IN THE AlUD OF THE LEVANT. 113 



east of England,"* and that whilst it has invested 

 some objects, it has filled the cavities of others, 

 and shewn a manifest preference for combining 

 with, and replacing animal substances. This, 

 however, is as yet a very obscure and difficult 

 subject; one that must probably be treated very 

 differently according to the strata we may be 

 examining. 



In some cases, as in the examples of silici- 

 fied fossil woods, the flint appears to have been 

 deposited atom by atom, since, though the carbon 

 is replaced by silica, all the original microscopic 

 structure appears to be preserved. At the same 

 time, all the interstices and fissures in the wood 

 are often filled up by clear chalcedony, which 

 bears every appearance of having run into the 

 fissures in a fluid state. 



At other times the original elements of the 

 organism have been wholly or partially removed, 

 leaving a cavity, which has been filled up by 

 infiltration of siliceous matter, subsequent probably 

 to the consolidation of the rock ; the interior 

 of such organisms, when the filling up has not 



* Annals and Mag. Nat. Hist. August, 1845. 

 Q 



