and often for
a long time subsequently to the discharge of scoriae and lava: these
vapors are condensed in the cold atmosphere surrounding the high
volcanic peak, and heavy rains are thus caused. The floods thus
occasioned, sweep along the impalpable dust and light scoriae, till a
current of mud is produced, which is called in Campania "lava d' acqua,"
and is often more dreaded than an igneous stream (lava di fuoco), from
the greater velocity with which it moves. So late as the 27th of
October, 1822, one of these alluviums descended the cone of Vesuvius,
and, after overspreading much cultivated soil, flowed suddenly into the
villages of St. Sebastian and Massa, where, filling the streets and
interior of some of the houses, it suffocated seven persons. It will,
therefore, happen very frequently that, towards the base of a volcanic
cone, alternations will be found of lava, alluvium, and showers of
ashes.

To which of these two latter divisions the mass enveloping Herculaneum
and Pompeii should be referred, has been a question of the keenest
controversy; but the discussion might have been shortened, if the
combatants had reflected that, whether volcanic sand and ashes were
conveyed to the towns by running water, or through the air, during an
eruption, the interior of buildings, so long as the roofs remain entire,
together with all underground vaults and cellars, could be filled only
by an alluvium. We learn from history, that a heavy shower of sand,
pumice, and lapilli, sufficiently great to render Pompeii and
Herculaneum uninhabitable, fell for eight successive days and nights in
the year 79, accompanied by violent rains.[548] We ought, therefore, to
find a very close resemblance between the strata covering these towns
and those composing the minor cones of the Phlegraean Fields, accumulated
rapidly, like Monte Nuovo, during a continued shower of ejected matter;
with this difference however, that the strata incumbent on the cities
would be horizontal, whereas those on the cones are highly inclined; and
that large angular fragments of rock, which are thrown out near the
vent, would be wanting at a distance where small lapilli only can be
found. Accordingly, with these exceptions, no identity can be more
perfect than the form, and distribution of the matter at the base of
Monte Nuovo, as laid open by the encroaching sea, and the appearance of
the beds superimposed on Pompeii. That city is covered with numerous
alternations