147 
tion, have left their forms impressed on the mass, which has now 
assumed a stony hardness. This opinion is, however, contradicted by 
the appearances which these coralline substances yield, since the 
most accurate examination proves that these fossils are actually 
the changed animal bodies themselves, and not merely casts or im- 
pressions. It is very true that some difficulty occurs in endeavouring 
to explain why the shells, &c. should have been removed, and 
should have left merely their casts and impressions, whilst the whole 
substance of those coralline bodies and zoophytes have been left, 
still possessing their particular forms. I at first imagined that this 
might have been occasioned by the tophaceous mass which involved 
these bodies having been so impregnated with silex as to resist the 
action of such agents as might gradually decompose the shells and 
those Other bodies which it had inclosed. This opinion, however, also 
proved to be totally unfounded, when tried by the touchstone of 
analysis. 
The whole substance of the mass of which these fossils were formed 
proved to be completely soluble in nitric acid, carbonic acid gas being 
at the same time liberated in considerable quantity. The addition ot 
sulphuric acid to this solution yielded a copious precipitate of sulphate 
of lime, which, being separated, ammonia was added to the residual 
solution, and no precipitate was formed : evaporation, however, yielding 
a farther deposition of selenite. Hence it appeared that this substance 
was a very pure carbonate of lime ; and although accurate examination 
has proved the fossils which have been here treated of to be really 
changed organic substances, nothing in the composition of the mass 
assists in explaining why the shells, &c. should have been entirely 
removed, and these fossils have remained, with the rest of the mass, 
unimpaired. 
The Maestricht fossil, which next claims our careful attention, is 
that which is shewn Plate XII. Fig. 11. It is formed of the same cal- 
