FOSSILS, AND HOW THEY ARE FORMED 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 lime 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 
*“ fossilization”” 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 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- 
