526 



The Earth a?id Its hihabitmits Cbmige unit x 



Fig. 474 Part of a petrified 

 root and tr^mk from the Pet- 

 rified Forest of Arizona. 



(grant — U. S. DEPARTMENT 

 OF interior) 



Fig. 475 These petrified eggs, laid by a dino- 

 saur ages ago, were found in the Gobi Desert. 



(AMERICAN MUSEUM OK NATURAL HISTORY) 



that it became a nc\\' branch of science 

 known as paleontology (pay-lee-on-tol'- 

 o-jee). Geologists and paleontologists 

 now help one another in reconstructing 

 the history of the earth throughout its 

 various eras. 



Fossils. You can imagine the confusion 

 of the early scientists at finding stone 

 animals and plants! Yet in the fifteenth 

 century the great Italian artist and sci- 

 entist, Leonardo da Vinci, was convinced 

 that these fossils must have come from 

 animals and plants that had once lived. 

 It took a long time to convince the 

 world of this, and longer still to explain 

 how they could turn to stone. The ex- 

 planation is simple. Before sedimentary 

 rock became rock it was soft sand or 

 mud. Any orgranism that died on or 

 above the sand or mud would sink into it. 

 If conditions were favorable the hard 

 parts of the organism remained in the 

 mud without decaying. During the lony 

 period of time during which the sediment 

 changed to rock the bongs or shells of 

 animals and the harder woody portions 

 of plants might also change to rock if 

 mineral matter replaced the parts which 

 were once living organic matter. This re- 

 placing of organic matter by mineral 

 matter is called petrifcictioji. 



Some oriranisms left nothing but an 

 imprint in the sand or mud. By chance 

 the imprint remained, so that we now 

 find it in the rock. There are huge num- 

 bers of such imprints just as there are 



