WHAT MAY BE LEARNED FROM EOZOON 



139 



fourth stage, though this is not uncommon in Silurian and 

 Devonian fossils. I have further to remark that the reason 

 why wood and the cells of corals so readily become silicified is 

 that the organic matter which they contain, becoming oxidized 

 in decay, produces carbon dioxide, which, by its affinity for 

 alkalies, can decompose soluble silicates and thus throw down 

 their silica in an insoluble state. Thus a fragment of decay- 

 ing wood imbedded in a deposit holding water and alkaline 

 silicates almost necessarily becomes silicified. It is also to be 

 remarked that the ordinary specimens of Eozoon have actually 

 not attained to the extreme degree of mineralization seen in 

 some much more recent silicified woods and corals, inasmuch 



a 



UJ 



iin 



f! 



FIG. 13. Diagram showing different States of Fossilization of a cell of 

 a Tubulate Coral, (a) Natural condition walls calcite, cell empty, (b) 

 Walls calcite, celt filled with the same, (c) Walls calcite, cell filled with 

 silica or silicate, (d) Walls silicified, cell filled with calcite. (<?) Walls 

 silicified, cell filled with silica or silicate. 



as the portion believed to have been the original calcareous 

 test has not usually been silicified, but still remains in the state 

 of calcium carbonate. 



With regard, then, to the calcareous organisms with which we 

 have now more especially to do, when these are embedded in 

 pure limestone and filled with the same, so that the whole rock, 

 fossils and cavities, is one in composition, and when meta- 

 morphic action has caused the whole to become, crystalline, 

 and has perhaps removed the remains of carbonaceous matter, 

 it may be very difficult to detect any traces of structure. But 



