396 



THE CHANGING GENERATIONS 



Fossils. The life of the past has left a great variety of records, and 

 we can define a fossil as being any trace of prehistoric life. For obvious 

 reasons fossils are never found in igneous or in strongly metamorphosed 

 rocks. Most fossils have been preserved by burial in sediments, and 

 many sedimentary rocks are literally filled with them. The things most 

 commonly preserved are the hard parts of organisms, such as shells, 

 bones, and teeth. Under exceptional circumstances even the more perish- 

 able soft parts may leave imprints on fine-textured muds, or be preserved 

 by petrifaction, by impregnation with tar or resin, or by other means. 

 Tracks left by worms or dinosaurs, mud-filled burrows, fossil excre- 

 ment, tooth marks on bones and 

 holes drilled by snails in mollusk 

 shells, deformities produced by 

 parasites, and even objects made 

 or used by animals, including 

 ancient man, must all be regarded 

 as fossils. Because fossils are so 

 varied, both with respect to what is 

 preserved and the means of pres- 

 ervation, the following scheme of 

 classification will prove helpful. 



I. Fossils giving evidence as to the 

 form or structure of the organism : 

 A. With preservation of both 

 form and structure. 

 1. Preservation of the actual 

 remains, unaltered. 



a. By simple burial of hard 

 parts. (Common.) 



b. By freezing. (Rare; Siberian 

 mammoths, etc.) 



c. By desiccation. (Rare; 

 ground-sloth mummies in 

 caves, etc.) 



2. Preservation of the actual remains, altered by impregnation 



a. With mineral substances, commonly lime or silica, filling the pores of 

 bones and other hard parts; permineralized fossils. (Common; fossil 

 bones that are heavier and denser than fresh bones, etc.) 



b. With asphalt. (Rare; animals trapped in tarpits.) 



c. With resin, which, on hardening, turns to copal or amber. (Rare; 

 beautifully preserved amber and copal insects, flowers, and other 

 minute fossils, mostly of Tertiary age. It should be noted, however, 

 that many amber fossils are really molds, the impregnation not having 

 been complete and the original material having almost disappeared.) 



3. Petrifaction, or molecular replacement of the original substance with 

 another material, commonly silica, giving rise to fossils in which fine struc- 

 tural details are preserved, occasionally extending to microscopic cellular 



Fig. 25.3. An aquatic relative of the scor- 

 pions, Eurypterus lacustris, preserved as an 

 imprint in Silurian rocks at Buffalo, N.Y. 

 (Courtesy Carnegie Museum, Pittsburgh.) 



