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has taken its form by exactly filling the cavities of the original or- 

 ganized fossil, which also owed its solidity to the same earthy sub- 

 stance, a carbonate of lime, disposed on, and held together by, a very 

 small portion of gelatin. The difficulty of the explanation consists 

 in the ascertaining of what agent could have so completely removed 

 the mould, without having manifested the least influence on the cast, 

 whilst the component parts of both so nearly agreed. 



The consideration of the actual difference in the constitution of 

 the cast and of the mould leads, in my opinion, to the desired expla- 

 nation, and shews that the only agent employed in the removal of the 

 mould was water. Let us suppose a part of the vertebral column of 

 the encrinus to be so situated as to be immersed in, or constantly ex- 

 posed to, the instillation of a fully impregnated solution of car- 

 bonate of lime. From this circumstance, after a certain period, the 

 cavity of the vertebral column may be expected to become accurate- 

 ly filled with a crystalline calcareous spar ; whilst the original animal 

 matter in which it is formed may be considered, either as impregnated 

 or not with the same substance. After another lapse of time, this 

 body thus filled, it is to be supposed, may be subjected to the action 

 of water. To determine what would here take place, it seems to be 

 only necessary to consider that calcareous spar exposed to the action 

 of water suffers, through a long period, little or no change or diminu- 

 tion ; whilst, on the other hand, animal substances, such as shells, 

 the crustaceous parts of animals and other bodies formed by the in- 

 termixture of animal membrane, or gelatine, with carbonate of lime, 

 suffer a very rapid decomposition by the agency of water alone, as is 

 the case with dead shells, &c. on the sea or river shores. In these in- 

 stances it may be ascertained, that the animal membrane, suffering 

 resolution, particle after particle, and layer after layer ; the carbonate 

 of lime, deprived of its cement and support, gradually separates, 

 and moulders away. 



Thus also it is likely to happen with shells or trochitae, enveloped 



