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.