ROCK-WEATHERING IN CHURCHYARDS. 657 



In the majority of cases superficial solution has been retarded by 

 the formation of a peculiar gray 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 trans- 

 lucent than the ordinary marble. So far as the few weathered speci- 

 mens I have seen enable me to judge, it appears to be either Carrara 

 marble or one of the strongly saccharoid, somewhat translucent varie- 

 ties employed instead of it. This stone, however, though it forms no 

 crust, suffers marked superficial solution. But it escapes the internal 

 disintegration which, so far as I have observed, is always an 'accom- 

 paniment of the crust. But the few examples of it I have met with 

 hardly suftice for any comparison between the varieties. 



2. Internal Disintegration. Many of the marble monuments in 

 our older churchyards are covered with a dirty crust, beneath which 

 the stone is found on examination 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 inscrip- 

 tion may be perfectly 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 sand. 



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 color from a dirty gray 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 por- 

 tions 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-chiseled 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, I found it to consist 



TOL. XVIII. 42 



