52 
ROTrEN-STONE. 
Black n arble 
at Ashfoid-in- 
the-watei 8 . 
proportion of alumine, which the former of these substance#' 
contains, and its comparative, or, in some instances, its total 
want of the carbonate of lime. The particular varieties of 
limestone now alluded to, are those which Mr.Kirwan has 
denominated argilliferous marliies, on account of their holding 
a large proportion of orgill (alumine) in their compositions, 
(v. E. Min. V. 1 . p. 99 .) — Some of these stones, though 
affording lime, contain 30 per ct, of alumine, together with 
small quantities of silex, iron, &c. : — and our Derbyshire 
black marble, or limestone, undoubtedly' belongs to this 
class. — 
The greatest quantity of this stone is quarried at Ashford-in- 
the-waters ; and, as the quarry is situated at no great distance 
from the depot of Rolten-slone, and affords an excellent exam- 
ple of this formation, I shall here describe the state in which it 
is found, and some of its principal varieties. It occurs in beds, 
which vary in thickness, from a few inches to two or three feet, 
w'ilh interposed seams (semistrataj of black, bituminous shale 
and clay. The substance of these beds, though throughout of 
the same general aspect, and constantly burning to lime, more 
or less pure, differs greatly in the proportion of its constituent 
parts, as well as somewjjat in its external characters. The 
limestone of those beds, immediately worked as marble, is of 
a deep greyish black, which, on the stones being polished, 
becomes perfect, or dark-black :* — texture close, fine earthy : 
* Its volmir must be ascribed to tlie t)itnrricn or carbon, which it 
contains, as it Ijeromes perfectly white, wlicn calciiK-d, and also acquires 
u white, cr ns -colmired, crust, 011 exposure to the weather. In many 
instances T have found tlie crust of a considerable tliickness, and become 
perfect Kotieu-stone. And tlicre is no doubt but in walls, which are 
sometimes built of black marble, and in otlier exposed situations, thi.s 
would frequently be the case, if a furtlier decay of the stones wen- not 
prevented by a timely and friendly covering of lichens and mosses. I 
have observed, however, that pieces of polished marble, though equally 
expose'll with tho'.c in tlic uni'oli.slied state, do not so soon acquire a 
white enist. — Polishing, by filling up tlie inimitc interstices, induces a 
greater degree of e xternal hardness of the stone, and prevents for a 
longer time the decomposition of the surface. 
fracture 
