LAURKNTIAN LIFE 



I I I 



of the animal which may be supposed to have dis- 

 appeared by decay before or during the mineraliza- 

 tion of its skeleton. 



So long as the imbedding mass continues soft 

 and incoherent, shells, corals, etc., can be recovered 

 in a condition similar to that of recent specimens, 

 except that they may have become bleached in colour 

 and brittle in texture, owing to the removal of organic 

 matter intimately associated with the lime, and that 

 their cavities may have been filled with sand or silt 

 washed into them, or with calcite or calcareous spar 

 introduced in solution in water. But if the contain- 

 ing mass has become a hard stone, the material 

 filling the interior of our shell or coral has exi)e- 

 rienced a similar change ; and when we break open 

 the stone, we may obtain the specimen, now hard, 

 solid, and heavy, but still showing more or less 

 of its outer surface and markings, and possiljjy in 

 some extent also its internal structure when it is 

 sliced and studied under the microscope. Ikit if the 

 whole mass has been metamorphosed, and has be- 

 come crystalline, the contained fossil and its contents 

 may have experienced a similar change, and may 

 have so coalesced with the containing matrix that it 

 is no longer separable from it. Even in this case, 



