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. The only places within the actual walls of Rome where these tertiary marine strata are to be found, are the Vatican and the Janiculum. At the base of the Capitoline, in the subterranean vaults of the Ospitale della Consolazione, under the volcanic rock which forms the upper part of the hill, Brocchi found a stratum of calcareous rock and clay, which he affirms to be of marine origin, and to resemble the limestone of the Apennines. [Sidenote: Volcanic formation.]