The stone used during the Empire was travertine, _lapis Tiburtinus_, but brick was the material generally used then. They are of two colours, red and yellow, according to the clay from which they were made. The walls were not of solid brick all through; but the interior was made of pieces--rubble-work--the outside course being entire brick, whilst at every four or five feet all through the construction were laid the great tie-bricks to keep the rubble-work from shifting. The brickwork was called _opus lateritium_. The great tie-bricks are usually stamped with the names of the consul or emperor and the maker, and these date the walls by measuring the number of bricks there are in a foot. In the fourth century another system--_opera decadence_--came into vogue, and walls were built with layers of brick and pieces of tufa-stone a little larger than our English bricks. This work continued down to the thirteenth century, when _opera Saracenesca_--tufa-stones without the bricks between--came into use. In the stone walls no cement was used; one stone was simply placed upon another, its weight keeping it in its place, and clamps were inserted to keep it from shifting. In the walls of Roma Quadrata we know of no clamps having been found; but in the wall of the two kings wooden clamps were found. In the walls of Servius Tullius iron clamps were found; and in the Colosseum clamps can still be seen in several places where pieces of the facing of the stone have been split off. Tufa is found all over the Campagna, and is of volcanic origin. When the Alban Hills were active volcanoes, the ashes and scoriæ thrown up fell into the sea, now the Campagna. The pressure of water on it formed it into stone: where there has been a great pressure, it is very hard; where little pressure, it is softer; and where there was no pressure, it still remains a sort of sand--this mixed with live lime is the celebrated Roman cement. The softer tufa was used by the Greek colonists, and the hard stone by the kings of Rome. Some tufa from the neighbourhood of Gabii is dark gray, the other is brown and reddish. Peperino is also volcanic. It was ejected in the shape of hot mud from the volcano, and on cooling formed a good stone: this comes from the Alban hills, and was used in the time of the Republic.