Page:Popular Science Monthly Volume 18.djvu/680

662 about a third of an inch (or roughly nine millimetres) in a century, where it is effected by internal displacement, a curvature of two and a half inches with abundant rents, a partial effacement of the inscription and a reduction of the marble to a pulverulent condition may be produced in about forty years, and a total disruption and effacement of the stone within one hundred. It is evident that white marble is here utterly unsuited for out-of-door use, and that its employment for really fine works of art which are meant to stand in the open air in such a climate ought to be strenuously resisted. Of course, I am now referring not to the durability of marble generally, but to its behavior in a large town with a moist climate and plenty of coal-smoke.

II. —These, being the common building materials of the country, are of most frequent occurrence as monumental stones. When properly selected, they are remarkably durable. By far the best varieties are those which consist of a nearly pure fine silicious sand, with little or no iron or lime, and without trace of bedding structure. Some of our sandstones contain 98 per cent, of silica. A good illustration of their power of resisting the weather is supplied by Alexander Henderson's tomb in Grey friars Churchyard. He died in 1646, and a few years afterward the present tombstone, in the form of a solid square block of freestone, was erected at his grave. It was ordered to be defaced in 1662 by command of the Scottish Parliament, but after 1688 it was repaired. Certain bullet-marks upon the stone are pointed out as those of the soldiery sent to execute the order. Be this as it may, the original chisel-marks on the polished surface of the stone are still perfectly distinct, and the incised lettering remains quite sharp. Two hundred years have effected hardly any change upon the stone, save that on the west and south sides, which are those most exposed to wind and rain, the surface is somewhat roughened, and an internal fine parallel jointing begins to show itself.

Three obvious causes of decay in arenaceous rocks may be traced among our monuments. In the first place, the presence of a soluble or easily removable matrix in which the sand-grains are imbedded. The most common kinds of matrix are clay, carbonates of lime and iron, and the anhydrous and hydrous peroxides of iron. The presence of the iron reveals itself by its yellow, brown, or red color. So rapid is disintegration from this cause, that the sharply-incised date of a monument erected in Greyfriars Church to an officer who died only in 1863 is no longer legible. At least one eighth of an inch of surface has here been removed from a portion of the slab in sixteen years, or at the rate of about three fourths of an inch in a century.

In the second place, where a sandstone is marked by distinct laminæ of stratification, it is nearly certain to split up along these lines under the action of the weather if the surface of the bedding planes is directly exposed. This is well known to builders, who are quite aware of the importance of "laying a stone on its bed." Examples may be observed