252 REPORT OF NATIONAL MUSEUM, 1892. 



cases the form of some of them has been preserved in fine sediments 

 in the condition of imprints and molds.* Therefore in the study of 

 fossil zoology we are, with the rare exceptions just indicated, confined 

 to an investigation of the skeletal and protective parts of animals, be- 

 cause these parts alone are capable of true fossilization. 



Those parts of the living animal are largely composed of mineral 

 substances, and they are of various kinds and character, some being 

 chitinous and some corneous, but the greater part are composed of 

 lime compounds, the most common of which are bones and shells. They 

 often are of different composition in different families or other divisions 

 of the animal kingdom, and often thus different in different parts of the 

 same animal. Being originally composed of mineral substances in inti- 

 mate association with a small proportion of animal matter, and being 

 usually still further mineralized by replacement of the animal by min- 

 eral matter in the process of fossilization, they become nearly or quite 

 as indestructible as are inorganic minerals. It is, however, true that 

 all kinds of hard parts of animals, even those originally containing the 

 greatest proportion of mineral substance, if exposed continuously to the 

 atmosphere after the death of the animal, will, within a few years at 

 most, become as completely decomposed as will the soft parts. That 

 is, the hard parts of animals may become permanently fossilized under 

 favorable conditions, or they may become as completely decomposed 

 under those that are unfavorable as will the soft parts under all con- 

 ditions. 



Compared with animals, the proportion of the component substance 

 of plants, except that of a few kinds which quickly decompose after 

 death, is very small. Much the greater proportion of the substance of 

 all of them, aside from w T ater, is carbonaceous and comparatively slow to 

 decompose, but none of it resists decomposition so fully as does most of 

 the skeletal and protective parts of animals. Still, the complete decom- 

 position of all plants is certain unless they fall under conditions which 

 are specially favorable to their preservation. Therefore in the study of 

 fossil botany we are confined to an investigation of imprints, mostly of 

 leaves, and of such of the woody parts as may have become antiseptically 

 changed by saturation with certain acids or with soluble salts, or com- 

 pletely mineralized by a process to which I have applied the term histo- 

 metabasist. Immense quantities of vegetable substance have in past 

 geological time been accumulated and reduced to the fixed condition of 

 carbon and thus permanently preserved in the form of coal, but this 

 substance has seldom been found of material use in the study of fossil 

 botany. 



See the close of this essay for an explanation of the different forms and con- 

 ditions i]i which fossils are found and the different methods by which they have 

 reached those conditions and acquired those forms. 



tSee remarks on conditions of fossilization at the close of this essay. 



