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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 
decomposition 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 
argillaceous 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 involving 
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 entrochai 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 
specimens 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 nitric 
acid, when a very considerable effervescence took place, which was 
soon found to proceed from one specimen. This specimen being 
removed, it was directly ascertained that the acid exerted no action 
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 this fossil, aware 
of the difficulty of the task, as was observed, when speaking of some 
of the fossil substances from St. Peters’s Mountain. This fossil body 
is entirely composed of a carbonate of lime in a spathose state, and 
