hat when the occasion for such a discharge
comes, the vapours of the eruption will seek a vent through some other
of the many volcanic openings which lie to the westward of this great
cone. The history of these lesser volcanoes points to the conclusion
that when the path by way of Vesuvius is obstructed they may give
relief to the steam which is forcing its course to the surface. Two or
three times since the eruption of Pliny, during periods when Vesuvius
had long been quiet, outbreaks have taken place on Ischia or in the
Phlaegraen Fields, a region dotted with small craters which lies to the
west of Naples. The last of these occurred in 1552, and led to the
formation of the beautiful little cone known as Monte Nuovo. This
eruption took place near the town of Puzzuoli, a place which was then
the seat of a university, the people of which have left us records of
the accident.

[Illustration: Fig. 16.--Diagrammatic sections through Mount Vesuvius,
showing changes in the form of the cone. (From Phillips.)]

The outbreak which formed Monte Nuovo was slight but very
characteristic. It occurred in and beside a circular pool known as the
Lucrine Lake, itself an ancient crater. At the beginning of the
disturbance the ground opened in ragged cavities, from which mud and
ashes and great fragments of hard rock were hurled high in the air,
some of the stones ascending to a height of several thousand feet.
With slight intermissions this outbreak continued for some days,
resulting in the formation of a hill about five hundred feet high,
with a crater in its top, the bottom of which lay near the level of
the sea. Although this volcanic elevation, being made altogether of
loose fragments, is rapidly wearing down, while the crater is filling
up, it remains a beautiful object in the landscape, and is also
noteworthy for the fact that it is the only structure of this nature
which we know from its beginning. In the Phlaegraen Field there are a
number of other craters of small size, with very low cones about them.
These appear to have been the product of brief, slight eruptions. That
known as the Solfatara, though not in eruption during the historic
period, is interesting for the fact that from the crevices of the
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 lim