662 THE POPULAR SCIENCE MONTHLY. 



about a third of an inch (or roughly nine millimetres) in a century, 

 AVhere 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 pro- 

 duced 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 re- 

 ferring not to the durability of marble generally, but to its behavior 

 in a large town with a moist climate and plenty of coal-smoke. 



11. Sandstones and Flagstones. These, being the common build- 

 ing materials of the country, are of most frequent occurrence as monu- 

 mental 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 bed- 

 ding 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 forpi 

 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 ex- 

 jjosed 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 monu- 

 ment erected in Greyfriars Church to an oflicer who died only in 1863 

 is no longer legible. At least one eighth of an inch of surface has here 

 been removed from a poi'tion 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 laminae 

 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." ExamjDles may be observed 



