disclosed, in the manner of ordinary caverns, by the falling of their
roofs; but the greater part are naturally hidden beneath the
ever-increasing materials of the cone.

The lava-stream caves of Ætna are not only interesting because of
their peculiarities of form, which we shall not undertake to describe,
but also for the reason that they help us to account for a very
peculiar feature in the history of the great cone. On the slopes of
the volcano, below the upper cindery portion, there are several
hundred lesser cones, varying from a few score to seven hundred feet
in height. Each of these has its appropriate crater, and has evidently
been the seat of one or more eruptions. As the greater part of these
cones are ancient, many of them being almost effaced by the rain or
buried beneath the ejections which have surrounded their bases since
the time they were formed, we are led to believe that many thousands
of them have been formed during the history of the volcano. The
history of these subsidiary cones appears to be connected with the
lava caves noted above. These caverns, owing to the irregularities of
their form, contain water. They are, in fact, natural cisterns, where
the abundant rainfall of the mountain finds here and there storage.
When, during the throes of an eruption, dikes such as we know often to
penetrate the mountain, are riven outward from the crater through the
mass of the cone, and filled with lava, the heated rock must often
come in contact with these masses of buried water. The result of this
would inevitably be the local generation of steam at a high
temperature, which would force its way out in a brief but vigorous
eruption, such as has been observed to take place when these
peripheral volcanoes are formed. Sometimes it has happened that after
the explosion the lava has found its way in a stream from the fissure
thus opened. That this explanation is sufficient is in a measure shown