Fresh-water formations cover the bottoms of all the valleys in the district of Rome and in the whole of the Campus Martius, and ascend to a considerable height on the flanks of the hills and into the Campagna. They consist chiefly of sand, clay, gravel, and the stone called travertine, and of tufa beds which have been disturbed and then re-deposited. This re-deposited tufa has been the subject of some controversy. It was at one time thought to indicate that the lower tufa was also a fresh-water deposit, since it is sometimes found overlying the fresh-water formations. But no doubt now remains that it must have been formed by a re-arrangement in fresh water of previously deposited marine tufa beds. The water of the Tiber, at the time when these fluviatile formations took place, stood at such a height as to leave deposits upon the intermontium of the Capitol, and as high as the Church of S. Isidoro on the Pincian, and it must have partially removed and shifted the previously existing light and porous volcanic soil of the sea-bottom. Even the top of the Pincian was covered by this fresh water; for modules of calcareous matter, such as are deposited in fresh water alone, were found in digging the excavations for the fountain on the public promenade. The surface of the broad river which then existed, seems, in fact, to have been at from 130 to 140 feet above the present surface level of the Tiber, and its water must have been more surcharged with alluvium, derived from sources with which the present river is no longer connected. Among the fluviatile deposits, argillaceous marl beds now play an important part. They intercept the water as it descends from the hills, and impede its descent to the river, thus furnishing supplies to the wells in Rome, but rendering the soil less dry and healthy. The greater portion of these strata consist of a mixture of sand and clay. The ridge between the Campo Vaccino and the Coliseum, on which the Arch of Titus stands, is formed almost entirely of these mixed strata of clay and sand. To prove the fresh-water origin of these deposits, we need only refer to the modules of travertine and the shells of lacustrine animals which they contain. Such species of fresh-water shell-fish could not live in turbid and rapid water like that of the Tiber as it now is, and we must therefore conclude from their presence that the waters of the Tiber valley where such fossils are found were once in a semi-stagnant state. That there was also a period of violent movement during the prevalence of this lacustrine era is testified by the quantities of matter brought from a distance and accumulated at considerable altitudes, and by the size of the pebbles and boulders which have been rolled along by the stream. But before a more accurate investigation of facts shall have been made, it will be impossible to distinguish these two periods of stagnation and rapid movement from each other. [Sidenote: Tiber water.]