molten lava would rise in the bore, and flow out over the ashes
and the sea-bottom--perhaps in one direction, perhaps all round.
Then, usually, the volcano, having vented itself, would be quieter
for a time, till the heat accumulated below, and more ash was blasted
out, making a second ash-bed; and then would follow a second lava
flow. Thus are produced the alternate beds of lava and ash which are
so common.

Now suppose that at this point the volcano was exhausted, and lay
quiet for a few hundred years, or more. If there was any land near,
from which mud and sand were washed down, we might have layers on
layers of sediment deposited, with live shells, etc., living in them,
which would be converted into fossils when they died; and so we
should have fossiliferous beds over the ashes and lavas. Indeed,
shells might live and thrive in the ash-mud itself, when it cooled,
and the sea grew quiet, as they have lived and thriven in Snowdonia.

Now suppose that after these sedimentary beds are laid down by water,
the volcano breaks out again--what would happen?

Many things: specially this, which has often happened already.

The lava, kept down by the weight of these new rocks, searches for
the point of least resistance, and finds it in a more horizontal
direction. It burrows out through the softer ash-beds, and between
the sedimentary beds, spreading itself along horizontally. This
process accounts for the very puzzling, though very common case in
Snowdon and elsewhere, in which we find lavas interstratified with
rocks which are plainly older than those lavas. Perhaps when that is
done the volcano has got rid of all its lava, and is quiet. But if
not, sooner or later, it bores up through the new sedimentary rocks,
faulting them by earthquake shocks till it gets free vent, and begins
its layers of alternate ash and lava once more.

And consider this fact also: If near the first (as often happens)
there is another volcano, the lava from one may run over the lava
from the other, and we may have two lavas of different materials
overlying each other, which have come from different directions. The
ashes blown out of the two craters may mingle also, and so, in the
course of ages, the result may be such a confusion of ashes, lavas,
and sedimentary rocks as we find throughout most mountain ranges in
Snowdon, in the Lake mountains, in the Auvergne in France, in Sicily
round Etna, in Italy r