4 HISTORICAL GEOLOGY 



to light, it must be evident that, even where conditions of fossili- 

 zation were most favorable, only a small part of the life of any 

 period is represented by its fossils. Comparatively few remains 

 of organisms now inhabiting the earth are being deposited under 

 conditions favorable for their preservation as fossils. So it has 

 been throughout the long periods of earth history, though the fossils 

 in the rocks known and unknown are a fair average of the groups 

 of organisms to which they belong. In spite of such imperfec- 

 tions in the life record, it is, nevertheless, remarkable that so vast 

 a number of fossils are embedded in the rocks, and from these we 

 are enabled to draw many fundamental conclusions regarding the 

 history of life on our planet. 



Preservation of fossils. — 1. Preservation of the entire organism 

 by freezing. Fossilization by this method is rare, though remark- 

 able examples are afforded by extinct species of the Mammoths 

 and Rhinoceroses, the bodies of which, with flesh, hide, and hair 

 intact, have been found in frozen soils in Siberia. 



2. Preservation of the entire organism by natural embalmment. 

 Fine examples are the perfectly preserved Insects in the famous 

 amber of the Baltic Sea region. This amber is a hardened resin, 

 the Insects having been caught in it while it was still soft and exud- 

 ing from the trees. 



3. Preservation of only the hard parts of the organisms. This is 

 a very common kind of fossilization in which the soft parts have 

 disappeared by decomposition, while the hard parts, such as bones, 

 shells, etc., remain. Fossils of this kind are abundant in rocks of 

 later geological time, though original shell material is frequently 

 found, even in very ancient rocks. 



4. Preservation of carbon only (carbonization) . This is particu- 

 larly true of plants where, as a result of slow chemical change or 

 decomposition, the hydrogen and oxj^gen mostly disappear, leav- 

 ing much of the carbon, but with the original structure often 

 beautifully preserved. Many excellent examples are furnished 

 by the fossil plants of the great coal (Pennsylvanian) age. 



5. Preservation of original form only (casts and molds) . Fossils 

 of this class, which are very abundant, show none of the original 

 material, but only the shape or form has been preserved. When 

 a fossil becomes embedded in sediment, which hardens around the 

 entire organism or any part of it, and the organism then decom- 

 poses or dissolves away, a cavity only is left and this is called a 



