PALAEONTOLOGY. 131 



spar; in other cases the replacing material is Iron- 

 pyrites, Selenite, or Silica.] 



The agency which produces such changes is simply 

 water charged with carbonic acid and holding in solution 

 the various mineral substances above mentioned ; this 

 acidulated water, percolating through the rocks, slowly 

 and gradually dissolves away the carbonate of lime com- 

 posing the shell, and in many cases replaces it particle 

 by particle with the other mineral which it happens to 

 contain in solution, and which is thus exchanged for the 

 carbonate of lime ; in other cases the material of the 

 shell may have been entirely removed without any con- 

 comitant replacement, the calcite, silica, or other mine- 

 ral having been subsequently introduced into the vacant 

 space thus left, and filling it up as it would any other 

 cavity in the rock. 



But to explain the change of structure in some cal- 

 careous shells which are not wholly replaced, it seems 

 necessary to suppose that the percolating water con- 

 tained a saturated solution of carbonate of lime, some 

 of which crystallized out in the cellular interspaces of 

 the shell, binding the whole and imparting to it a crys- 

 talline structure, just as the sand of Montmartre, being 

 infiltrated by carbonate of lime, is gathered together by 

 crystalline action into rhombohedral prisms, taking the' 

 form of calcite crystals. 



Sand 4. Casts and Impressions. The internal cast and 

 external impression may be considered together, as they 

 nearly always exist, whether the shell be unaltered, re- 

 placed, or removed ; they are of course very much better 

 seen a"nd realized in the latter case, when the cast is 

 loose or nearly so, and may be taken out of the cavity,. 



92 



