398 THE CHANGING GENERATIONS 



B. Lesions and deformities. Such as bone lesions, giving evidence of microorgan- 

 isms causing disease, and malformations due to the presence of animal 

 parasites. (Relatively rare.) 



C. Habitations and artifacts. Worm tubes, the cases of insect larvae, wood show- 

 ing insect tunnels, mud nests built by insects, mammal burrows, and the 

 like. This category also must include tools and ornaments made and used by 

 prehistoric man. (Relatively rare.) 



III. Fossil-like objects (pseudofossils) . Objects are occasionally found in the rocks 

 that are mistaken by untrained persons for fossils or are spoken of by geologists 

 as "fossils" through analogy. Among the first class are chemically-formed nodules 

 called concretions, which sometimes bear a striking resemblance to organic 

 structures. 1 In the second class are such objects as "fossil " raindrop impressions, 

 "fossil" ripple marks, "fossil" stream courses, and other normally ephemeral 

 features of the physical world that have been preserved in some manner. 



1 An example is the so-called "dinosaur backbone," shown at the bottom of Silver 

 Springs, Fla., by the guides. This is apparently the edge of a very large flint concre- 

 tion. Dinosaurs had long been extinct when the upper Eocene Ocala limestone (in 

 which the spring arises) was formed beneath the sea. 



