ng through one or more fissures, and meeting with water-charged
strata or scoriae, will convert the water into steam at high pressure,
resulting in explosions more or less violent and prolonged, in
proportion to the quantity of water and the depth to which it has
penetrated. In this manner we may suppose that ashes, scoriae, and blocks
of rock torn from the sides of the crater-throat, and hurled into the
air, are piled around the vent, and accumulate into hills or mountains
of conical form. After the explosion has exhausted itself, the molten
lava quietly wells up and fills the crater, as in the cases of those of
Auvergne and Syria, and other places. We may, therefore, adopt the
general principle that in volcanic eruptions _where water in large
quantities is present, we shall have crater-cones built up of ashes,
scoriae, and pumice; but where absent, the lava will be extravasated in
sheets to greater or less distances without the formation of such cones;
or if cones are fanned, they will be composed of solidified lava only,
easily distinguishable from crater-cones of the first class_.

(_g._) _Nature of the Interior Reservoir from which Lavas are
derived._--We have now to consider the nature of the interior reservoir
from which lavas are derived, and the physical conditions necessary for
their eruption at the surface.

Without going back to the question of the original condition of our
globe, we may safely hold the view that at a very early period of
geological history it consisted of a solidified crust at a high
temperature, enfolding a globe of molten matter at a still higher
temperature. As time went on, and the heat radiated into space from the
surface of the globe, while at the same time slowly ascending from the
interior by conduction, the crust necessarily contracted, and pressing
more and more on the interior molten magma, this latter was forced from
time to time to break through the contracting crust along zones of
weakness or fissures.

(_h._) _The Earth's Crust in a State of both Exterior Thrust and of
Interior Tension._--As has been shown by Hopkins,[4] and more recently
by Mr. Davison,[5] an exterior crust in such a condition must eventually
result in being under a state of horizontal thrust towards the exterior
and of tension towards the interior surface. For the exterior portion,
having cooled down, and consequently contracted to its normal state,
will remain rigid up to a certain point of resistance; b