eroidal by the gradual operation of centrifugal force acting on
yielding materials brought successively within its action by aqueous and
igneous causes.

The heat in mines and artesian wells increases as we descend, but not in
uniform ratio in different regions. Increase at a uniform ratio would
imply such heat in the central nucleus as must instantly fuse the crust.

Assuming that there are good astronomical grounds for inferring the
original fluidity of the planet, yet such pristine fluidity need not
affect the question of volcanic heat, for the volcanic action of
successive periods belongs to a much more modern state of the globe, and
implies the melting of different parts of the solid crust one after the
other.

The supposed great energy of the volcanic forces in the remoter periods
is by no means borne out by geological observations on the quantity of
lava produced by single eruptions in those several periods.

The old notion that the crystalline rocks, whether stratified or
unstratified, such as granite and gneiss, were produced in the lower
parts of the earth's crust at the expense of a central nucleus slowly
cooling from a state of fusion by heat has now had to be given up, now
that granite is found to be of all ages, and now that we know the
metamorphic rocks to be altered sedimentary strata, implying the
denudation of a previously solidified crust.

The powerful agency of steam or aqueous vapour in volcanic eruptions
leads us to compare its power of propelling lava to the surface with
that which it exerts in driving water up the pipe of an Icelandic
geyser. Various gases also, rendered liquid by pressure at great depths,
may aid in causing volcanic outbursts, and in fissuring and convulsing
the rocks during earthquakes.

The chemical character of the products of recent eruptions suggests that
large bodies of salt water gain access to the volcanic foci. Although
this may not be the primary cause of volcanic eruptions, which are
probably due to the aqueous vapour intimately mixed with molten rock,
yet once the crust is shattered through, the force and frequency of
eruptions may depend in some measure on the proximity of large bodies of
water.

The permanent elevation and subsidence of land now observed, and which
may have been going on through past ages, may be connected with the
expansion and contraction of parts of the solid crust, some of which
have been cooling from time to time, while others have be