rocks about it there comes forth a continued efflux of carbonic-acid
gas. This substance probably arises from the effect of heat contained
in old lavas which are in contact with limestone in the deep
under-earth. We know such limestones 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
Phlægræn 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.