The whitish limestone has a fine texture, without impressions of leaves, and is of an even color; it was to some extent employed as a substitute for marble. It was not quarried at Pompeii, and was not extensively used; the most important example of its use is in the later colonnade about the Forum. The white Carrara marble (_marmor lunense_) was preferred for columns, pilasters, and architraves; but colored marbles of many varieties, cut into thin slabs and blocks, were used as a veneering for walls and in the mosaic floors. Bricks were used only for the corners of buildings, for doorposts, and in a few instances, as in the Basilica and the house of the Labyrinth, for columns; brick walls are not found in Pompeii. The bricks seen in corners and doorposts (Figs. 11, 95) are simply a facing for rubble work. They are ordinarily less than an inch thick; they have the shape of a right-angled triangle, and are so laid that the side representing the hypothenuse--about six inches long--appears in the surface of the wall. Sometimes fragments of roof tiles, more or less irregular in shape, were used instead. The bricks of the earlier time contain sea sand and have a granular surface, with a less uniform color; the later bricks are smooth and even in appearance. The flat oblong roof tiles (_tegulae_), measuring ordinarily 24 by 19 or 20 inches, had flanges at the sides; over the joints where the flanges came together, joint tiles in the form of a half-cylinder (_imbrices_) were laid, like those in use at the present day (Figs. 114, 117). The styles of masonry are characteristic and interesting. We may distinguish them as masonry with limestone framework, rubble work, reticulate work, quasi-reticulate work, ashlar work, and, in the case of columns and entablatures, massive construction. The masonry with limestone framework dates from the earliest period. The walls were built without mortar, clay being used instead. Since this served only as a filling, without strength as a binding material, it was necessary to arrange the stones themselves in such a way that the wall would stand firm. This result was accomplished by using large, oblong blocks, not only for corners and doorposts, but also for a framework in the body of the wall; as shown in our illustration, alternate vertical and horizontal blocks were built up into pillars which would hold in place the courses of smaller stones that filled the intervening spaces. The material of the larger, hewn blocks, as well as of the smaller fragments, was Sarno limestone, with occasional pieces of cruma or slag. [Illustration: Fig. 9.--Wall with limestone framework.]