te as at this spot.[7]

(_i._) _Dykes: Conditions under which they were Erupted._--No one can
visit the geological sections in Co. Antrim and the adjoining districts
of Down, Armagh, Derry, and Tyrone, without being struck by the great
number and variety of the igneous dykes by which the rocks are
traversed. The great majority of these dykes are basaltic, and they are
found traversing all the formations, including the Cretaceous and
Tertiary basaltic sheets. The Carlingford and Mourne Mountains are
seamed with such dykes, and they are splendidly laid open to view along
the coast south of Newcastle in Co. Down, as also along the Antrim coast
from Belfast to Larne. The fine old castle of Carrickfergus has its
foundations on one of those dyke-like intrusions, but one of greater
size than ordinary. All the dykes here referred to are not, however, of
the same age, as is conclusively proved by sections amongst the Mourne
Mountains where cliffs of Lower Silurian strata, superimposed on the
intrusive granite of the district, exhibit two sets of basaltic
dykes--one (the older) abruptly terminated at the granite margin, the
other and newer penetrating the granite and Silurian rocks alike. It is
not improbable that the older dykes belong to the Carboniferous or
Permian age, while the newer are with equal probability of Tertiary age.
Sir A. Geikie has shown that the Tertiary dykes of the North of Ireland
are representatives of others occurring at intervals over the North of
England, and Central and Western Scotland, all pointing towards the
central region of volcanic activity; or in a parallel direction thereto,
approximating to the N.W. in Ireland, the Island of Islay, and East
Argyleshire, but in the centre of Scotland generally ranging from east
to west.[8] The area affected by the dykes of undoubted Tertiary age
Geikie estimates at no less than 40,000 square miles--a territory
greater than either Scotland or Ireland, and equal to more than a third
of the total land-surface of the British Isles;[9] and he regards them
as posterior "to the rest of the geological structures of the regions
which they traverse." It is clear that the dykes referred to belong to
one great system of eruption or intrusion; and they may be regarded as
the manifestation of the final effort of internal forces over this
region of the British Isles. They testify to the existence of a
continuous _magma_ (or shell) of augitic lava beneath the crust; and as
the