interior wall of Monte di Somma, the original
crater of Vesuvius, presents a good illustration of such fragmental
beds. I shall have occasion further on to describe more fully the
structure of this remarkable mountain; so that it will suffice to say
here that this old prehistoric crater, the walls of which enclose the
modern cone of Vesuvius, is seen to be formed of irregular beds of ash,
scoriae, and fragmental masses, traversed by numerous dykes of lava, and
sloping away outwards towards the surrounding plains.

Of similar materials are the flanks of Etna composed, even at great
distances from the central crater; the beds of ash and agglomerate
sometimes alternating with sheets of solidified lava and traversed by
dykes of similar material of later date, injected from below through
fissures formed during periods of eruptive energy. Numerous similar
examples are to be observed in the Auvergne region of Central France and
the Eifel. And here we find remarkable cases of "breached cones," or
craters, which will require some special description. Standing on the
summit of the Puy de Dome, and looking northwards or southwards, the eye
wanders over a tract formed of dome-shaped hills and of extinct
crater-cones rising from a granitic platform. But what is most peculiar
in the scene is the ruptured condition of a large number of the cones
with craters. In such cases the wall of the crater has been broken down
on one side, and we observe that a stream of lava has been poured out
through the breach and overflowed the plain below. The cause of this
breached form is sufficiently obvious. In such cases there has been an
explosion of ashes, stones, and scoriae from the volcanic throat, by
which a cone-shaped hill with a crater has been built up. This has been
followed by molten lava welling up through the throat, and gradually
filling the crater. But, as the lava is much more dense than the
material of which the crater wall is composed, the pressure of the lava
outwards has become too great for the resistance of the wall, which
consequently has given way at its weakest part and, a breach being
formed, the molten matter has flowed out in a stream which has inundated
the country lying at the base of the cone. In one instance mentioned by
Scrope, the original upper limit of the lake of molten lava has left its
mark in the form of a ring of slag on the inside of the breached
crater.[4]

_Craterless Domes._--These differ essentially both