526 
Proceedings of the Royal Society 
marble. The rents in some cases proceed from the margin inwards, 
more particularly from the upper and under edges of the stone, 
pointing unmistakably to an increase in volume as the cause of 
fracture. In other cases the rents appear in the central part of the 
swelling where the tension from curvature has been greatest. 
Some exceedingly interesting examples of this singular process 
of weathering are to be seen in Greyfriars’ Churchyard. On the 
south wall, in the enclosure of a well-known county family, there 
is an oblong upright marble slab (PI. XVI., A, measuring 30J inches 
in height, by 22§ inches in breadth and J inch in thickness, and 
facing west. The last inscription on it bears the date 1838, at 
which time, of course, it was no doubt still smooth and upright. 
Since then, however, it has escaped from its fastenings on either 
side, though still held firmly at the top and bottom. It conse- 
quently projects from the wall like a well-filled sail. The axis 
of curvature is, of course, parallel to the upper and lower margins, 
and the amount of deviation from the original vertical line is fully 
2J inches, so that the hand and arm can be inserted between the 
curved marble and the perfectly vertical and undisturbed wall to 
which it was fixed. At the lower end of the slab a minor curvature 
to the extent of |fh of an inch is observable, coincident with the 
longer axis of the stone. In both cases the direction of the bending 
has been determined by the position of the enclosing solid frame 
of sandstone which resisted the internal expansion of the marble. 
Preed from its fastenings at either side the stone had assumed a 
simple wave-like curve. But the tension has become so great that 
a series of rents has appeared along the crest of the fold. One of 
these has a breadth of xg-th of an inch at its opening.* Not only has 
the slab been ruptured, but its crust has likewise yielded to the 
strain, and has broken up into a network of cracks, and some of the 
isolated portions are beginning to curl up at the edges, exposing the 
crumbling decayed marble below. I should add that such has been 
the expansive force of the marble that the part of the sandstone 
block in the upper part of the frame, exposed to the direct pressure, 
has begun to exfoliate, though elsewhere the stone is quite sound. 
* It is a further curious fact that the slab measures ^ inch more in breadth 
across the centre, where it has had room to expand, than at the top, where it 
has been tightly jammed between the sandstone slabs. 
