138 WHAT MAY BE LEARNED FROM EOZOON 



I have seen even the minute cells of woody structures, each 

 with several bands of differently coloured materials deposited 

 in succession, like the coats of an onyx agate. 



A further stage of mineralisation occurs when the substance 

 of the organism is altogether removed and replaced by foreign 

 matter, either little by little, or by being entirely dissolved or 

 decomposed, leaving a cavity to be filled by infiltration. In 

 this state are some silicified woods, and those corals which 

 have been not filled with but replaced by silica, and can thus 

 sometimes be obtained entire and perfect by the solution in 

 an acid of the containing limestone, or by its removal in 

 weathering. In this state are the beautiful silicified corals ob- 

 tained from the corniferous limestone of Lake Erie, which are 

 so perfectly replaced by flinty matter that when, weathered out 

 of the limestone, or treated with acid till the latter is removed, 

 we find the coral as perfect as when recent. It may be well 

 to present to the eye these different stages of fossilization. I 

 have attempted to do this in Fig. 13, taking a tabulate coral of 

 the genus Favosites for an example, and supposing the material 

 employed to be calcite and silica. Precisely the same illustra- 

 tion would apply to a piece of wood, except that the cell wall 

 would be carbonaceous matter instead of carbonate of lime. 

 In this figure the dotted parts represent carbonate of lime, 

 the diagonally shaded parts silica or a silicate. Thus we have 

 in the natural state the walls of carbonate of lime and the 

 cavities empty {a). When fossilized the cavities may be merely 

 filled with carbonate of lime, or they may be filled with silica 

 (^, c) ; or the walls themselves may be replaced by silica, and 

 the cavities may remain filled with carbonate of lime {d) ; or 

 both the walls and cavities may be represented by or filled 

 with silica or silicates (e). The ordinary specimens of Eozoon 

 are supposed to be in the third of these stages, though some 

 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 



