INTRODUCTION 9 



though silica, lime carbonate, and iron pyrites are the most 

 common replacing minerals, 30 to 40 others replace more rarely. 



1. Through silicification. — Varying from the merely partial 

 filling of cavities in fossils (even if composed of silica) to entire 

 replacement with silica. Examples : Shells, plants. 



2. Through calcification. — Varying from the merely partial fill- 

 ing of cavities in fossils (even if composed of calcium carbonate) 

 to entire replacement with calcium carbonate. Examples : Most 

 fossil corals, brachiopods, echinoderms, mollusks. 



3. Through pyritization. — Varying from the merely partial 

 filling of cavities to entire replacement with pyrite or mar- 

 casite. Examples : Shells, Crustacea, plants. 



4. Through carbonization. — Animal or vegetable tissue de- 

 composed under water, resulting in a giving off of comparatively 

 more hydrogen and oxygen than carbon, with a resultant con- 

 centration of carbon. Examples : Fish, graptolites, plants. 



II. Objects Indicating the Former Presence of Organisms 



A. Molds, external and internal. 



1. Imprints of shells, feathers, tree trunks, entire animal 

 (as dog), etc. 



2. Tracks of amphibians, reptiles, birds, etc. 



3. Trails of worms, crustaceans, etc. 



4. Burrows of worms, echinoids, mammals, etc. 



B. Coprolites. 



C. Artificial structures. Examples : Birds' nests, early 

 human implements. 



Fossils unaltered (original) . ^ All organic remains last longer 

 in a region of almost continuous dry or cold than in one which 

 is moister and warmer. In very cold, or very dry, regions, even 

 the softer portions may remain- unchanged for a long time, pre- 

 serving to future ages the entire animal. Thus, examples of 

 the ancient elephant — the mammoth — entombed in the ice 

 and frozen earth of Siberia since the Pleistocene, are so well 



