ones 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
by observations on certain effects of lava flows from Vesuvius. The
writer was informed by a very judicious observer, a resident of
Naples, who had interested himself in the phenomena of that volcano,
that the lava streams when they penetrated a cistern, such as they
often encounter in passing over villages or farmsteads, vaporized the
water, and gave rise, through the action of the steam, to small
temporary cones, which, though generally washed away by the further
flow of the liquid rock, are essentially like those which we find on
AEtna. Such subsidiary, or, as they are sometimes called, parasitic
cones, are known about other volcanoes, but nowhere are they so
characteristic as on the flanks of that wonderful volcano.

A very conspicuous feature in the AEtnean cone consists of a great
valley known as the Val del Bove, or Bull Hollow, which extends from
the base of the modern and ever-changeable cinder cone down the flanks
of the older structure to near its base. This valley has steep sides,
in places a thousand or more feet high, and has evidently been formed
by the down-settling of portions of the cone which were left without
support by the withdrawal from beneath them of materials cast forth in
a time of explosion. In