For a foundation of a lighter character, rubble stone may be used. This consists of stone split from a ledge with no preparation other than breaking up with a stone hammer, and squaring one edge and face. As these stones are irregular in shape and thickness, considerable mortar will have to be used and the character of the mortar and the filling of voids will need especial attention. An important element in the strength of a rubble wall, is the bonding or the lapping of stones over each other. The stones should be laid in irregular courses leveled off to a horizontal joint about every two feet, and a bond stone should be put in at intervals of four or five feet. Most city laws require a certain proportion of bond stones, and, if nothing is said, a through bond stone should be inserted into every five or six square feet. Care must be taken that the stones are bonded also in the direction of the length of the wall, so that no vertical joints may appear running through three or four courses. Outside of the bonding, the manner of laying the stones must have our attention. All the stones must be laid as nearly as possible on their natural bed, that is, with their splitting surface horizontal. Angles and corners must be carefully laid with large stones bonding across each way, as in Fig. 102.

Fig. 100. Brick Footing.

Fig. 100. Brick Footing.

Fig. 101. Block Stone Wall.

Fig. 101. Block Stone Wall.

With a heavy foundation wall of block stones, where each stone is of the full thickness of the wall, the main care will he to obtain a good bond, with level and plumb joints. Openings for pipes should be left when the wall is being built, as it may he difficult to break an opening after the walls are carried up.

Fig. 102. Bonding of Corner.

Fig. 102. Bonding of Corner.