The rubble work, _opus incertum_, consists of fragments irregular in shape, of the size of the fist and larger, laid in mortar. The material used in the earlier times was ordinarily lava; later, Sarno limestone. Corners and doorposts at first were built of hewn blocks; afterwards bricks and blocks of stone cut in the form of bricks were used for this purpose, and in the latest period frequently brick and stone combined, _opus mixtum_ or _opus compositum_--a course of stone alternating with every two or three courses of brick. An example of the _opus mixtum_ is seen in the entrances of the Herculaneum Gate (Fig. 113). Rubble work is the prevailing masonry at Pompeii; in comparison the other kinds described may be considered exceptional. The reticulate work, _opus reticulatum_, formed the outer surface of a wall, the inner part of which was built up with rubble. It was composed of small four-sided pyramidal blocks, of which only the base, cut square and smooth, showed on the surface; the tapering part served as a key to bind the block into the wall. These blocks, which measured from three to four inches square at the base, were laid on their corners, so that the edges ran diagonally to the horizontal and vertical lines of the wall; the pattern thus formed had the appearance of a net, hence the name. The material was in most cases gray, occasionally yellow, tufa. The corners and doorposts were at first made of the same kind of stone cut in the shape of bricks; later of bricks. This style of masonry was in vogue at Rome, and apparently also at Pompeii, in the time of Augustus (Fig. 12; see also the pedestal in the foreground of Plate I). The quasi-reticulate work belongs to the early years of the Roman colony. In appearance it lies between rubble and reticulate work, differing from the latter in that the small blocks are less carefully finished and are laid with less regularity. The material is generally lava, but tufa and limestone are also found. The corners and doorposts are of brick, or of brick-shaped blocks of tufa or limestone (Fig. 11). Ashlar work, of carefully hewn oblong blocks laid in courses, is found in the older portions of the city wall (Fig. 109) and in the walls of the Greek temple in the Forum Triangulare; it was used otherwise only for the fronts of houses (Fig. 10). The material in the earliest times was Sarno limestone, later gray tufa. With the coming of the Roman colony ashlar work went out of use, even for the corners of houses and doorposts. In the construction of columns and many architraves large blocks were used. Previous to the time of the Roman colony these were of gray tufa, or, in rare instances, of limestone; a coating of white stucco was laid on the surface. From the advent of the colony to the time of the Early Empire, the whitish limestone was used; after that, Carrara marble.