8 AN INTRODUCTION TO THE STUDY OF FOSSILS 



replacement, to the formation of external molds or to the pro- 

 cess of carbonization (page 13). 



Classification of Fossils. — A fossil is the remains oj a plant 

 or animal, or the record of its presence, preserved in the rocks of 

 the earth. 



The word is derived from the Latin fodere, to dig, and hence 

 has associated with it the thought of something dug up, for in 

 most cases the preservation of a form depends upon its burial. 

 Many geologists restrict the term fossil so as to include evidences 

 of life to the close of the Pleistocene only. According to this 

 definition all post-Pleistocene remains are spoken of as recent, 

 not fossil. 



Fossilization is the sum of the phenomena by which the re- 

 mains of animals or plants, or the evidences of their presence 

 are preserved in the earth's strata. 



Fossils may be divided according to the above definition into, 



I. Fossilized remains of organisms. 



II. Objects indicating the former presence of organisms. 

 Sometimes these fossils preserve their original composition 



as when first buried, that is, they are unaltered. At other times 

 they become more or less completely altered by infiltration of 

 minerals from the surrounding rocks, or the more volatile parts 

 are gradually given off, leaving a residue composed very largely 

 of carbon. It is to such fossils only that the term petrifaction, 

 i.e. " turned to stone," should be applied. 



I. Fossilized Remains of Organisms 



A. Unaltered (i.e. original). 



1. Originally soft portions of the animal preserved. Ex- 

 amples : insects in amber ; mammoths in frozen earth of 

 Siberia. 



2. Only hard parts preserved. Examples: many Ceno- 

 zoic shells, bones, teeth, horns. 



B. Altered {i.e. petrifications) ; these are divided into four 

 main divisions according to the material petrifying them. Al- 



