rable
breadth when first they overflow, and since, in some eruptions, a
considerable part of the upper portion of the cone breaks down at once,
may form a sheet extending as far as the space which the eye usually
takes in, in a single section.

The high inclination of some of the beds, and the firm union of the
particles even where there is evidently no cement, is another striking
feature in the volcanic tuffs and breccias, which seems at first not
very easy of explanation. But the last great eruption afforded ample
illustration of the manner in which these strata are formed. Fragments
of lava, scoriae, pumice, and sand, when they fall at slight distances
from the summit, are only half cooled down from a state of fusion, and
are afterwards acted upon by the heat from within, and by fumeroles or
small crevices in the cone through which hot vapors are disengaged. Thus
heated, the ejected fragments cohere together strongly; and the whole
mass acquires such consistency in a few days, that fragments cannot be
detached without a smart blow of the hammer. At the same time sand and
scoriae, ejected to a greater distance, remain incoherent.[521]

Sir William Hamilton, in his description of the eruption of 1779, says
that jets of liquid lava, mixed with stones and scoriae, were thrown up
to the height of at least ten thousand feet, having the appearance of a
column of fire.[522] Some of these were directed by the winds towards
Ottajano, and some of them falling almost perpendicularly, still red-hot
and liquid, on Vesuvius, covered its whole cone, part of the mountain of
Somma, and the valley between them. The falling matter being nearly as
vividly inflamed as that which was continually issuing fresh from the
crater, formed with it one complete body of fire, which could not be
less than two miles and a half in breadth, and of the extraordinary
height above mentioned, casting a heat to the distance of at least six
miles round it. Dr. Clarke, also, in his account of the eruption of
1793, says that millions of red-hot stones were shot into the air full
half the height of the cone itself, and then bending, fell all round in
a fine arch. On another occasion he says that, as they fell, they
covered nearly half the cone with fire.

The same author has also described the different appearance of the lava
at its source, and at some distance from it, when it had descended into
the plains below. At the point where it issued, in 1793, from a