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 mate- 

 rials as were of an animal nature. 2. The filling of all, even its 

 minutest interstices, 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 tilling up of the void thus left, and the im- 

 pregnation 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 this 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 sili- 

 cious earth, might, as it took up the former, deposit the latter in its 

 place. But whether these suppositions processes be admitted or not : 

 and whether the abstraction of the original calcareous matter and the 



