sequence of the
curve described by the uprushing vapour. The expansion in the
ascending column causes it to enlarge at a somewhat steadfast rate,
while the speed of the ascent is ever diminishing. Precisely the same
action can be seen in the like rush of steam and other gases and
vapours from the cannon's mouth; only in the case of the gun, even of
the greatest size, we can not trace the movement for more than a few
hundred feet. In this column of ejection the outward movement from the
centre carries the bits of lava outwardly from the centre of the
shaft, so that when they lose their ascending velocity they are drawn
downward upon the flanks of the cone, the amount falling upon each
part of that surface being in a general way proportional to the
thickness of the vaporous mass from which they descend. The result is,
that the thickest part of the ash heap is formed on the upper part of
the crater, from which point the deposit fades away in depth in every
direction. In a certain measure the concentration toward the centre of
the cone is brought about by the draught of air which moves in toward
the ascending column.

Although, in general, ejections of volcanic matter take place through
cones, that being the inevitable form produced by the escaping steam,
very extensive outpourings of lava, ejections which in mass probably
far exceed those thrown forth through ordinary craters, are
occasionally poured out through fissures in the earth's crust. Thus in
Oregon, Idaho, and Washington, in eastern Europe, in southern India,
and at some other points, vast flows, which apparently took place from
fissures, have inundated great realms with lava ejections. The
conditions which appear to bring about these fissure eruptions of lava
are not yet well understood. A provisional and very probable account
of the action can be had in the hypothesis which will now be set
forth.

Where any region has been for a long time the seat of volcanic action,
it is probable that a large amount of rock in a more or less fluid
condition exists beneath its surface. Although the outrushing steam
ejects much of this molten material, there are reasons to suppose that
a yet greater part lies dormant in the underground spaces. Thus in the
case of AEtna we have seen that, though some thousands of miles of
rock matter have come forth, the base of the cone has been uplifted,
probably by the moving to that region of more or less fluid rock. If
now a region thus u