ad of the Valley of Calanna.
_a_, Lava of 1819 descending the precipice and flowing through
the valley.
_b_, Lavas of 1811 and 1819 flowing round the hill of Calanna.]

It happened in 1811 and 1819 that the flows of lava overtopped the ridge
intervening between the hills of Zocolaro and Calanna, so that they fell
in a cascade over a lofty precipice, and began to fill up the valley of
Calanna (_a_, fig. 53). Other portions of the same lava-current (_b_)
flowed round the promontory, and they exhibit one of the peculiar
characteristics of such streams, namely that of becoming solid
externally, even while yet in motion. Instead of thinning out gradually
at their edges, their sides may often be compared to two rocky walls
which are sometimes inclined at an angle of between thirty and forty
degrees. When such streams are turned from their course by a projecting
rock, they move right onwards in a new direction; and in the Valley of
Calanna a considerable space has thus been left between the steep sides
of the lavas _b b_, so deflected, and the precipitous escarpment of
Zocolaro, A, which bounds the plain C.

_Lavas and breccias._--In regard to the volcanic masses which are
intersected by dikes in the Val del Bove, they consist in great part of
graystone lavas, of an intermediate character between basalt and
trachyte, and partly of porphyritic lava resembling trachyte, but to
which that name cannot, according to Von Buch and G. Rose, be in
strictness applied, because the felspar belongs to the variety called
Labradorite. There is great similarity in the composition of the ancient
and modern lavas of Etna, both consisting of felspar, augite, olivine,
and titaniferous iron. The alternating breccias are made up of scoriae,
sand, and angular blocks of lava. Many of these fragments may have been
thrown out by volcanic explosions, which, falling on the hardened
surface of moving lava-currents, may have been carried to a considerable
distance. It may also happen that when lava advances very slowly, in the
manner of the flow of 1819, the angular masses resulting from the
frequent breaking of the mass as it rolls over upon itself, may produce
these breccias. It is at least certain that the upper portion of the
lava-currents of 1811 and 1819 now consist of angular masses to the
depth of many yards. D'Aubuisson has compared the surface of one of the
ancient lavas of Auvergne to that of a river suddenly frozen over by the
st