6 ANIMALS OF THE PAST 



During this process the animal matter of bones or other 

 objects would disappear and its place be taken by lime 

 or silica, and thus would be formed a layer of rock 

 containing fossils. The exact manner in which this re- 

 placement is effected and in which the chemical and me- 

 chanical changes occur is very far from being definitely 

 known especially as the process of " f ossilization " 

 must at times have been very complicated. 



In the case of fossil wood greater changes have taken 

 place than in the fossilization of bone, for there is not 

 merely an infiltration of the specimen but a complete re- 

 placement of the original vegetable by mineral matter, 

 the interior of the cells being first filled with silica and 

 their walls replaced later on. So completely and 

 minutely may this change occur that under the micro- 

 scope the very cellular structure of the wood is visible, 

 and as this varies according to the species, it is possible, 

 by microscopical examination, to determine the rela- 

 tionship of trees in cases where nothing but fragments 

 of the trunk remain. 



The process of fossilization is at best a slow one, and 

 soft substances such as flesh, or even horn, decay too 

 rapidly for it to take place, so that all accounts of 

 petrified bodies, human or otherwise, are either based 

 on deliberate frauds or are the result of a very erroneous 

 misinterpretation of facts. That the impression or cast 

 of a body might be formed in nature, somewhat as casts 

 have been made of those who perished at Pompei, is 

 true ; but, so far, no authentic case of the kind has come 

 to light, and the reader is quite justified in disbelieving 

 any report of "a petrified man." 



Natural casts of such hard bodies as shells are com- 

 mon, formed by the dissolving away of the original shell 

 after it had become enclosed in mud, or even after this 



