anic mountains may be regarded as in a
dormant, or partially moribund, condition; and if the volcanic forces
have to some extent lost their strength, so it would appear have those
of elevation.

(_c._) _Areas of Volcanic Action in the British Isles._--In the case of
the British Islands it may be observed that the later Tertiary volcanic
districts lie along very ancient depressions, which may indicate zones
of weakness in the crust. Thus the Antrim plateau, as originally
constituted, lay in the lap of a range of hills formed of crystalline,
or Lower Silurian, rocks; while the volcanic isles of the Inner Hebrides
were enclosed between the solid range of the Archaean rocks of the Outer
Hebrides on the one side, and the Silurian and Archaean ranges of the
mainland on the other. And if we go back to the Carboniferous period, we
find that the volcanic district of the centre of Scotland was bounded by
ranges of solid strata both to the north and south, where the resistance
to interior pressure from molten matter would have been greater than in
the Carboniferous hollow-ground, where such molten matter has been
abundantly extruded. In all these cases, the outflow of molten matter
was in a direction somewhat parallel to the plications of the strata.

(_d._) _Special Conditions under which the Volcanic Action
operates._--Assuming, then, that the molten matter, forming an interior
magma or shell, is constantly exerting pressure against the inner
surface of the solid crust, and can only escape where the crust is too
weak (owing to faults, plications, or fissures) to resist the pressure,
we have to inquire what are the special conditions under which outbursts
of volcanic matter take place, and what are the general results as
regards the nature of the _ejecta_ dependent on those conditions.

(_e._) _Effect of the Presence or Absence of Water._--The two chief
conditions determining the nature of volcanic products, considered in
the mass, are the presence or absence of water. Such presence or absence
does not of course affect the essential chemical composition of the
_ejecta_, but it materially influences the form in which the matter is
erupted. The agency of water in volcanic eruptions is a very interesting
and important subject in connection with the history of volcanic action,
and has been ably treated by Professor Prestwich.[3] At one time it was
considered that water was essential to volcanic activity; and the fact
that the great