roduced by
this spring, have an oolitic texture.

_Valley of the Elsa._--If we pass from the volcanic district of France
to that which skirts the Apennines in the Italian peninsula, we meet
with innumerable springs which have precipitated so much calcareous
matter, that the whole ground in some parts of Tuscany is coated over
with tufa and travertin, and sounds hollow beneath the foot.

In other places in the same country, compact rocks are seen descending
the slanting sides of hills, very much in the manner of lava currents,
except that they are of a white color and terminate abruptly when they
reach the course of a river. These consist of a calcareous precipitate
from springs, some of which are still flowing, while others have
disappeared or changed their position. Such masses are frequent on the
slope of the hills which bound the valley of the Elsa, one of the
tributaries of the Arno, which flows near Colle, through a valley
several hundred feet deep, shaped out of a lacustrine formation,
containing fossil shells of existing species. I observed here that the
travertin was unconformable to the lacustrine beds, its inclination
according with the slope of the sides of the valley. One of the finest
examples which I saw was at the Molino delle Caldane, near Colle. The
Sena, and several other small rivulets which feed the Elsa, have the
property of encrusting wood and herbs with calcareous stone. In the bed
of the Elsa itself, aquatic plants, such as Charae, which absorb large
quantities of carbonate of lime, are very abundant.

[Illustration: Fig. 21.

Section of travertin, San Vignone.]

_Baths of San Vignone._--Those persons who have merely seen the action
of petrifying waters in England, will not easily form an adequate
conception of the scale on which the same process is exhibited in those
regions which lie nearer to the active centres of volcanic disturbance.
One of the most striking examples of the rapid precipitation of
carbonate of lime from thermal waters, occurs in the hill of San Vignone
in Tuscany, at a short distance from Radicofani, and only a few hundred
yards from the high road between Sienna and Rome. The spring issues from
near the summit of a rocky hill, about 100 feet in height. The top of
the hill stretches in a gently inclined platform to the foot of Mount
Amiata, a lofty eminence, which consists in great part of volcanic
products. The fundamental rock, from which the spring issues, is a black