ot so when we include earthquakes
among the effects of igneous agency. A large proportion of the land
hitherto explored by geologists can be shown to have been rent or shaken
by subterranean movements since the oldest tertiary strata were formed.
It will also be seen, in the sequel, that new springs have burst out,
and others have had the volume of their waters augmented, and their
temperature suddenly raised after earthquakes, so that the description
of these springs might almost with equal propriety have been given under
the head of "igneous causes," as they are agents of a mixed nature,
being at once igneous and aqueous.

But how, it will be asked, can the regions of volcanic heat send forth
such inexhaustible supplies of water? The difficulty of solving this
problem would, in truth, be insurmountable, if we believed that all the
atmospheric waters found their way into the basin of the ocean; but in
boring near the shore we often meet with streams of fresh water at the
depth of several hundred feet below the sea level; and these probably
descend, in many cases, far beneath the bottom of the sea, when not
artificially intercepted in their course. Yet, how much greater may be
the quantity of salt water which sinks beneath the floor of the ocean,
through the porous strata of which it is often composed, or through
fissures rent in it by earthquakes. After penetrating to a considerable
depth, this water may encounter a heat of sufficient intensity to
convert it into vapor, even under the high pressure to which it would
then be subjected. This heat would probably be nearest the surface in
volcanic countries, and farthest from it in those districts which have
been longest free from eruptions or earthquakes.

It would follow from the views above explained, that there must be a
twofold circulation of terrestrial waters; one caused by solar heat, and
the other by heat generated in the interior of our planet. We know that
the land would be unfit for vegetation, if deprived of the waters raised
into the atmosphere by the sun; but it is also true that mineral springs
are powerful instruments in rendering the surface subservient to the
support of animal and vegetable life. Their heat is said to promote the
development of the aquatic tribes in many parts of the ocean, and the
substances which they carry up from the bowels of the earth to the
habitable surface, are of a nature and in a form which adapts them
peculiarly for the nutr