochre, vol canic ash, and agglomerate, the whole attaining a thickness of nearly 2000 feet. These rocks reappear in the Isles of Mull and Staffa, and are considered by Mr. A. Geikie to be continued through the Faroe Islands into Iceland, where the volcanoes, long since extinct in Britain, are still in activity. 2 In texture and composition, these basaltic rocks are extremely variable. In some places they are soft, earthy, and amygdaloidal; in others compact, or highly crystal line. In the largely crystalline dolerite of Fair Head, in Antrim, the ordinary augitic mineral is replaced by hyperstliene. A similar hypersthenic dolerite 1 Geikie, Geol. of Edinb. Mem. Geol. Survey, p. 22 (1861). 2 Address, Brit. Assoc., 1867.
