8. Many good specimens of columns, ancient ornamental sculpture, and other monuments may be seen in the following places. The Churches of S. Lorenzo and S. Agnese fuori le Mura; S. Maria in Cosmedin, di Aracœli, degli Angeli, and in Trastevere. The Villas Ludovisi, Borghese, Albani, and Spada.

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INTRODUCTION TO CHAPTER IX.

The geological strata found on the site of Rome and in its immediate neighbourhood divide themselves into three principal groups. The oldest of these is a marine formation, and exhibits itself upon the Vatican, Janiculum, and Monte Mario. The second, of which all the hills on the eastern bank and the district of the Campagna are composed, is of volcanic origin, and consists chiefly of beds of tufaceous matter erupted from submarine volcanoes and more or less solidified. The third, which appears in the hollows of the Tiber valley, is a fresh-water formation, and is found on the slope of the hills on both banks of the river.

Marine formation.

The oldest of these three groups belongs to the division of the tertiary period, called by Lyell the older pleiocene, as having had a fauna and flora in which the greater number of species were identical with those now living on the earth. These strata are of marine formation, and are similar to those which extend over a great breadth of Italy on both flanks of the Apennine mountains, reaching as far south as the point of Reggio in Calabria. Their lower bed consists of a bluish-grey clayey marl, which will be found in the valley between the Janiculum and the Vatican. Its marine origin is sufficiently proved by the fossils found in it, and the remains of sea-weed. This bed of clay is of a plastic nature, and is still used for making pottery, as it was in the time of Juvenal. Above it lies a stratum of yellow calcareous sand, which sometimes takes the form of loose sand with boulders, sometimes of a stratified arenaceous rock, and sometimes of a rough conglomerate. This may be seen outside the Porta Angelica, on the left, under the walls of the city, and in the Belvedere Gardens on the Vatican Hill. The Church of S. Pietro in Montorio is said to derive its name Montorio, monte aureo, from the yellow colour of this sand.

On Monte Mario an abundance of fossil shells, of the Ostrea hippopus and other varieties of sea shells, may be seen, plainly indicating the marine origin of this formation.