and increase at the surface of the
earth, sink down and penetrate into subterranean cavities, so as to be
ejected from the volcanic orifices? We have of late years become
familiar with the fact, in the process of boring Artesian wells, that
the seeds of plants, the remains of insects, and even small fish, with
other organic bodies, are carried in an uninjured state by the
underground circulation of waters, to the depth of many hundred feet.
With still greater facility in a volcanic region we may conjecture, that
water and mud full of invisible infusoria may be sucked down, from time
to time, into subterranean rents and hollows in cavernous lava which has
been permeated by gases, or in rocks dislocated by earthquakes. It often
happens that a lake which has endured for centuries in a volcanic
crater, disappears suddenly on the approach of a new eruption. Violent
shocks agitate the surrounding region, and ponds, rivers, and wells are
dried up. Large cavities far below may thus become filled with fen-mud
chiefly composed of the more indestructible and siliceous portions of
infusoria, destined perhaps to be one day ejected in a fragmentary or
half-fused state, yet without the obliteration of all traces of organic
structure.[553]

_Herculaneum._--It was remarked that no lava has flowed over the site of
Pompeii, since that city was built, but with Herculaneum the case is
different. Although the substance which fills the interior of the houses
and the vaults must have been introduced in a state of mud, like that
found in similar situations in Pompeii; yet the superincumbent mass
differs wholly in composition and thickness. Herculaneum was situated
several miles nearer to the volcano, and has, therefore, been always
more exposed to be covered, not only by showers of ashes, but by
alluviums and streams of lava. Accordingly, masses of both have
accumulated on each other above the city, to a depth of nowhere less
than 70, and in many places of 112 feet.[554]

The tuff which envelops the buildings consists of comminuted volcanic
ashes, mixed with pumice. A mask imbedded in this matrix has left a
cast, the sharpness of which was compared by Hamilton to those in
plaster of Paris; nor was the mask in the least degree scorched, as if
it had been imbedded in heated matter. This tuff is porous; and, when
first excavated, is soft and easily worked, but acquires a considerable
degree of induration on exposure to the air. Above this lowest