of Edinburgh, Session 1879-80. 
521 
veloped. Not a trace exists of any amorphous granular matrix or 
base holding the crystalline grains together. These seem moulded 
into each other, but have evidently no extraordinary cohesion. A 
small fragment placed in dilute acid was entirely dissolved. There 
can be no doubt that this marble must be very nearly pure carbonate 
of lime. 
The process of weathering in the case of this white marble presents 
three phases sometimes to be observed on the same slab, — viz., super- 
ficial solution, internal disintegration, and curvature with fracture. 
(1.) Superficial Solution is effected by the carbonic acid, and 
partly by the sulphuric acid of town-rain. When the marble is 
first erected it possesses a well-polished surface, capable of affording 
a distinct reflection of objects placed in front of it. Exposure for 
not more than a year or two to our prevalent westerly rains suffices 
to remove this polish, and to give the surface a rough granular 
character. The granules which have been cut across or bruised in 
the cutting and polishing process are first attacked and removed in 
solution, or drop out of the stone. An obelisk in Greyfriars’ 
Churchyard, erected in memory of a lady who died in 1864, has so 
rough and granular a surface that it might readily be taken for a 
sandstone. So loosely are the grains held together that a slight 
motion of the finger will rub them off. In the course of solution 
and removal, the internal structure of the marble begins to reveal 
itself. Its harder nests and veinings of calcite and other minerals 
project above the surrounding surface, and may be traced as pro- 
minent ribs and excrescences running across the faint or illegible 
inscriptions. On the other hand, some portions of the marble are 
more rapidly removed than others. Irregular channels, dependent 
partly on the direction given to trickling rain by the form of the 
monumental carving, but chiefly on original differences in the 
internal structure of the stone, are gradually hollowed out. In this 
way the former artificial surface of the marble disappears, and is 
changed into one that rather recalls the bare bleached rocks of some 
mountain side. 
The rate at which the transformation takes place seems to depend 
primarily on the extent to which the marble is exposed to rain. 
Slabs which have been placed facing to north-east, and with a suffi- 
ciently projecting architrave to keep off much of the rainfall, retain 
3s 
VOL. X. 
