The volcanic tufa used by the earliest Roman builders was discarded gradually in favour of better materials. _Peperino_, a grey-green volcanic stone from the Alban hills, began to take its place, and was used for the construction of the Tabularium in 78 B.C. and for Hadrian's mausoleum. It was cut in the same way in large rectangular blocks, clamped together during the Republican and early Imperial periods with iron. Mortar was not used till later, and at first served only to level the surfaces of the stones; it came into use for binding bricks together only at a later and degenerate period of architecture. Travertine was adopted towards the first century B.C. It is a cream-coloured stone hard and durable though easily calcined by fire, formed by deposit in running water. It was quarried at Tivoli and on the banks of the river Anio, where it is still plentiful. To the present day the quarries are worked at Tivoli, and the stone is brought to Rome on waggons drawn by immense white oxen which pace majestically along the dusty roads beneath the goad of their wild-looking drivers. The chocolate-brown earth imported from Pozzuoli or dug from beds in the campagna, is known as _pozzolana_, and early in the history of Rome her builders discovered that when mixed with lime it made a remarkably strong cement. As such they used it for foundations, for the lining of walls and ceilings. With pieces of brick and stone a concrete was formed which was poured in a liquid state between wooden casings, and when set proved to be one of the hardest and most durable of the materials used. It was the strength of this concrete which enabled the Roman builders to give the vaults of their baths and basilicas such an enormous span; and it could be used for the flooring of upper stories without beams or supports. When especial lightness was required, the concrete was made with broken pumice stone. [Illustration: TEMPLE OF MARS ULTOR The temple erected by Augustus in his Forum to the God of War under the title of Mars the Avenger. Only the upper part of the ancient arch of the Forum, now known as _Arco de' Pantani_, is visible. This represents the first imperial building in Rome. See pages 3, 30.]