PRESERVATION OF HARD STRUCTURES 1079 



Sometimes various minerals, as pyrite, or chlorite, or even talc, 

 replace them. The same thing may be said of the cellulose com- 

 ])osing the tissues of plants, where decomposition is a slower process 

 than in the fleshy tissues of animals. The cell structure of plants 

 may thus be conserved for a long period, and tliis is especially the 

 case where there is a nearly complete exclusion of air, as in fine 

 sediments or in peat bogs. 



The first requisite in fossilization is the burial or inhumation 

 of the remains. Even the hard parts of animals will be destroyed 

 if exposed too long to the atmosphere. Thus the bones of the 

 American bison, which, during the process of extinction that this 

 animal was undergoing on the western plains, were abundantly 

 scattered about, are fast disappearing by decay, so that shortly 

 no traces of them will remain except where they have been buried.* 

 This fact must be borne in mind in considering the remains of 

 earlier mammals. Those found can constitute but a small portion 

 of the skeletons once scattered about but which disintegrated before 

 the slow process of burial by continental waste or by dust saved 

 them. The soft tissues of animals and the tissues of plants decay, 

 of course, rapidly, and even inhumation, except in the cases noted 

 above, will not check the process of decay. Immersion in water 

 likewise results in the decay of organic matter, for bacteria here 

 become an active agent in the dissolution of tissues. Hard struc- 

 tures, such as shells or bones, will also suffer destruction by solu- 

 tion, especially if the waters are rich in carbon dioxide. Thus, as 

 already noted in an earlier chapter, the shells of many Protozoa 

 are dissolved after the death of the animal before they settle down 

 to the abyssal portions of the sea, and hence deposits of these shells 

 are generally absent from the greater deeps, though abundant in 

 regions of lesser depth. Solution may continue even after burial 

 if the beds are raised above the sea-level, and if they are per- 

 meable. 



The buried hard parts of animals generally undergo a process 

 of petrifaction, which most commonly is either calcification or 

 silicifi cation, or sometimes the first followed by the second, i. e., 

 a replacement of the lime by silica, or. more rarely, the reverse. 

 Pyritization, or the replacement of the remains by iron pyrites 

 (or, more frequently, by marcasite) and replacement by iron oxide, 

 sphalerite, barite, vivianite, glauconite, or other minerals, also 

 occurs. The process of replacement differs in different groups 

 of organisms. 



* A certain percenta.£;e of these bones, however, has been picked up and 

 burned for commercial and other purposes. 



