06 



THE DAWN OF LIFE. 



Thus we have^ in the natural state^ the walls of car- 

 bonate of lime and the cavities empty. When fossil- 

 ized the cavities may be merely filled with carbonate 

 of lime, or they may be filled with silica ; or the walls 

 themselves may be replaced by silica and the cavities 

 may remain filled with carbonate of lime; or both, 

 the walls and cavities may be represented by or filled 

 with silica or silicates. The ordinary specimens of 

 Eozoon are in the third of these stages, though some 



a 



ind 



If « » » 





i! 



I > » » » « f 



;C 



'■/■///y^\ 



a 



Fig. 22. Diagram showing different States of Fossilization of a Cell 

 of a Tabulate Coral. 



a.) Natural condition — walls calcite, cell empty, (b.) Walls calcite, cell filled 

 with the same, (c.) Walls calcite, cell filled with silica or silicate, (d.) Walls 

 silicified, cell filled with calcite. (e.) Walls silicified, cell filled with sihca 

 or silicate. 



exist in the second, and I have reason to believe that 

 some have reached to the fifth. I have not met with 

 any in the fourth stage, though this is not uncommon 

 in Silurian and Devonian fossils. 



With regard to the calcareous organisms with which 

 we have now to do, when these are imbedded in pure 

 limestone and filled with the same^ so that the whole 

 rock, fossils and all, is identical in composition, and 

 when metamorphic action has caused the whole to 

 become crystalline, and perhaps removed the remains 

 of carbonaceous matter, it may be very difficult to 



