PART IV 



HISTORICAL GEOLOGY 



CHAPTER XX 

 FOSSILS 



A fossil is the impression or remains of an animal or plant 

 which has been entombed in the rocks by natural causes. 



A knowledge of fossils is indispensable to the geologist because 

 they give him the means of establishing a consecutive chronology 

 of the earth, and teach him much concerning the changes of 

 land and sea, of climate, and of the distribution of living things 

 upon the globe. To comprehend the lessons taught by fossils, it 

 is essential not only that the student should familiarize himself 

 with actual specimens, but also that he should have some ac- 

 quaintance with the elements of zoology and botany, else he can- 

 not appreciate the distinctions which obtain between the fossils of 

 widely separated periods of time. 



I. How Fossils -were embedded in the Rocks 



The conditions of the preservation of fossils are much more 

 favourable to some kinds of organisms than to others. It is only 

 under the rarest circumstances that soft, gelatinous animals, which 

 (like jelly-fish) have no hard parts, can leave traces in the rocks. 

 The vast majority of fossilized animals are those which have hard 

 shells, scales, teeth, or bones ; and of plants, those which contain 

 a sufficient amount of woody tissue. 



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