One of the most remarkable instances of Roman brick construction is the Pile Cinq-Mars, as it is called, a tower still standing on the right bank of the Loire, near Tours. It is about 95 feet high and 13 feet square, expanding at the base, being built of tiles to a depth of 3 feet each side, with a body of concrete; the tiles are set in mortar composed of chalk, sand, and pounded tiles. On one side there are eleven rectangular panels with tile-work of various patterns, like those on the flue-tiles (see p. 348), and as also seen on the Roman wall at Cologne; the patterns include squares, triangles, and rosettes. The history and purpose of this building are quite unknown.[2429] At Pompeii bricks are used only for corners of buildings or doorposts, and sometimes for columns, as in the Basilica and the house of the Labyrinth.[2430] There are also late examples of brick columns with capitals in tiers of bricks as in the tomb mentioned above. Brick walls are not found, but bricks occur as facing for rubble-work. These are less than an inch thick, triangular in form, with the hypotenuse (about 6 inches long) showing in the face of the wall. Sometimes fragments of roof-tiles are used (cf. p. 334). The earlier bricks contain sea-sand, and have a granular surface; the later are smooth and even in appearance. Later, what is known as _opus mixtum_ (see below) is used, as in the entrance of the Herculaneum gate; this implies courses of stone and brick alternating,[2431] which, as we have seen, was common in military works, as in the Roman walls in Britain. In this country, owing to the absence of good material for concrete, the use of stones or brick throughout for building was general from the first; hence, too, the bricks are always flat and rectangular in form (_bipedales_).[2432] The arrangement of triangular bricks (made by dividing a medium-sized brick into four before baking), laid flat in regular horizontal courses, is characteristic of the earliest examples of Roman methods. It is found in the Rostra (44 B.C.) and in the Regia (35 B.C.), the earliest existing examples.[2433] The back wall of the Rostra is of concrete faced with triangular bricks 1½ inch thick, the sides 10 inches long. The same arrangement may be seen in the Pantheon, in the Thermae of Diocletian, and in some of the aqueducts (see below). The brickwork in the Pantheon was formerly thought to belong to the building of Agrippa in 27 B.C., but has been now shown to belong to the second century.[2434] At Ostia, in the temple of Honos and Virtus, the walls are built of triangular bricks or with red and yellow bricks with moulded cornices. [Illustration: Section of Angle. FIG. 187. CONCRETE WALL, Faced with (A) _opus incertum_, (B) _opus reticulatum_. C shows the horizontal section, similar in both. ] [Illustration: FIG. 188. CONCRETE WALL (VERTICAL SECTION), FACED WITH BRICK.]