THE PRESERVATION OF EOZOOX. 95 



Often they are stained with oxide of iron or coaly 

 matter. They may consist of carbonate of lime, silica 

 or silicates, sulphate of baryta, oxides of iron, car- 

 bonate of iron, iron pyrite, or sulphides of copper or 

 lead, all of which are common materials. They are 

 sometimes so complicated that 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 mineralization 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 converted into 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 beauti- 

 ful silicified corals obtained from the corniferous lime- 

 stone of Lake Erie. It may be well to present to 

 the eye these different stages of fossilization. I have 

 attempted to do this in fig. 22, taking a tabulate 

 coral of the genus Favosites for an example, and 

 supposing the materials employed to be calcite and 

 silica. Precisely the same illustration 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. 



