68 
is generally attended with the evolution of sulphuretted 
hydrogen gas ; hence these metallic oxides have been con- 
verted into sulphurets, or reduced to a lower state of oxida- 
tion ; the organic matter itself has been converted into a 
bituminous coal, which, disseminated through the whole 
mass, gives the stone, in some instances, a perfectly black 
appearance, as in black marble, and at other times a grey 
or slate colour. 
A time at length arrived when the formation of this par- 
ticular deposit, and the existence of its parasites, were brought 
to a close. Violent convulsions of nature uplifted the car- 
boniferous bed of the ocean and converted it into dry land. 
The dislocations produced by this uplifting are still exhibited 
to us in the numerous fissures, caverns, &c, which exist in 
the limestone strata. These caverns and fissures were for a 
series of ages the receptacles for water and all soluble com- 
pounds which filtered through the new strata above. One of 
the constituents of such water would probably be that pro- 
duct of putrefactive decomposition, — sulphuretted hydrogen, 
and this would enable the water to dissolve a portion of 
these metallic sulphurets which I have shown the rock to 
contain. Now, if from any cause, such as the application 
of heat, or exposure to the atmosphere, this sulphuretted 
hydrogen should escape, then would sulphuret of lead and 
lime become insoluble ; and, provided this escape was very 
slow, they would crystallize, as we now find them. 
It is impossible to conceive that fissures like those in the 
mountain limestone could exist without some portion being 
exposed to atmospheric air ; hence we have this simple mode 
of accounting for the formation of some of the minerals 
which accompany it. There is another cause, however, 
which, in the Derbyshire district, has been in extensive 
operation, and which enables us to account for the contents 
of these fissures more satisfactorily, viz., the agency of 
