4 XORTH AMERICAN INDEX FOSSILS. 



impression made by the animal, as in case of the molds of a shell. 

 Burrows of worms or other animals are more or less permanent 

 homes, and are often reenforced by walls made of agglutinated sand 

 particles. They then partake of the character of artificial struc- 

 tures, and they may be considered as intermediate between classes 

 2 and 3. 



3. Artificial Structures. — Here belong the tubes built of agglu- 

 tinated sand grains by worms, the " shells " built of similar foreign 

 particles by many Foraminifera, and more especially the imple- 

 ments, habitations and other structures left by primitive man in the 

 later rocks of the earth's crust, as well as the structures buried in 

 historic times. 



4. Coprolites. — The excrements of many animals are character- 

 istic, serving to determine their presence in the formation in ques- 

 tion. Typical examples of these are the coprolites of Jurassic and 

 Cretacic reptiles. 



Mode of Preservation.* 



Under this heading we need to consider only the preservation of 

 the actual remains (hard parts) of animals and plants. 



Burial and Removal of Organic Matter. — The first step in fos- 

 silization is of course burial, for without that even the hard parts 

 will soon be destroyed. This is well illustrated by the disappear- 

 ance, largely through decay, of the bones of the American bison, 

 which were left unburied in countless numbers on the western 

 plains. It is also illustrated by the annual decay of the fallen 

 leaves. After burial the organic tissue which forms the frame- 

 work of all hard animal structures is removed by decay, thus ren- 

 dering the remains more or less porous. This is well seen when a 

 fresh bone is compared with one buried for some time. Tertiary 

 shells often show the porosity due to the removal of the organic 

 framework. This porosity is indicated by adhesion to the tongue, 

 to the touch of which fresh shells feel perfectly smooth. 



Infiltration. — Sooner or later the pores left by the removal of 

 the organic matter are infiltrated with lime, silica or other mineral 

 matter, the organism thus becoming completely petrified or turned 

 to stone. When silica infiltrates it tends to replace the original 



*For a full discussion of this see Grabau, A. W., " Principles of Stratigraphy. 



