Natural Beds

All stones in walls, but especially those that are of a laminated structure, should be placed "on their natural bed," - that is, either in the same position in which they were originally deposited in the quarry, or turned upside down, so that the layers are parallel to their original position, but inverted. If they are placed with the layers parallel to the face of the wall, the effect of the wet and frost will be to scale off the face layer by layer, and the stone will bo rapidly destroyed. In arches, such stones should be placed with the natural bed as nearly as possible at right angles to the thrust upon the stone, - that is, with the "grain" or lamina) parallel to the centre lines of the arch stones, and perpendicular to the face of the arch. In cornices with undercut mouldings the natural bed is placed vertically and at right angles to the face, for if placed horizontally, layers of the overhanging portion would be liable to drop off. There are, in elaborate work, other exceptions to the general rule. It must be remembered that the beds are sometimes tilted by upheaval subsequent to their deposition, and that it is the original position in which the stone was deposited that must be ascertained.

The natural bed is easily seen in some descriptions of stone by the position of imbedded shells, which were of course originally deposited horizontally. In others it can only be traced by thin streaks of vegetable matter, or by traces of laminae, which generally show out more distinctly if the stone is wetted. In other cases, again, the stone shows no signs of stratification, and the natural bed cannot be detected by the eye. A good mason can, however, generally tell the natural bed of the stone by the "feel" of the grain in working the surface. A stone placed upon its proper natural bed is able to bear a much greater compression than if the laminae are at right angles to the bed joints. Fairbairn found by experiment that stones placed with their strata vertical bore only 6/7 the crushing stress which was undergone by similar stones on their natural bed.

Destructive Agents

The 2 principal classes of agents which destroy stone have already been described: they are - chemical agents, consisting of acids, etc., in the atmosphere; and mechanical agents, such as wind, dust, rain, frost, running water, force of the sea etc. There are other enemies to the durability of stones, which may be glanced at, viz. - lichens, and worms or molluscs.

Lichens

In the country, lichens and other vegetable substances collect and grow upon the faces of stones. These are in many cases a protection from the weather, and tend to increase the durability of the stone. In the case of limestones, however, the lichens sometimes do more harm than good, for they7 give out carbonic acid, which is dissolved in rain-water, and then attacks the lime carbonate in the stone.

Molluscs

The Pholas dactylus is a boring mollusc found in sea-water, which attacks limestone, hard and soft argillaceous shales, clay, and sandstones; but granite has been found to resist it successfully. The animals make a number of vertical holes close together, so that they weaken and eventually destroy the stone. By some it is supposed that they secrete a corrosive juice, which dissolves the stone; others consider that the boring is mechanically done by the tough front of the shell covering the Pholas. These animals are generally small, but sometimes attain a length of 5 in. - the softer the rock the bigger they become.

The Saxicava is another small mollusc, found in the crevices of rocks and corals, or burrowing in limestone, the holes being sometimes 6 in. deep. It has been known to bore the cement stone (clay-ironstone) at Harwich, the Kentish Rag at Folkestone, and the Portland stone used at Plymouth Breakwater.