into the Arno increased enormously. Frisi,
alluding to such occurrences, observes, that as soon as the bushes and
plants were removed, the waters flowed off more rapidly, and, in the
manner of floods, swept away the vegetable soil.[996]

This effect of vegetation is of high interest to the geologist, when he
is considering the formation of those valleys which have been
principally due to the action of rivers. The spaces intervening between
valleys, whether they be flat or ridgy, when covered with vegetation,
may scarcely undergo the slightest waste, as the surface may be
protected by the green sward of grass; and this may be renewed, in the
manner before described, from elements derived from rain-water and the
atmosphere. Hence, while the river is continually bearing down matter in
the alluvial plain, and undermining the cliffs on each side of every
valley, the height of the intervening rising grounds may remain
stationary.

In this manner, a cone of loose scoriae, sand, and ashes, such as Monte
Nuovo, may, when it has once become densely clothed with herbage and
shrubs, suffer scarcely any further dilapidation; and the perfect state
of the cones of hundreds of extinct volcanoes in France, the Neapolitan
territory, Sicily, and elsewhere, may prove nothing whatever, either as
to their relative or absolute antiquity. We may be enabled to infer,
from the integrity of such conical hills of incoherent materials, that
no flood can have passed over the countries where they are situated,
since their formation; but the atmospheric action alone, in spots where
there happen to be no torrents, and where the surface was clothed with
vegetation, could scarcely in any lapse of ages have destroyed them.

During a tour in Spain, in 1830, I was surprised to see a district of
gently undulating ground in Catalonia, consisting of red and gray
sandstone, and in some parts of red marl, almost entirely denuded of
herbage; while the roots of the pines, holm oaks, and some other trees,
were half exposed, as if the soil had been washed away by a flood. Such
is the state of the forests, for example, between Oristo and Vich, and
near San Lorenzo. But, being overtaken by a violent thunder-storm, in
the month of August, I saw the whole surface, even the highest levels of
some flat-topped hills, streaming with mud, while on every declivity the
devastation of torrents was terrific. The peculiarities in the
physiognomy of the district were at once exp