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bedded in, stone. The next stage in this assumed process would be 

 the filtration through this stony mass of some menstruum, probably 

 water slightly impregnated with sulphuric acid, yielded by the de- 

 composition of superincumbent pyrites, carrying with it a portion of 

 iron. The effect of the filtration of this fluid through the mass may be 

 easily conceived : the acid, combining with the lime, would part 

 with the iron, which would be left united with the silicious and ar- 

 gillaceous particles, whilst the lime would be gradually removed. 

 Thus, by the perpetual accession of this very dilute acid metallic 

 solution, the whole of the calcareous matter, as well of the involv- 

 ing matrix, as of the animal remains, would be carried away : 

 and iron, with perhaps a minute portion of silex, deposited in the 

 smaller interstices of the investing stone, and of that which fills the 

 cavities of the entrochal column. By this removal of the calcareous 

 earth, the space which it possessed would, of course, be left empty, 

 and a solid substance would remain, exactly answering to the cavities 

 which the vertebral column originally possessed. 



The doubt expressed by Mr. Walch with respect to the presence of 

 silex in the screw-stone, led me to make an examination of such spe- 

 cimens as I possessed. 



All the detached specimens which I could obtain, being thirteen in 

 number, were placed in a glass vessel, and covered with diluted ni- 

 tric acid, when a very considerable effervescence took place, which 

 was soon found to proceed from one specimen. This specimen be- 

 ing removed, it was directly ascertained that the acid exerted no ac- 

 tion on the remainder ; and that they, like their containing mass, were 

 chiefly formed of silex and alumine, whilst this specimen alone was 

 formed entirely of a very pure calcareous spar. 



I will now endeavour to explain the formation of thi-s fossil, aware 

 of the difficulty of the task, as was observed, when speaking of some 

 of the fossil substances from St. Peter's mountain. This fossil body 

 is entirely composed of a carbonate of lime in a spathose state, and 



