here are large areas of volcanic rock
overspread by the waters of the ocean.

(_a._) _Geological Age._--The British volcanic eruptions now under
consideration are all later than the Cretaceous period. Throughout
Antrim, and in parts of Mull, the lavas are found resting on highly
eroded faces either of the Upper Chalk (Fig. 27), or, where it has been
altogether denuded away, on still older Mesozoic strata. From the
relations of the basaltic sheets of Antrim to the Upper Chalk, it is
clear that the latter formation, after its deposition beneath the waters
of the Cretaceous seas, was elevated into dry land and exposed to a long
period of subaerial erosion before the first sheets of lava invaded the
surface of the ground. We are, therefore, tolerably safe in considering
the first eruptions to belong to the Tertiary period; but the evidence,
derived as it is exclusively from plant remains, is somewhat conflicting
as to the precise epoch to which the lavas and beds of tuff containing
the plant-remains are to be referred. The probabilities appear to be
that they are of Miocene age; and if so, the trachytic lavas, which in
Antrim are older than those containing plants, may be referred to a
still earlier epoch--namely, that of the Eocene.[1] As plant remains are
not very distinctive, the question regarding the exact time of the first
volcanic eruptions will probably remain for ever undecided; but we are
not likely to be much in error if we consider the entire volcanic period
to range from the close of the Eocene to that of the Miocene; by far the
greater mass of the volcanic rocks being referable to the latter epoch.

In describing the British volcanic districts it will be most convenient
to deal with them in three divisions--viz., those of Antrim, Mull, and
Skye, commencing with Antrim.[2]

(_b._) _Volcanic Area._--The great sheets of basalt and other volcanic
products of the North-east of Ireland overspread almost the whole of the
County Antrim, and adjoining districts of Londonderry and Tyrone,
breaking off in a fine mural escarpment along the northern shore of
Belfast Lough and the sea coast throughout the whole of its range from
Larne Harbour to Lough Foyle; the only direction in which these features
subside into the general level of the country being around the shores of
Lough Neagh. Several outliers of the volcanic sheets are to be found at
intervals around the great central plateau; such as those of Rathlin
Island, Isl