estones are covered by the lavas of
Vesuvius, for the reason that numerous blocks of the rock are thrown
out during eruptions, and are often found embedded in the lava
streams. It is an interesting fact that these craters of the
Phlaegraen Field, lying between the seats of vigorous eruption on
Ischia and at Vesuvius, have never been in vigorous eruption. Their
slight outbreaks seem to indicate that they have no permanent
connection with the sources whence those stronger vents obtain their
supply of heated steam.

The facts disclosed by the study of the Vesuvian system of volcanoes
afford the geologist a basis for many interesting conclusions.

In the first place, he notes that the greater part of the cones, all
those of small size, are made up of finely divided rock, which may
have been more or less cemented by the processes of change which
go on within it. It is thus clear that the lava flows are
unessential--indeed, we may say accidental--contributions to the mass.
In the case of Vesuvius they certainly do not amount to as much as one
tenth of the elevation due to the volcanic action. The share of the
lava in Vesuvius is probably greater than the average, for during the
last six centuries this vent has been remarkably lavigerous.[8]
Observation on the volcanoes of other districts show that the Vesuvian
group is in this regard not peculiar. Of nearly two hundred cones
which the writer has examined, not more than one tenth disclose
distinct lavas.

[Footnote 8: I venture to use this word in place of the phrase
"lava-yielding" for the reason that the term is needed in the
description of volcanoes.]

An inspection of the old inner wall of Monte Somma in that portion
where it is best preserved, on the north side of the Atria del
Cavallo, or Horse Gulch--so called for the reason that those who
ascended Vesuvius were accustomed to leave their saddle animals
there--we perceive that the body of the old cone is to a considerable
extent interlaced with dikes or fissures which have been filled with
molten lava that has cooled in its place. It is evident that during
the throes of an eruption, when the lava stands high in the crater,
these rents are frequently formed, to be filled by the fluid rock. In
fact, lava discharges, though they may afterward course for long
distances in the open air, generally break their way underground
through the cindery cone, and first are disclosed at the distance of a
mile or more from the inne