erest from the assemblage of old crater-lakes which it contains; as,
for instance, those of Albano, Vallariccia, Nemi, Juturna, and the lake
of Gabii. The lake of Albano, one of the most beautiful sheets of water
in the world, is about six miles in circumference, and surrounded by
beds of peperino, a variety of tuff presenting a bright, undecomposed
aspect when newly broken. The level of this lake was lowered by the
Romans during the siege of Veii by means of a tunnel, so that the waters
are 200 feet lower than the level at which they originally stood. In the
same district is the lake of Nemi, very regular in its circular outline;
that of Juturna lying near the foot of the Alban Hills, and that of
Ariccia lying in a deep hollow eight miles in circumference;--all may be
supposed to have been the craters of extinct volcanoes, both by reason
of their shape and of the materials of which they are formed. All these
old craters are, however, according to Daubeny, "only the dependencies
and offshoots, as it were, of the great extinct volcano, the traces of
which still remain upon the summit of the Alban Hills, and which is
comparable in its form to that of Vesuvius, as it is surrounded by an
outer circle of volcanic rock comparable to that of Somma."[5]

To the north of the city of Rome are several crateriform lakes, some of
which are of great size, such as that of Bolsena, over twenty miles in
circumference, and the Lago di Bracciano, almost as large, and lying
about twelve miles from the city. These extensive sheets of water are
surrounded by banks of tuff and volcanic sand, in which fragments of
augite, leucite, and crystals of titanite are distributed. The town of
Viterbo is built up at the foot of a steep hill called Monte Cimini, the
lower part of which is composed of trachyte; this is surmounted by
tuff, which appears to have been ejected from an extinct crater
occupying the summit of the mountain, and now converted into a lake
called the Lake of Vico. This crater is perfectly circular, and from its
centre rises a little conical hill covered by trees.

(_c._) _Physical History._--Space does not permit of a fuller
description of the remarkable volcanic features of the tract lying along
the western slope of the Apennines; but from what has been stated it
will be clear that volcanic forces have been in operation at one time on
a grand scale in the Roman States and the South of Tuscany, over a tract
extending from Mount Anna