478 



Tllli: CHANGING WORLD 



Mammoths preserved in arctic ice. 



amber, which is hardened Oligocene pitch ; or mammoths frozen 

 centuries ago in arctic mud and ice, with no opportunity since then 



to thaw out, of which 

 at least a score of 

 authentic instances 

 are known. 



Petrifactions of bone 

 or shell or wood are 

 another kind of fossils, 

 formed by the filtra- 

 tion of dissolved min- 

 erals into spaces left 

 after the decay of the 

 original organic mat- 

 ter. In such fossils 

 the inorganic part has 

 resisted disintegration 

 long enough to serve as a matrix or a mold, and thus to preserve the 

 original shape. Sometimes the mineral replacement of minute parts 

 may be so gradual and complete that the bone or shell or tree-trunk 

 is said to be "turned to stone," often with histological details faith- 

 fully retained. Limestone is often composed of innumerable shells 

 of minute organisms, such as foraminifera and the skeletons of corals 

 that extract from the water the necessary calcareous materials. 



Still other types of fossils are 

 casts and molds in which the 

 organisms or parts of them re- 

 main undestroyed long enough 

 to permit the taking of a perma- 

 nent death mask of some kind, 

 which is then all that is finally 

 preserved. Some beautiful ex- 

 amples, which may reproduce in 

 great detail the character of the 

 original, are impressions of ferns 

 and leaves, or of insect wings, 

 occasionally to be found when 

 shale or slate rock is split open. 

 Under favorable circum- 

 stances, tracks and trails left by A Paleozoic fernlike plant. 



