101 



matter in the fossil. The flesh of animals, ;is already noted, 

 will commonly disappear quickly, not even an impression 



remaining. The presence of petroleum in corals and in 

 cavities of shells, however, indicates a probable result of the 

 decomposition of buried fleshy portions of animals. A more 

 complete preservation of soft organic tissues is brought 

 about by the process of 



CARBONIZATION. 



This occurs in plants and in those animals which have a 

 chitinous skeleton, e. g., Hydrozoa; a carbonaceous film, 

 seldom showing structure, will usually remain. 



The most important changes which buried hard structures 

 of animals undergo are as follows : 



1. Infiltration. 



Skeletal structures are commonly more or less porous, 

 and the first change which is likely to occur is the filling of 

 the pores by mineral matter, usually by carbonate of lime, 

 though infiltration of silica often occurs. The filling of the 

 pores occurs upon the decay and removal of the perishable 

 organic matter which occupied them. The structure then is 

 solid, and completely "petrified" or turned to stone. 



Fossils made solid by infiltration of carbonate of lime are 

 commonly among the best preserved organic remains, for 

 they will retain, in an unaltered condition, the minutest 

 structural features. This is the normal condition of the fos- 

 sils in limestones and shales, and hence specimens obtained 

 from such rocks are in the best condition for critical study. 

 All classes of organisms are, however, not equally well pre- 

 served by this process. Very porous structures, such as the 

 plates of echinoderms (e. g., crinoid stems and plates), are 

 commonly affected by the crystallization of the infiltrated 

 calcite. Such crystallization usually affects the whole plate, 

 thus obliterating the original microscopic structure, though 

 the external form may be perfectly retained. 



