Travertine comes from Tivoli, and is a petrifaction formed by the action of lime and sulphur on vegetable decay. This was not used as a building material to any great extent before the time of Cæsar. It is white, and becomes yellow on exposure. Silex is another volcanic stone very little used for building, but entirely for paving the roads both ancient and modern. This came out of the volcano as a red-hot stream of lava, and on cooling down became a capital paving material. The bed of the road was first properly prepared, and then it was paved with polygonal blocks of blue basalt called silex. The stones fitted close to one another. Many of the roads are in a good condition to this day; the best specimen is opposite the Temple of Saturn in the Forum, B.C. 175. This stone is used for _opus reticulatum_ in some of the tombs on the Appian Way and at the Temple of Hercules; also for concrete. TABLE OF CONSTRUCTION. _TUFA OF THE KINGS._ STYLE. SPECIMEN. DATE. Polygonal Tusculum ---- { Veii ---- Opus quadratum. First period, { Gabii ---- squared edges { Palatine Hill 753 B.C. { Second Wall of Rome 746 B.C. Second period, bevelled edges { Aventine Hill 600 B.C. { Ostia 600 B.C. _PEPERINO OF THE REPUBLIC._ { Tomb of Scipio 298 B.C. Opus quadratum { Temple of Hope 240 B.C. Opus incertum Temple of Cybele 191 B.C. Opus incertum, polygonal Emporium 190 B.C. Opus quadratum Tabularium 78 B.C. _TRAVERTINE AND BRICK OF THE EMPIRE._ { Tomb of Cecilia Metella 78 B.C. { Theatre of Marcellus 13 B.C. Opus quadratum { Arch of Dolabella 10 A.D. { Colosseum 80 A.D. { Muro Morto 80 B.C. { Tomb of Augustus 10 B.C. Opus reticulatum { Palatine Tiberius' House ---- { Palatine Germanicus' House ---- { Hadrian's Villa ---- { Hadrian's Ostia ----