'2fi4 REPORT <>K NATIONAL MUSEUM, 1892. 



The following explauatiou of the different conditions in which fossil 

 remains arc found will supplement the preceding discussions and add 

 to the description of their character which has already been given. 



There are seven different natural conditions in which fossil remains 

 are recognizable, three of which relate to substance, three to form, and 

 one to both. To those relating to substance 1 have applied the terms 

 permineralization, histometabasis, and carbonization: to those relating 

 to form, the terms molds, imprints, and casts; and to the one relating 

 to both form and substance, the term pseudomorphism. 



The term permineralization applies to that condition of fossil remains 

 of animals which <liffers least from their original condition as parts of 

 living animals, such, for example, as bones of vertebrates, shells of 

 mollusks, tests of crustaceans, etc. It is in this condition that the 

 greater part of all fossil remains are found. In their original condition 

 they were all composed of both mineral and animal matter. Mineral 

 matter greatly preponderated in allot them, but the proportions differed 

 much in the case of different branches of the animal kingdom. For 

 example, the proportion of animal matter is much greater in bones, 

 even in their most solid portions, than in shells of mollusks or tests of 

 most crustaceans. In all cases, however, the proportion of mineral mat- 

 terwas sufficient to perfectly preserve the original form of each specimen 

 during the process of fossilization. Their only material change in this 

 process was the removal by decomposition of the animal matter and 

 its replacement by mineral matter, the latter having been added as a 

 precipitate from its solution in the waters in which the fossilization 

 took place. This having been continued until all the minute inter- 

 stices originally occupied by the animal matter were filled, the fossils 

 became wholly mineralized and as indestructible as are other minerals 

 of like composition. Indestructibility of these fully mineralized fos- 

 sils, however, is not in all cases absolute, as will appear by remarks in 

 following paragraphs. 



The term histometabasis* is applied to that condition of fossilization 

 in which an entire exchange of the original substance for another has 

 occurred in such a maimer as to retain or reproduce the minute and 

 even the microscopic texture of the original. It is especially applicable 

 to silicified wood. In such cases of fossilization the exchange has 

 been made by destructive decomposition, molecule by molecule, of the 

 woody tissues and their immediate replacement by precipitated mole- 

 cules of the silex held in solution in the water in which the wood was 

 immersed. By this remarkable process not only the original cell struc- 

 ture of various kinds of wood but the characteristic cell markings of 

 each kind are often found to have been so perfectly preserved in the 

 solid agate-like mass that it may be as completely studied as if the 

 specimens were taken from living trees. 



Etym. : cafb'c, tissue; /mraf-idmr, exchange. 



