[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 inner walls of the crater. Their path is
probably formed by riftings in the compacted ashes, such as we trace
on the steep sides of the Atria del Cavallo, as before noted. For the
further history of these fissures, we shall have to refer to facts
which are better exhibited in the cone of Ætna.

The amount of rock matter which has been thrown forth from the
volcanoes about the Bay of Naples is very great. Only a portion of it
remains in the region around these cones; by far the greater part has
been washed or blown away. After each considerable eruption a wide
field is coated with ashes, so that the tilled grounds appear as if
entirely sterilized; but in a short time the matter in good part
disappears, a portion of it decays and is leached away, and the most
of the remainder washes into the sea. Only the showers, which