162 GEOLOGY. 



fossils. The term comes from the Latin word which 

 means to dig. The remains are generally more or less 

 defective, the softer parts being removed, and only por- 

 tions of those which are hard being preserved, although 

 in some cases there has been entire preservation, as in 

 the case of ancient mammoths found imbedded in frozen 

 earth in Siberia. Sometimes a fossil is a mere trace, as 

 of a leaf, or simply a track of an animal. Sometimes in 

 the fossil there is not a particle of the original organized 

 substance, as when perfect petrifaction takes place (118). 

 The mineral substance in these petrified fossils differs in 

 different cases. The most common substances are silica, 

 carbonate of lime, and clay. Analogous to petrifaction 

 is the substitution of some mineral substance for the 

 whole of an organic body, as a shell, the new substance 

 making merely a cast of it, This differs from a petrifac- 

 tion in that there is no trace of the texture of the shell 

 in the cast. Coal, strictly speaking, is a fossil, for it is 

 the remains of the vegetable substances from which it 

 came, their texture being retained in it more or less, as 

 shown in 41. 



There is much difference between different organized 

 substances in regard to their preservation as fossils. 

 Bones are very durable, but the bones of birds are sel- 

 dom found, because they were borne up by their feath- 

 ers as they decayed, and the bones, being hollow, were 

 easily broken. Their tracks, however, have been exten- 

 sively observed in the layers of rock, as you will see far- 

 ther on, and the investigation of these, especially in this 

 country, by Hitchcock and others, has furnished one of 

 the most interesting chapters in geology. Insects, too, 

 are seldom found, because they are so light, and decay 

 so rapidly from the presence of air in the air tubes that 

 pervade their bodies. Such hard substances as shells 

 are more largely preserved than any other substances. 



246. Abundance of Fossils. Although many organ- 

 ized substances, from the softness of their structure, 



