FOSSILS, AND HOW THEY ARE FORxMED 9 



and sand would keep on accumulating until 

 an enormously thick layer was formed. The 

 lime or silica contained in the water would 

 tend to cement the particles of mud and grains 

 of sand into a solid mass, while the process 

 would be aided by the pressure of the overly- 

 ing sediment, the heat created by this press- 

 ure, and that derived from the earth beneath. 

 During this process the animal matter of bones 

 or other objects would disappear and its place 

 be taken by hme or silica, and thus would be 

 formed a layer of rock containing fossils. The 

 exact manner in which this replacement is 

 effected and in which the chemical and me- 

 chanical changes occur is very far from being 

 definitely known — especially as the process of 

 " fossiUzation " must at times have been very 

 compHcated. 



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 replacement of 

 the original vegetable by mineral matter, the 

 interior of the cells being first filled with silica 

 and their walls replaced later on. So com- 



