Stone masonry is masonry is which the units consist of natural stone. In RUBBLE stone masonry, the stones are left in their natural state, without any kind of shaping. In ASHLAR masonry, the faces of stones that are to be placed in surface positions are squared so that the surfaces of the finished structure will be more or less continuous plane surfaces. Both rubble and ashlar work may be either RANDOM or COURSED.

Random rubble is the crudest of all types of stonework. Little attention is paid to laying the

Figure 7-48.-Layers of bond stones in random stone masonry.

stones in courses, as shown in figure 7-47. Each layer must contain bonding stones that extend through the wall, as shown in figure 7-48. This produces a wall that is well tied together. The bed joints should be horizontal for stability, but the "builds" or head joints may run in any direction. Coursed rubble consists of roughly squared stones assembled in such a manner as to produce approximately continuous horizontal bed joints, as shown in figure 7-49.

The stone for use in stone masonry should be strong, durable, and cheap. Durability and strength depend upon the chemical composition and physical structure of the stone. Some of the

more commonly found stones that are suitable are limestone, sandstone, granite, and slate. Un-squared stones obtained from nearby ledges or quarries or even fieldstone maybe used. The size of the stone should be such that two people can easily handle it. A variety of sizes is necessary to avoid using large quantities of mortar.

The mortar for use in stone masonry may be composed of portland cement and sand in the proportions of one part cement to three parts sand by volume. Such mortar shrinks excessively and does not work well with the trowel. A better mortar to use is portland cement-lime mortar. Mortar made with ordinary portland cement will stain most types of stone. If staining must be prevented, nonstaining white portland cement should be used in making the mortar. Lime does not usually stain the stone.