J7me 3, 1880] 



NA TURE 



105 



data bearing on the general question of rock-weathering 

 we must admit the kind and amount of such weathering 

 visible in a town to be in some measure different from 

 what is normal in nature. So far as the disintegration of 

 rock-surfaces is effected by mineral acids, for example, 

 there must be a good deal more of such chemical change 

 where sulphuric acid is copiously evolved into the atmo- 

 sphere from thousands of chimneys than in the pure air of 

 country districts. In these respects we may regard the 

 disintegration in towns as an exaggeration of the normal 

 rate. Still the difl'erence between town and country may 

 be less than might be supposed. Surfaces of stone are 

 apt to get begrimed with dust and smoke, and the crust of 

 organic and inorganic matter deposited upon them may 

 in no small measure protect them from the greater chemi- 

 cal activity of the more acid town rain. In regard to 

 the effect of daily or seasonal changes of temperature, on 

 the other hand, any ditierence between town and country 

 may not impossibly be on the side of the town. Owing 

 probably to the influence of smoke in retarding radiation, 

 thermometers placed in open spaces in town commonly 

 mark an extreme nocturnal temperature not quite so low 

 as those similarly placed in the suburbs, while they show 

 a maximum day temperature not quite so high. 



The illustrations of rock-weathering presented by city 

 graveyards are necessarily limited to the few kinds of 

 rock ernployed for monumental purposes. In this district 

 the maferials used are of three kinds : ist, Calcareous, 

 including marbles and limestones ; and, Sandstones and 

 flagstones ; 3rd, Granites. 



I. Calcareous. — With extremely rare exceptions, the 

 calcareous tombstones in our graveyards are constructed 

 of ordinary white saccharoid Italian marble. I have 

 also observed a pink Italian shell-marble and a finely 

 fossiliferous limestone containing fragments of shells, 

 foraminifera, &c. 



In a few cases the white marble has been employed by 

 itself as a monolith in the shape of an obelisk, urn, or 

 other device ; but most commonly it occurs in slabs which 

 have been tightly fixed in a framework of sandstone. 

 These slabs, from less than one to fully two inches thick, 

 are generally placed vertically; in one or two examples 

 they have been inserted in large horizontal sandstone 

 slabs or "through-stanes." The form into which it has 

 been cut and the position in which it has been erected 

 have had considerable influence on the weathering of the 

 stone. 



A specimen of the common white marble employed for 

 monumental purposes was obtained from one of the 

 marble works of the city, and examined microscopically. 

 It presented the well-known granular character of true 

 saccharoid marble, consisting of rounded granules of 

 clear transparent calcite, averaging about yJu inch in 

 diameter. Each granule has its own system of twin 

 lamellations, and not unfrequently gives interference 

 colours. The fundamental rhombohedral cleavage is 

 everywhere well developed. Not a trace exists ot 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., Superficial Solution, Internal Dis- 

 integration, and Curvature with Fracture. 



(l) 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. E.xposure 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 surfiice 

 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 oft'. 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 prominent 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 dif- 

 ferences in the internal structure of the stone, are gradu- 

 ally 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 this 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 sufficiently projecting architrave to 

 keep oft' much of the rainfall, retain their inscriptions 

 legible for a century or longer. But even in these cases 

 the progress of internal disintegration is distinctly visible. 

 Where the marble has been less screened from rain the 

 rapidity of waste has been sometimes very marked. A 



good illustration is supplied by the tablet of G G , 



on the south side of Greyfriars Churchyard, who died in 

 17S5.' This monument had become so far decayed as to 

 require restoration in 1S03. It is now, and has been for 

 some years, for the most part utterly illegible. The 

 marble has been dissolved away over the centre of the 

 slab to a depth of about a quarter of an inch. Yet this 

 monument is by no means in an exposed situation. It 

 faces eastward in a rather sheltered corner, where, however, 

 the wind eddies in such a way as to throw the rain against 

 the part of the stone which has been most corroded. 



In the majority of cases superficial solution has been 

 retarded by the formation of a peculiar grey or begrimed 

 crust, to be immediately described. The marble employed 

 here for monumental slabs appears to be peculiarly liable 

 to the development of this crust. Another kind of white 

 marble, sometimes employed for sculptured ornaments on 

 tombstones, dissolves without crust. It is snowy white, 

 and more translucent than the ordinary marble. So far as 

 the few weathered specimens I have seen enable me to 

 judge, it appears to be either Carrara marble or one of 

 the strongly saccharoid, somewhat translucent varieties 

 employed instead of it. This stone, however, though it 

 forms no crust, sufi'ers marked superficial solution. But it 

 escapes the internal disintegration which, so far as I have 

 observed, is always an accompaniment of the crust. But 

 the few examples of it I have met with hardly suffice for 

 any comparison between the varieties. 



(3) IiitLi-ncil Disintegration.— 'hlxTiy of the marble 

 monuments in our older churchyards are covered with 

 a dirty crust, beneath which the stone is found on exami- 

 nation to be merely a loose crumbling sand. This crust 

 seems to form chiefly where superficial solution is feeble. 

 It may be observed to crack into a polygonal network, the 

 individual polygons occasionally curling up so as to reveal 

 the yellowish white crumbling material underneath. It 

 also rises in blisters, which, when they] break, expose 

 the interior to rapid disintegration. 



So long as this begrimed film lasts unbroken the 

 smooth face of the marble slab remains with apparently 

 little modification. The inscription may be perfectly 



» For obvious reasons I withhold the names can-ed on the tombstones 

 referred to in this communication. 



