To accomplish so prodigious an amount of building the Romans systematised the methods of construction in regard to both the labour and the material. The labour was mainly of an unskilled kind, including soldiers of the legions, slaves, and subjects liable through debt or other causes to statute labour. This employment of unskilled labour was made possible by the Roman habit of carrying the principle of repetition of motives to its utmost limit, and also by the methods of construction which they invented. This was the extended use of concrete. During the Republic the Romans had followed the Greek method of building with large blocks of stone, unconnected with mortar. Their practical spirit, however, urged them to make a more economical use of materials and instead of composing the walls entirely of blocks of stone or marble, they used these or bricks as a facing, filling in the thickness of the wall with small fragments of stone mixed with lime or mortar. They had been led to this practice by the existence of _pozzolana_, a volcanic product of clean, sandy earth, found in Rome and in greater quantities at Pozzuoli on the Bay of Naples, which, when mixed with lime, formed a concrete of exceptional hardness, strength, and durability. Material, approximating the properties of pozzolana and lime, was procurable in all parts of the Empire. Accordingly the use of this method of construction gave a similarity to Roman building everywhere. While the chief, and almost sole building material in Greece was marble, the geological formation of Italy supplied stone as well as marble and plentiful supplies of clay, which was converted into terra-cotta or bricks. The bricks were of two shapes: either square, from 1 to 2 feet in size and 2 inches thick or triangular in plan and of about 1½ inches in thickness. The latter were especially used for the facing of the walls, their pointed ends being driven into the concrete to form the smooth surfaces, while at the corners the points projected. In Rome itself the following materials were available: _travertine_, a hard limestone from Tivoli; _tufa_, a volcanic substance of which the hills of Rome are mainly composed; and _peperino_, a stone of volcanic origin from Mount Albano. * * * * * While Roman architecture was developed under the stimulus of Greek art and culture it probably owes its origin to the example of the Etruscans.