59 
the superior part of the specimen. From observing this we are led to 
the following conjectures respecting the formation of this fossil : 
The changes which it has undergone in the superior part appear to be 
of four kinds. 1. The abstraction of such of its constituent materials as 
were of an animal nature. 2. The filling of all, even its minutest inter- 
stices, with the finest and most impalpable argillaceous particles, which 
by hardening acquired the exact form of every cavity of the coral. 
3. The removal, by some appropriate menstruum, of the calcareous 
remains of the coral, which formed the mould that had been thus filled. 
And 4. The filling up of the void thus left, and the impregnation of the 
argillaceous cast with silicious matter. 
The coral imbedded in an argillaceous matrix, would be secured 
from the introduction of any other extraneous matters besides those 
which the constant percolation of water through the mass might con- 
vey. The fine reticular internal structure of the coral would prevent 
the intrusion of coarse particles : those only which were held in solu- 
tion, and those which were suspended in the fluid, in the finest degree 
of tenuity, would enter and be there detained. Thus, by a slow process, 
would all the internal cavities of the coral become filled, by the depo- 
sition of the argillaceous, and the crystallization of the impregnating 
silicious matter ; whilst by the action of the water perpetually passing 
through the mass, almost the whole of the remaining animal princi- 
ples would be removed. To give to tbis mass the properties pos- 
sessed by the fossil whose formation is here endeavoured to be ex- 
plained, no more seems to have been required than that the water 
which permeated it and passed through it should have held in solution 
a portion of silicious earth, by the influence of such a menstruum, as 
possessing a stronger degree of attraction for calcareous than for sih- 
cious earth, might, as it took up the former, deposit the latter in its 
place. But whether these supposititious processes be admitted or not ; 
and whether the abstraction of the original calcareous matter and the 
