That the materials of the tuff were assorted under water is suggested by their stratified structure. This inference is strengthened by the intercalation among them of sheets of sand, gravel, clay and marl. The layers of gravel are especially important, for their component pebbles of limestone and other non-volcanic stones are unmistakably fragments of Mesozoic rocks, which have been rolled along by running water from their original resting places among the Apennines so as to acquire smoothed and rounded forms. But though the tuff was accumulated under water, it presents a strong contrast to the clays and sands below it by its generally unfossiliferous character. Leaves, branches and stems of ilex, oak and other land-vegetation have been obtained from it at various places, sometimes as mere hollow moulds or in carbonised forms, but occasionally with the internal structure still preserved. Less frequently it has yielded the bones, antlers or tusks of terrestrial quadrupeds. But both the plants and animals have obviously been drifted from the land, and did not live where their remains have been found. It is worthy of remark that though so many observers have been at work in successive generations among the rocks of the Campagna, no undoubted example of marine mollusk has been recorded from the tuff in the interior of the Campagna. The crowds of shells in the Pliocene strata underneath are there absent. That the conditions required for the existence of an abundant marine fauna continued over this site until the beginning of the volcanic period is manifest from the crowded pteropods, lamellibranchs and gasteropods of the clays and sands. But as the eruptions increased in area and in intensity these conditions were eventually destroyed. The descent of continued showers of hot dust, ashes and stones over the sea-bottom, the rise of mephitic gases from below, as well as of hot springs that deposited sheets of travertine, must have made that sea-floor no congenial home for either plant or animal.