to to Velletri and Segni.

This tract was separated from that of the Neapolitan volcanic region by
a range of limestone hills of Jurassic age between Segni and Gaeta, a
protrusion of the Alban Hills westward; but the general structure and
physical history of both regions are probably very similar, with the
exception that the igneous forces still retain their vitality in the
more southerly region. In the case of the Roman volcanic district, a bay
seems to have been formed about the close of the Miocene period, bounded
on all sides but the west by hills of limestone, over whose bed strata
of marl, sandstone, and conglomerate were deposited. This tract was
converted by subsequent movements into a fresh-water lake, and
contemporaneously volcanic operations commenced over the whole region,
and beds of tuff, often containing blocks of rock ejected from
neighbouring craters, were deposited over those of marine origin.
Meanwhile numerous crater-cones were thrown up; and, as the land
gradually rose, the waters of the lake were drained off, leaving dry
the Campagna and plain of the Tiber. Ultimately the volcanic fires
smouldered down and died out, whether within the historic epoch or not
is uncertain; lakes were formed within the now dormant craters, and the
face of nature gradually assumed a more placid and less forbidding
aspect over this memorable region, destined to be the site of Rome, the
Mistress of the World.

[1] As determined by Daubeny in 1825.

[2] Including the ruins of the Temple of Serapis, whose pillars are
perforated by marine boring shells up to a height of about 16 feet from
their base; indicating that the land had sunk down beneath the sea, and
afterwards been elevated to its present level.

[3] The account of Falconi, and another by Pietro Giacomo di Toledo, are
given by Sir W. Hamilton, _op. cit._, p. 198, and also reproduced by Sir
C. Lyell, _Principles_, vol. i. p. 608.

[4] Guiseppe Ponzi, "Sulla storia fisica del Bacino di Roma," _Annali di
Scienze Fisiche_ (Roma, 1850).

[5] Daubeny, _Volcanoes_, p. 171.

CHAPTER VI.

EXTINCT VOLCANOES OF CENTRAL FRANCE.

(_a._) _General Structure of the Auvergne District._--From a granitic
and gneissose platform situated near the centre of France, and separated
from the western spurs of the Alps by the wide valley of the Rhone,
there rises a group of volcanic mountains surpassing in variety of form
and structure any similar mountain group in Europe,