530 Proceedings of the Royal Society 
where the stone, set on its edge, has peeled off in successive layers. 
In flagstones, which are merely thinly bedded sandstones, this 
minute lamination is often fatal to durability. These stones, from 
the large size in which slabs of them can be obtained, and from the 
ease with which they can be worked, form a tempting material for 
monumental inscriptions. The melancholy result of trusting to 
their permanence is strikingly shown by a tombstone at the end 
of the south burying-ground in Greyfriars’ Churchyard. The date 
inscribed on it is 1841, and the lettering that remains is as sharp 
as if cut only recently. The stone weathers very little by surface 
disintegration. It is a laminated flagstone set on edge, and large 
portions have scaled off, leaving a rough, raw surface where the 
inscription once ran. In- this instance a thickness of about J-rd of 
an inch has been removed in forty years. 
In the third place, where a sandstone contains concretionary masses 
of different composition or texture from the main portion of the 
stone, these are apt to weather at a different rate. Sometimes they 
resist destruction better than the surrounding sandstone so as to be 
left as permanent excrescences. More commonly they present less 
resistance, and are therefore hollowed out into irregular and often 
exceedingly fantastic shapes. Examples of this kind of weathering 
abound in our neighbourhood. Perhaps the most curious to which 
a date can be assigned are to be found in the two sandstone pillars, 
which until recently flanked the tomb of Principal Carstares in 
Greyfriars’ Churchyard. They were erected some time after the year 
1715. Each of them is formed of a single block of stone about 8 
feet long. Exposure to the air for about 150 years has allowed the 
original differences of texture or composition to make their influence 
apparent. Each column is hollowed out for almost its entire length 
on the exposed side into a trough 4 to 6 inches deep and 6 to 8 
inches broad. As they lean against the wall, beneath the new 
pillars which have supplanted them, they suggest some rude form 
of canoe rather than portions of a sepulchral monument. 
Where concretions are of a pyritous kind their decomposition 
gives rise to sulphuric acid, some of which combines with the iron 
and gives rise to dark stains upon the corroded surface of the stone. 
Some of the sandstones of the district, full of such impurities, 
ought never to be employed for architectural purposes. Every 
