io6 



NATURE 



\ynne 3, 1880 



legible ; the moment the crust is broken up, however, the 

 decay of the stone is rapid. For we then see that the 

 cohesion of the individual crystalline granules of the 

 marble has already been destroyed, and that the merest 

 touch causes them to crumble into a loose smd. 



It appears therefore that two changes take place in 

 upright marble slabs freely exposed to rain in our burial- 

 grounds — a superficial, more or less firm crust is formed, 

 and the cohesion of the particles beneath is destroyed. 



The crust varies in colour from a dirty grey to a deep 

 brown black, and in thickness from that of writing-paper 

 up to sometimes at least a millimetre. One of the most 

 characteristic examples of it was obtained from an utterly 

 decayed tomb (erected in the year 1792), on the east side 

 of Canongate Churchyard. No one would suppose that 

 the pieces of flat dark stone lying there on the sandstone 

 plinth were once portions of white marble. Yet a mere 

 touch suffices to break the black crust, and the stone at 

 once crumbles to powder. Nevertheless the two opposite 

 faces of the original polished slab have been preserved, and 

 I even found the sharply-chiselled socket-hole of one of 

 the retaining nails. The specimen was carefully removed 

 and soaked in a solution of gum, so as to preserve it from 

 disintegration. On submitting the crust of the marble to 

 microscopic investigation, 1 found it to consist of particles 

 of coal, grains of quartz sand, angular pieces of broken 

 glass, fragments of red brick or tile, and organic fibres. 

 This miscellaneous collection of town dust was held 

 together by some amorphous ce.iicnt which was not dis- 

 solved by hydrochloric acid. At my request my friend 

 Mr. B. N. Peach tested it with soda on charcoal, and at 

 once obtained a strong sulphur reaction. There can be 

 little doubt that it is mainly sulphate of lime. The crust 

 ■which forms upon our marble tombstones is thus a pro- 

 duct of the reaction of the sulphuric acid of the town 

 rain upon the carbonate of lime. A pellicle of amor- 

 phous gypsum is deposited upon the marble and incloses 

 the particles of dust which give the characteristic sooty 

 aspect to the stone. This pellicle, of course, when once 

 formed, is comparatively little affected by the chemical 

 activity of rain-water. Hence the conservation of the 

 even surface of the marble. It is liable, however, to be 

 cracked by an internal expansion of the stone to which 

 I shall immediately refer, and also to rise in small blisters, 

 and as I have said, its rupture leads at once to the rapid 

 disintegration of the monument. 



The cause of this disintegration is the next point for 

 consideration. Chemical examination revealed the pre- 

 sence of a slight amount of sulphate in the heart of the 

 crumbling marble ; but the quantity appeared to me to 

 be too small seriously to affect the cohesion of the stone. 

 I submitted to microscopic examination a portion of a 

 crumbling urn of white marble in Canongate Churchyard. 

 The tomb bears a perfectly fresh date of" I792"cut in sand- 

 stone over the top ; but the marble portions are crumbling 

 into sand, though the structure faces the east, and is pro- 

 tected from vertical rain by arching mason-work. A small 

 portion of the marble retaining its crust was boiled in 

 Canada balsam, and was then sliced at right angles to 

 its original polished surface. By this means a section of 

 the crumbled marble was obtained which could be com- 

 pared with one of the perfectly fresh stone. From the 

 dark outer amorphous crust with the carbonaceous and 

 other miscellaneous particles fine rifts could be seen 

 passing down between the separated calcite granules, 

 which in many cases were quite isolated. The black 

 crust descends into these rifts, and likewise passes 

 along the cleavage planes of the granules. Towards 

 the outer surface of the stone immediately beneath the 

 crust the fissures are chiefly filled with a yellowish, 

 structureless substance, which gave a feeble glimmering 

 reaction with polarised light, and inclosed minute amor- 

 phous aggregates like portions of the crust. It probably 

 consists chiefly of sulphate of lime. But the most 



remarkable feature in the slide was the way in which 

 the calcite granules had been corroded. Seen with 

 reflected light, they resembled those surfaces of spar 

 which have been placed in weak hydrochloric acid to lay 

 bare inclosed crystals and zeolites. The solution had 

 taken place partly along the outer surfaces, so as to pro- 

 duce the fine passage or rifts, and partly along the 

 cleavage. Deep cavities, defined by intersecting cleav- 

 age planes, appeared 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 

 sulphate 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 lamella; 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 com 

 ponent crystalline granules of the stone are partially dis- 

 solved and their mutual adhesion is destroyed. This 

 process goes on in all exposures, and with every variety 

 in the thickness of the outer crust. It is distinctly trace- 

 able 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 positions, 

 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 eftacement of the 

 monuments. 



3. Ciirvaluic 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 cannot be explained as mere sagging by gravita- 

 tion, for it usually appears as a swelling up of the centre 

 of the slab, which continues until the large, blister-like 

 expansion is disrupted. Nor is it by any means excep- 

 tional ; it occurs as a rule on all the older upright marble 

 tablets, and is only found to be wanting in those cases 

 where the marble has evidently not been fitted tightly into 

 its sandstone frame. Wherever 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 

 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 Church- 

 yard. On the south wall, in the inclosure of a well-known 

 county family, there is an oblong upright marble slab 

 measuring 30] inches in height by 22! inches in breadth, and 

 I inch in thickness, facing west. The last inscription on it 

 bears the date 1838, at which time 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 consequently projects from the 

 wall like a wcU-fiUei sail. The axis of carvature is of 

 course parallel to the upper and lower margms, and the 



