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.
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