ROCK-WEATHERING IN CHURCHYARDS. 659 



to descend into the heart of some of the granules. In no case did I 

 observe any white pellicle such as might indicate a redeposit of lime 

 from the dissolved carbonate. Except for the veining of probable sul- 

 phate just referred to, the lime when once dissolved had apparently 

 been wholly removed in solution. There was further to be observed a 

 certain dirtiness, so to speak, which at the first glance distinguished 

 the section of crumbled marble from the fresh stone. This was due 

 partly to corrosion, but chiefly to the introduction of particles of soot 

 and dust, which could be traced among the interstices and cleavage 

 lamellse of the crystalline granules, for some distance back from the 

 crust. 



It may be inferred, therefore, that the disintegration of the marble 

 is mainly due to the action of carbonic acid in the permeating rain- 

 water, whereby the component crystalline granules of the stone are 

 partially dissolved and their mutual adhesion is destroyed. This pro- 

 cess goes on in all exposures, and with every variety in the thickness 

 of the outer crust. It is distinctly traceable in tombstones that have 

 not been erected for more than twenty years. In those which have 

 been standing for a century it is, save in exceptionally sheltered posi- 

 tions, so far advanced that a very slight pressure suffices to crumble 

 the stone into powder. But with this internal disintegration we have 

 to take into consideration the third phase of weathering to which I 

 have alluded. In the upright marble slabs it is the union of the two 

 kinds of decay which leads to so rapid an effacement of the monu- 

 ments. 



3. Curvature and Fracture. This most remarkable phase of rock- 

 weathering is only to be observed in the slabs of marble which have 

 been firmly inserted into a solid framework of sandstone and placed in 

 an erect or horizontal position. It consists in the bulging out of the 

 marble, accompanied with a series of fractures. The change can not 

 be explained as mere sagging by gravitation, for it usually appears 

 as a swelling up of the center of the slab, which continues until the 

 large, blister-like expansion is disrupted. Nor is it by any means 

 exceptional ; it occurs as a rule on all the older upright marble tab- 

 lets, and is only found to be wanting in those cases where the marble 

 has evidently not been fitted tightly into its sandstone frame. Wher- 

 ever there has been little or no room for expansion, protuberance of 

 the marble may be observed. Successive stages may be seen, from 

 the first gentle uprise to an unsightly swelling of the whole stone. 

 This change is accompanied by fracture of the marble. The rents in 

 some cases proceed from the margin inward, 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 



