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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 substance, 
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, 
Avithout 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 
exposed to, the instillation of a fully impregnated solution of carbonate 
of lime. From this circumstance, after a certain period, the cavity 
of the vertebral column may be expected to become accurately 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 dimi- 
nution; whilst, on the other hand, animal substances, such as shells, 
the crustaceous parts of animals and other bodies formed by the 
intermixture 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 trochitie, enveloped 
