529 
of Edinburgh, Session 1879 - 80 . 
building materials of the country, are of most frequent occurrence 
as monumental stones. Where properly selected they are remark- 
ably durable. By far the best varieties are those which consist of 
a nearly pure fine siliceous 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 afterwards 
the present tombstone, in the form of a solid square block of free- 
stone, 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 inscribed lettering remains quite 
sharp. Two hundred years have effected hardly any change upon 
the stone, save that on the west and north sides, which are 
those most exposed to wind and rain, the surface is somewhat 
roughened, and the 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 embedded. 
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 colour. 
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 Jth of an inch of 
surface has here been removed from a portion of the slab in 16 years, 
or at the rate of about three quarters 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.” 
Examples may be observed in our churchyards where sandstones of 
this character have been used for pilasters and ornamental work and 
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VOL. X. 
