Building-stones, whether of sandstone, limestone, or granite, are named according to their shape and finish. The principal classes are three, namely, rubble, squared rubble, and ashlar, but each of these has several subdivisions.

Bubble may be uncoursed, or (for better work) uncoursed but with hammer-dressed joints, a variety of rubble generally known as rustic work; when Hints are used they may be laid entire, or "polled", and laid with the fractured surfaces outward, the latter making the neater and better work.

The strength and imperviousness of rubble walls depend very much on the quality of the mortar used. If this is not good, the wall is bound to be more or less a failure. Much rubble is an inferior kind of stone, coarse, porous, and friable; on the other hand, flints and the rubble obtained from igneous and some other rocks, are exceedingly dense, non-absorbent, and practically proof against atmospheric attacks. With mortar of the best quality, used in sufficient quantity, walls built of good rubble are undoubtedly satisfactory and durable. Frequently rubble walls are finished externally with stucco, Douglas being a well-known example of a rubble-and-stucco town.

The angles of rubble walls, when these are not finished with stucco, ought to be formed with squared rubble, or ashlar, or good bricks.

Squared rubble gives a better and neater kind of wall than random rubble. Frequently, as in the case of the well-known Yorkshire wall-stones, the rock splits naturally along the planes of bedding into courses from 2 to 6 or even 8 inches thick, and the slabs are "nicked" to the required width (usually 6 inches) and roughly squared at the ends. The Yorkshire wall-stones are finished on the face principally in two ways, known as "straight-face" and "pitched-face", the latter (shown in Fig. 35) having a rough projecting face formed with hammer and pitching-tool or chisel. The pitched-face wall-stones are usually more expensive than the others, but they catch dirt and rain, and are therefore more liable to be blackened by the soot and smoke of towns. Flat-bedded wall-stones of this kind, when laid in regular courses like bricks, are known as regular coursed rubble.

A variety of squared rubble is known as irregular coursed, or snecked, rubble; it consists in the use of stones of different depths, all laid with horizontal beds, but with large stones at irregular intervals, breaking the courses of the other wall-stones.

Ashlar wall-stones are of freestone requiring the use of hammer and chisel on beds, joints, and faces. It is unnecessary to describe minutely the different kinds of finish given to ashlar wall-stones; the beds and joints are usually boasted, while the finish of the faces may vary from the rough rock-face known as "pitched", to the finest "tooled" and the smoothest "rubbed".1 The rougher the surface, the more opportunity does it afford for the lodgment of dirt and water, and the more likely is it to lead to the decay of the stone.

Ashlar should (except in certain exceptional positions) be laid on its natural bed, especially if the planes of bedding are easily discernible. Otherwise, the face of the stone will be likely to crumble or flake off. The ordinary mason is very fond of using one large stone (false jointed if required) instead of several smaller ones, thus saving labour in beds and joints; but in ninety-nine cases out of a hundred the large stone must be laid on edge with the natural bed of the stone vertical; whence comes decay.

The vertical joints of hand-worked ashlar wall-stones and of squared rubble are seldom squared back far enough.

Frequently the appearance of two adjacent stones, seen from above, is as shown in Fig. 31; unless such joints are all thoroughly flushed with good mortar (and how often is this the case?), driving rain is sure to saturate the wall: the wall will be a damp one and much time and money will be spent in attempts (more or less vain) to mend it.

Certainly the joints should be squared back from the face not less than 3 inches.

The beds are often worked in the same way, a fault which may lead to the cracking of the stones (as shown in Fig. 32 at a) on account of the weight being concentrated on the edge, and also facilitate the ingress of water by forming an inclined plane a b, down which gravity conducts the water to the interior of the wall.

In the days before hollow walls were known, and railways and good roads had brought everything everywhere, our forefathers were sorely tried in their attempts to build dry walls of porous materials. Among the hills which form the boundary between Lancashire and the West Riding of Yorkshire, - hills whereon fierce wet west winds play till the trees take a permanent "set" to the cast, - the difficulty was great The millstone grit (chief building-stone of the locality), hard and durable though it is, is undoubtedly porous, so porous sometimes that rain is driven quite through the through-stones and bubbles out here and there and trickles down the plaster inside. Having learnt by bitter experience the folly of inward-sloping bed-joints like that in Fig. 32, the old builders went to the other extreme, and not content with level beds, often laid the stones as shown in Fig. 33. The projecting-joint was always well pointed with mortar. The stone marked A is a through-stone. Many of these old millstone-grit buildings are damp to-day, while some have had their westerly faces covered with oil or other so-called "waterproof" solution, or with stucco or slates.

Fig. 31. Wall stones with Scamped Joints.

Fig. 31.-Wall stones with Scamped Joints.

Fig. 32  Wall Stone with Scamped Beds.

Fig. 32 -Wall Stone with Scamped Beds.

1 Known also as "cleaned" or "polished".

Ashlar wall-stones are, of course, a superior kind of stone used only for the face of the wall, the remainder of the wall being built of rubble or brick.

Stone walls are usually built with one "through" in every square yard, but as moisture is apt to follow the through, a better plan (where the thickness of the wall will allow) is to have two bond-stones in place of the through, as shown in Fig. 34 at a and B.