170 
dently from the introduction of a considerable proportion of carbonate 
or oxide of iron into their composition » Those which are obtained 
from Blackenburg are generally of a deeper colour than those which 
we find in England. 
The nature of these bodies was, for a long time, entirely unknown. 
In 1751, M. Lieberoth, of Hettstedt, published an essay on the 
origin of these substances in the Hamburgh Magazine, in which he 
endeavoured to prove that these cylindrical bodies were the remains 
of some totally unknown animals, which, like the earth-worm, had the 
power of contracting and of elongating themselves. This opinion was 
opposed by M. Lehman, who contended that these stones owed their 
peculiar forms to the rays of the Caput Medusae, impregnated with 
iron. M. Vogel hazarded a conjecture that these forms might arise 
from particular species of strombites. A satisfactory explanation of 
the origin of these bodies was first given by M. Schulz, who attri- 
buted their formation to the introduction and deposition of an 
ochrous matter in the cavities of entrochi. 
Whoever will take the pains to examine the internal structure, and 
the cavities existing in the vertebral column of the encrinite, and will 
also attentively view the screw-stone, cannot but perceive, how exactly 
the cavities of the one correspond with the projecting parts of the 
other; and how certain it is that the screw-stone is the cast of the 
internal part of this column. 
But little difficulty will, I believe, occur in explaining the manner 
in which this east is formed. These columns, being immersed in 
water, . holding earthy particles, merely in a state of diffusion, 
would gradually have every cavity filled with the earth which would 
be hence deposited ; and would also become surrounded with an 
earthy mass, which, having gradually acquired a pulpy state, would, 
as evaporation proceeded, obtain, with that which had filled all 
the cavities of the column, a stony hardness : thus we should 
have the calcareous animal remains completely filled with, and im- 
