Rotten-Stone. 321 
affording lime, contain 30 per ct. of alumine, 
together with small quantities of silex, iron, 
‘&e :—and our Derbyshire black marble, or 
limestone, undoubtedly belongs to this class.— 
The greatest quantity of this stone 1s quarried 
at Ashford-in-the-waters; and, as the quarry 
is situated at no great distance from the depot 
of Rotten-stone, and affords an excelient 
example of this formation, I shall here des- 
cribe 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, with interposed seams 
(semistrata) 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 consti- 
tuent parts, as well as somewhat in its exter- 
nal 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 * :— 
* Its colour must be ascribed to the bitumen or carbon, 
which it contains, as it becomes perfectly white, when 
caleined, and also acquires a white, or ash-coloured, crust, 
on exposure to the weather. In mauy instances I have 
found the crust of a considerable thickness and become 
perfect Rotten-stone. And there is no doubt but in walls, 
ss 
