Conclusion 201 



taking them up before frost, whon no man can 

 work in those beds. It will not do to let plaster 

 freeze, and without plaster we could not take up 

 any vertebrate fossil there. 



Owing to the fact that the clay in the strata 

 prevent water entering it, very little true petrifi- 

 cation has taken place. If you will refer to the 

 Life of a Fossil Hunter, page 258, you will see 

 there what I had learned up to the time of writ- 

 ing, the process by which fossils are made. I 

 ing, of the process by which fossils are made. I 

 all fossils. I found here in the Belly River 

 Series entirely different conditions. The bones 

 had not been replaced by silica and become petri- 

 fied. There was very little change in the bones 

 except that they \vere usually sheathed in a hard 

 layer of bog-iron. The spongy bone was as friable 

 as that in a dry, recent bone; the cells were not 

 filled with rocky material. The thin outer layer 

 of compact bone was filled with the iron simply. 

 I once said that if I could get my teeth on a 

 fossil bone I could tell its age almost, by the 

 amount of silica it contained. Here, however, I 

 find that nature has more than one way of pre- 

 serving her records, and that it depends largely 

 on the matrix in which the bones are entombed. 

 If clay prevents water passing through the bones, 

 there can be no true replacement, as water is the 

 vehicle used in transporting silica or lime or 

 whatever the petrifying material may be, and it 

 cannot pass through certain clays. This dis- 



