and seismic phenomena of the present day, they are restricted to
comparatively narrow limits; and that even within these limits the
volcanic forces are less powerful than they were in pre-historic times.

The middle part of the Tertiary period appears, in fact, to have been
one of extraordinary volcanic activity, whether we regard the wide area
over which this activity manifested itself, or the results as shown by
the great amount of the erupted materials. Many of the still active
volcanic chains, or groups, probably had their first beginnings at the
period referred to; but in the majority of cases the eruptive forces
have become dormant or extinct. With the exception of the lavas of the
Indian-Peninsular area, which appear, at least partially, to belong to
the close of the Cretaceous epoch, the specially volcanic period may be
considered to extend from the beginning of the Miocene down to the close
of the Pliocene stage. During the Eocene stage, volcanic energy appears
to have been to a great degree dormant; but plutonic energy was
gathering strength for the great effort of the Miocene epoch, when the
volcanic forces broke out with extraordinary violence over Europe, the
British Isles, and other regions, and continued to develop throughout
the succeeding Pliocene epoch, until the whole globe was surrounded by a
girdle of fire.

* * * * *

The reply, therefore, to the question with which we set out is very
plain; and is to the effect that the present epoch is one of
comparatively low volcanic activity. The further question suggests
itself, whether the volcanic phenomena of the middle Tertiary period
bear any comparison with those of past geological times. This, though a
question of great interest, is one which is far too large to be
discussed here; and it is doubtful if we have materials available upon
which to base a conclusion. But it may be stated with some confidence,
in general terms, that the history of the earth appears to show that,
throughout all geological time, our world has been the theatre of
intermittent geological activity, periods of rest succeeding those of
action; and if we are to draw a conclusion regarding the present and
future, it would be that, owing to the lower rate of secular cooling of
the crust, volcanic action ought to become less powerful as the world
grows older.

[1] Admirably illustrated in Prof. J. Milne's recently published work,
_The Great Earthquak