CONDITIONS OF FOSSILIZATION 



271 



Fig. 2. — -Chonostegites clap pi. 

 (After Edwards and Haime.) 



CONDITIONS IN WHICH FOSSILS OCCUR 



The following illustrations will show the various conditions in 

 which fossils are found, and they will also serve to show the necessity 

 for close observation and comparative study. 



A. The original skeleton may be preserved. If there be hollows 

 or spaces, they may become filled 

 with infiltrating material. In 

 a case of this kind little difficulty 

 will present itself in the determina- 

 tion of the fossil. 



B. The original skeleton may 

 be replaced by some mineral and 

 the cavities filled with the same, or 

 some other material. 



If the skeleton were composed 

 of aragonite, and were replaced 

 by calcite, the external form and 

 markings would be preserved; but 

 the internal organic structure would 

 be lost, and hence not seen under .the microscope. In this case we 

 should depend upon external markings for identification. 



If a skeleton composed of calcite 

 were replaced by pyrite, we could not 

 ascertain in thin section whether the 

 internal organic structure were lest or 

 preserved, because of the opaqueness 

 of the pyrite, and again we should 

 depend upon external markings. A 

 broken section, however, will show on 

 the fractured surface the minutest 

 details, in many cases. 



If the organism were replaced molec- 

 ularly by a mineral which transmits light, the internal organic 

 structure would be so well preserved as to be readily distinguished 

 under the microscope. In a case of this kind we should have a 

 double check — the internal as well as the external structure — and 

 its identification would be doubly sure. 



Fig. 3. — Chonostegites 

 clappi. (After Miller.) 



