ccording to this view, I shall consider
first, the volcano; secondly, the earthquake; thirdly, the rising or
sinking of land in countries where there are no volcanoes or
earthquakes; fourthly, the probable _causes_ of the changes which result
from subterranean agency.

It is a very general opinion that earthquakes and volcanoes have a
common origin; for both are confined to certain regions, although the
subterranean movements are least violent in the immediate proximity of
volcanic vents, especially where the discharge of aeriform fluids and
melted rock is made constantly from the same crater. But as there are
particular regions, to which both the points of eruption and the
movements of great earthquakes are confined, I shall begin by tracing
out the geographical boundaries of some of these, that the reader may
be aware of the magnificent scale on which the agency of subterranean
fire is now simultaneously developed. Over the whole of the vast tracts
alluded to, active volcanic vents are distributed at intervals, and most
commonly arranged in a linear direction. Throughout the intermediate
spaces there is often abundant evidence that the subterranean fire is at
work continuously, for the ground is convulsed from time to time by
earthquakes; gaseous vapors, especially carbonic acid gas, are
disengaged plentifully from the soil; springs often issue at a very high
temperature, and their waters are usually impregnated with the same
mineral matters as are discharged by volcanoes during eruptions.

VOLCANIC REGIONS.

_Region of the Andes._--Of these great regions, that of the Andes of
South America is one of the best defined, extending from the southward
of Chili to the northward of Quito, from about lat. 43 degrees S. to
about 2 degrees N. of the equator. In this range, however, comprehending
forty-five degrees of latitude, there is an alternation on a grand scale
of districts of active with those of extinct volcanoes, or which, if not
spent, have at least been dormant for the last three centuries. How long
an interval of rest may entitle us to consider a volcano as entirely
extinct is not easily determined; but we know that in Ischia there
intervened between two consecutive eruptions a pause of seventeen
centuries; and the discovery of America is an event of far too recent a
date to allow us even to conjecture whether different portions of the
Andes, nearly the whole of which are subject to earthquakes, may not
experie