of Edinburgh, Session 1879 - 80 . 
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block of stone in which they occur should be unhesitatingly con- 
demned. Want of attention to this obvious rule has led to the 
unsightly disfigurement of public buildings. 
III. Granites. — In Professor Pfaffs experiments, to which I 
have already referred, he employed plates of syenite and granite, 
both rough and polished. He found that they had all lost slightly in 
weight at the end of a year. The annual rate of loss was estimated by 
him as equal to 0*007 6 mm. from the unpolished, and 0*0085 from the 
polished granite. That a polished surface of granite should weather 
more rapidly than a rough one is perhaps hardly what might have 
been expected. The same observer remarks, that though the polished 
surface of syenite was still bright at the end of not more that three 
years, it was less so than at first ; and in particular, that some figures 
indicating the date, which he had written on it with a diamond, had 
become entirely effaced. Granite has been employed for too short a 
time as a monumental stone in our cemeteries to afford any ready 
means of measuring even appproximately its rate of weathering. 
Traces of decay in some of its felspar crystals may be detected, yet 
in no case that I have seen is the decay of a polished granite surface 
sensibly apparent after exposure for fifteen or twenty years. That 
the polish will disappear, and that the surface will gradually roughen 
as the individual component crystals are more or less easily attacked 
by the weather, is of course sufficiently evident. Even the most 
durable granite will probably be far surpassed in permanence by 
the best of our siliceous sandstones. But as yet the data do not 
exist for making any satisfactory comparison between them. 
[Note added 21st May 1880. Since the preceding paper was 
written, I have had an opportunity of examining the condition of 
the monumental stones in the graveyards of a number of towns and 
villages in the north-east of Scotland, where the population is sparse 
and where comparatively little coal-smoke passes into the atmos- 
phere. The marble tablets last longer there than in Edinburgh, 
but show everywhere indications of decay. They appear to be quite 
free from the black or grey sulphate-crust. They suffer chiefly 
from superficial erosion, but I observed a few cases of curvature and 
and fracture. As a contrast to the universal decay of the marble 
tombstones, reference may be made to the remarkable durability of 
the clay-slate which has been employed for monumental purposes 
