147 



tion, have left their forms impressed on the mass, which has now as- 

 sumed 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 endeavour- 

 ing 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 

 analvsis. 



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 of 

 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, yield- 

 ing a farther deposition of selenite. Hence it appeared that this sub- 

 stance was a very pure carbonate of lime ;. and although accurate ex- 

 amination 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, Sec. should have been en- 

 tirely 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- 



