mitted ravages, and from which it has now retired--estuaries
where high tides once rose, but which are now dried up--valleys hollowed
out by water, where no streams now flow, is no more than we should
expect; these and similar phenomena are the necessary consequences of
physical causes now in operation; and if there be no instability in the
laws of nature, similar fluctuations must recur again and again in time
to come.

But, however natural it may be that the force of running water in
numerous valleys, and of tides and currents in many tracts of the sea,
should now be _spent_, it is by no means so easy to explain why the
violence of the earthquake and the fire of the volcano should also have
become locally extinct at successive periods. We can look back to the
time when the marine strata, whereon the great mass of Etna rests, had
no existence; and that time is extremely modern in the earth's history.
This alone affords ground for anticipating that the eruptions of Etna
will one day cease.

Nec quae sulfureis ardet fornacibus, AEtna
Ignea semper erit, _neque enim fuit ignea semper_,

(Ovid, _Metam._ lib. 15-340,)

are the memorable words which are put into the mouth of Pythagoras by
the Roman poet, and they are followed by speculations as to the cause of
volcanic vents shifting their positions. Whatever doubts the philosopher
expresses as to the nature of these causes, it is assumed, as
incontrovertible, that the points of eruption will hereafter vary,
_because they have formerly done so_; a principle of reasoning which, as
I have endeavored to show in former chapters, has been too much set at
naught by some of the earlier schools of geology, which refused to
conclude that great revolutions in the earth's surface are now in
progress, or that they will take place hereafter, _because_ they have
often been repeated in former ages.

_Division of the subject._--Volcanic action may be defined to be "the
influence exerted by the heated interior of the earth on its external
covering." If we adopt this definition, without connecting it, as
Humboldt has done, with the theory of secular refrigeration, or the
cooling down of an original heated and fluid nucleus, we may then class
under a general head all the subterranean phenomena, whether of
volcanoes, or earthquakes, and those insensible movements of the land,
by which, as will afterwards appear, large districts may be depressed or
elevated, without convulsions. A