In this system, Fig. 1286, unsquared and undressed pieces of stone of all sizes are used indiscriminately, fitted into each other's broken surfaces as closely as possible, with large stones at intervals the full width of the wall, and all held firmly together by a plentiful use of first-class mortar, so as to make a compact mass when set. Very much stronger work can be done by substituting Portland cement for the mortar, thus forming a kind of coarse concrete.

Coursed Rubble

The same class of stone is used, but instead of mixing up the various sizes indiscriminately, pieces of like size are confined to one course, and those of a smaller size to the next above, and so on, commencing with the largest and finishing with the smallest, but adding a final course of larger size on the top. Each course is laid regularly and uniformly in good mortar, and solidity is given by occasionally laying a large stone crosswise so as to form a "binder" or "through."

Combined Rubbles

When the wall is sufficiently thick to admit of it, an economic yet substantial plan is to combine a facing of large coursed rubble with a backing of rough rubble, as in Fig. 1287: a is the rough rubble; b, "stretchers," or stones laid parallel with the wall; c, "headers," or stones laid at right angles to the line of the wall, and contributing to the solidity of the structure.