f the most splendid
edifices of ancient and modern Rome are built of travertin, derived from
the quarries of Ponte Lucano, where there has evidently been a lake at a
remote period, on the same plain as that already described.

[Illustration: Fig. 22.

Section of spheroidal concretionary Travertin under the Cascade of
Tivoli.]

_Travertin of Tivoli._--In the same neighborhood the calcareous waters
of the Anio incrust the reeds which grow on its banks, and the foam of
the cataract of Tivoli forms beautiful pendant stalactites. On the sides
of the deep chasm into which the cascade throws itself, there is seen an
extraordinary accumulation of horizontal beds of tufa and travertin,
from four to five hundred feet in thickness. The section immediately
under the temples of Vesta and the Sibyl, displays, in a precipice about
four hundred feet high, some spheroids which are from _six to eight feet
in diameter_, each concentric layer being about the eighth of an inch in
thickness. The preceding diagram exhibits about fourteen feet of this
immense mass, as seen in the path cut out of the rock in descending from
the temple of Vesta to the Grotto di Nettuno. I have not attempted to
express in this drawing the innumerable thin layers of which these
magnificent spheroids are composed, but the lines given mark some of the
natural divisions into which they are separated by minute variations in
the size or color of the laminae. The undulations also are much smaller
in proportion to the whole circumference than in the drawing. The beds
(_a_ _a_) are of hard travertin and soft tufa; below them is a pisolite
(_b_), the globules being of different sizes: underneath this appears a
mass of concretionary travertin (_c_ _c_), some of the spheroids being
of the above-mentioned extraordinary size. In some places (as at _d_)
there is a mass of amorphous limestone, or tufa, surrounded by
concentric layers. At the bottom is another bed of pisolite (_b_), in
which the small nodules are about the size and shape of beans, and some
of them of filberts, intermixed with some smaller oolitic grains. In the
tufaceous strata, wood is seen converted into a light tufa.

There can be little doubt that the whole of this deposit was formed in
an extensive lake which existed when the external configuration of this
country varied greatly from that now observed. The Anio throws itself
into a ravine excavated in the ancient travertin, and its waters give
rise to ma