utburst of
volcanic forces.[3]

(_b._) _Central Italy and the Roman States._--The tract bordering the
western slopes of the Apennines northward from Naples into Tuscany, and
including the Roman States, is characterised by volcanic rocks and
physical features of remarkable interest and variety. These occur in the
form of extinct craters, sometimes filled with water, and thus converted
into circular lakes; or of extensive sheets and conical hills of tuff;
or, finally, of old necks and masses of trachyte and basalt, sometimes
exhibiting the columnar structure. The Eternal City itself is built on
hills of volcanic material which some observers have supposed to be the
crater of a great volcano; but Ponzi, Brocchi, and Daubeny all concur in
the opinion that this is not the case, as will clearly appear from the
following account.

The geological structure of the valley of the Tiber at Rome is very
clearly described by Professor Ponzi in a memoir published in 1850, from
which the accompanying section is taken.[4] (Fig. 16.) From this it will
be seen that "the Seven-hilled City" is built upon promontories of
stratified volcanic tuff, of which the Campagna is formed, breaking off
along the banks of the Tiber, the hills being the result of the erosion,
or denudation, of the strata along the side of the river valley. As the
strata dip from west to east across the course of the river, it follows
that those on the western banks are below those on the opposite side;
and thus the marine sands and marls which underlie the volcanic tuff,
and are concealed by it along the eastern side of the valley, emerge on
the west, and form the range of hills on that side. Such being the
structure of the formations under Rome, it is evident that it is not
"built on a volcano."

[Illustration: Fig. 16.--Geological Section across the Valley of the
Tiber at Rome. 1. Alluvium of the Tiber; 2. Diluvium; 3. Volcanic tuff
(recent deposits); 4. Sands, etc.; 5. Blue marl (sub-Apennine
deposits).]

The tuff contains fragments of lava and pebbles of Apennine limestone,
and was deposited under the waters of an extensive lake at a time when
volcanic action was rife amidst the Alban Hills. This lacustrine
formation rests in turn on deposits of marine origin, containing
oysters, patellae, and other sea-shells, of which the chain of hills on
the right bank of the Tiber is chiefly formed.

The district around Albano lying to the south of Rome is of peculiar
int