§211. Paries Caementicius.--The materials varied with the place. At Rome lime and volcanic ashes (_lapis Puteolānus_) were used with pieces of stone as large or larger than the fist. Brickbats sometimes took the place of stone, and sand (§146) that of the volcanic ashes; potsherds crushed fine were better than the sand. The harder the stones the better the concrete; the very best was made with pieces of lava, the material with which the roads were generally paved. The method of forming the concrete walls was the same as that of modern times, familiar to us all in the construction of sidewalks. It will be easily understood from the illustration (Fig. 58). Upright posts, about 5 by 6 inches thick, and from 10 to 15 feet in height, were fixed about 3 feet apart along the line of both faces of the intended wall. On the outside of these were nailed horizontally boards, 10 or 12 inches wide, overlapping each other. Into the intermediate space the semi-fluid concrete was poured, receiving the imprint of posts and boards. When the concrete had hardened, the frame-work was removed and placed on top of it and the work continued until the wall had reached the required height. Walls made in this way varied in thickness from a seven-inch partition wall in an ordinary house to the eighteen-foot walls of the Pantheon of Agrippa. They were far more durable than stone walls, which might be removed stone by stone with little more labor than was required to put them together; the concrete wall was a single slab of stone throughout its whole extent, and large parts of it might be cut away without diminishing the strength of the rest in the slightest degree. [Illustration: FIGURE 59. WALL FACINGS] [Illustration: FIGURE 60. BRICK FOR FACING WALL]