slate, with serpentine (_b_ _b_, fig. 21), belonging to the older
Apennine formation. The water is hot, has a strong taste, and, when not
in very small quantity, is of a bright green color. So rapid is the
deposition near the source, that in the bottom of a conduit-pipe for
carrying off the water to the baths, and which is inclined at an angle
of 30 degrees, half a foot of solid travertin is formed every year. A more
compact rock is produced where the water flows slowly; and the
precipitation in winter, when there is least evaporation, is said to be
more solid, but less in quantity by one-fourth, than in summer. The rock
is generally white; some parts of it are compact, and ring to the
hammer; others are cellular, and with such cavities as are seen in the
carious part of bone or the siliceous millstone of the Paris basin. A
portion of it also below the village of San Vignone consists of
incrustations of long vegetable tubes, and may be called tufa. Sometimes
the travertin assumes precisely the botryoidal and mammillary forms,
common to similar deposits in Auvergne, of a much older date; and, like
them, it often scales off in thin, slightly undulating layers.

A large mass of travertin (_c_, fig. 21) descends the hill from the
point where the spring issues, and reaches to the distance of about half
a mile east of San Vignone. The beds take the slope of the hill at about
an angle of 6 degrees, and the planes of stratification are perfectly
parallel. One stratum, composed of many layers, is of a compact nature,
and fifteen feet thick; it serves as an excellent building stone, and a
mass of fifteen feet in length was, in 1828, cut out for the new bridge
over the Orcia. Another branch of it (_a_, fig. 21) descends to the
west, for 250 feet in length, of varying thickness, but sometimes 200
feet deep; it is then cut off by the small river Orcia, as some glaciers
in Switzerland descend into a valley till their progress is suddenly
arrested by a transverse stream of water.

The abrupt termination of the mass of rock at the river, where its
thickness is undiminished, clearly shows that it would proceed much
farther if not arrested by the stream, over which it impends slightly.
But it cannot encroach upon the channel of the Orcia, being constantly
undermined, so that its solid fragments are seen strewed amongst the
alluvial gravel. However enormous, therefore, the mass of solid rock may
appear which has been given out by this sin