ROCK-WEATHERING IN CHURCHYARDS. 661 



considerably in the center, and on the blister-like expansion has been 

 rent by numerous cracks which run on the whole in the direction of 

 the length of the stone. 



A further stage of decay is exhibited by a remarkable tomb on the 

 west wall of the Greyfriars Churchyard. The marble slab, bearing a 

 now almost wholly effaced inscription, on which the date 1779 can be 

 seen, is still held tightly within its inclosing frame of sandstone slabs, 

 which are firmly built into the wall. But it has swollen out into a 

 ghastly protuberance in the center, and is, moreover, seamed with rents 

 Avhich strike inward from the margins. In this and in some other ex- 

 amples the marble seems to have undergone most change on the top 

 of the swelling, partly from the system of fine fissures by which it is 

 broken up, and partly from more direct and effective access of rain. 

 Eventually the cohesion of the stone at that part is destroyed, and the 

 crumbling marble falls out, leaving a hole in the middle of the slab. 

 When this takes place disintegration proceeds rapidly. Three years 

 ago I sketched a tomb in this stage on the east wall of Canongate 

 Churchyard. In a recent visit to the place I found "that the whole of 

 the marble had since fallen out. 



. The first cause that naturally suggests itself, in explanation of this 

 remarkable change in the structure of a substance usually regarded as 

 so inelastic, is the action of frost. White statuary marble is naturally 

 porous. It is rendered still more so by that internal solution which 

 I have described. The marble tombstones in our graveyards are, 

 therefore, capable of imbibing a relatively large amount of moisture. 

 When this interstitial water is frozen, its expansive force as it passes 

 into the solid state must increase the isolation of the granules and 

 ausfment the dimensions of a marble block. I am inclined to believe 

 that this must be the principal cause of the change. Whatever may 

 be the nature of the process, it is evidently one Avhich acts from within 

 the marble itself. Microscopic examination fails to discover any 

 chemical transformation which would account for the expansion. Dr. 

 Angus Smith has pointed out that in towns the mortar of walls may be 

 observed to swell up and lose cohesion from a conversion of its lime 

 into the condition of sulphate. I have already mentioned that sul- 

 phate does exist within the substance of the marble, but that its quan- 

 tity so far as I have observed is too small to be taken into account in 

 this question. The expansive power is exerted in such a way as not 

 sensibly to affect the internal structure and composition of the stone, 

 and this, I imagine, is most probably the work of frost. 



The results of my observations among our burial-grounds show that, 

 save in exceptionally sheltered situations, slabs of marble exposed to 

 the weather in such a climate and atmosphere as those of Edinburgh are 

 entirely destroyed in less than a century. When this destruction takes 

 place by simple comparatively rapid superficial solution and removal of 

 the stone, the rate of lowering of the surface amounts sometimes to 



