BULLETIN 39, UNITED STATES NATIONAL MUSEUM. [6] 



CONDITIONS IN WHICH KOSSILS OCCUR. 



The conditions in which fossils occur in nature are various, and it is 

 important that the collector of extinct plants and animals should be 

 familiar with them. This subject has received much attention from Dr. 

 Charles A. White, an honorary curator of the j^ational Museum, and 

 his observations are given in his valuable essay on "The relation of 

 biology to geological investigation," which because of its completeness is 

 here quoted at length. He writes : ^ 



There are seven different natural conditions in wliich fossil remains are recogniz- 

 able, three of which relate to substance, three to form, and oiie to both. To those 

 relating to substance I have apjilied the terms permineralizatiou, histometabasis 

 [substitution], and carbonization; to those relating to form, the terms molds, 

 imjirints, and casts ; and to the one relating to both form and substance, the term 

 pseudomorphism. 



Permineralized fossils. — The term permineraliz;ation applies to that condition of 

 fossil remains of animals which differs 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 min- 

 eral and animal matter. Mineral matter greatly preponderated in nearly all of 

 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 matter was suf- 

 ficient 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 lat- 

 ter 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 tilled, the fossils became 

 wholly mineralized and as indestructible as are other minerals of like composition. 

 Indestructibility of these fully mineralized fossils, however, is not in all cases abso- 

 lute, as will appear by remarks in following jiaragraphs. 



Fossils in which the original mineral substance is exchanged for another. — 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 manner as to retain or 

 reproduce the minute and even the microscopic texture of the original. It is espe- 

 cially 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 molecules of the silex held in solu- 

 tion in the water in which the wood was immersed. By this remarkable process not 

 only the original cell structure 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. 



Fossil pseudomorjihs. — Pseudomorphism of fossils is so nearly like that of mineral 

 crystals that this term is equally applicable to both. It consists in the replacement 

 of the original substance of the fossil by a crystallizable or crystallized mineral, 

 such, for example, as calcite, pyrifce, quartz in the form of chalcedony, etc., the 

 original form of the fossil being perfectly retained. It is evident that at least a part 

 of the crystallized pseudomorphs were formed by the precipitation of the compo- 

 nent mineral from its solution within such cavities as are described as molds in 



1 Proc. U. S. Nat. Mus., xv, 1892, pp. 264-267. 



