Masonry may be classed either as "ashlar" or "rubble."

Ashlar is built from large blocks of stone, carefully worked, while rubble is composed of small stones, often irregular in shape, and in the roughest description, hardly worked at all.

Between these two there are many gradations.

Different kinds of masonry are sometimes combined. Thus, walls are built with ashlar facing and rubble backing, or even with stone facing and brick backing.

This leads to defective construction, the consideration of which forms part of the Advanced Course, and is entered upon in Part II.

General Remarks

Masonry recpiires more skill to build than brickwork. The bricks, being all of the same size, are laid according to regular rules, whereas with each stone judgment is required in order that it may be laid in the best way.

The more nearly the work approaches ashlar the more regular are the stones, and the more easily are they built.

Natural Bed

As a rule every stone in ordinary walls and arches should be laid upon its " natural bed" - that is to say, the bed upon which it rested when originally formed, should now be perpendicular to the principal pressure upon it.

When a stratified stone is placed vertically, and so that the layers of which it is composed are parallel to its face, they are apt to be split off in succession by the action of the weather. Moreover, a stone in this position has not so much strength to resist crushing as it has when placed on its natural bed.

In a cornice with overhanging or undercut mouldings the natural bed should be placed parallel to the side joints, for if placed horizontally layers of the overhanging portions will be liable to drop off. There are other exceptions to the general rule which occur in more elaborate work; also some dependent upon the nature of the stone, the quarry, etc. These will be noticed in Part III.