378 



PROGRESSIVE DEVELOPMENT 







Part of a fossil backbone. 



Am. Museum of Nat. History 

 Fossil footprints. 



The age of the earth. 

 Before examining the 

 evidences of evolution 

 of present living 

 things, it is necessary 

 to understand some- 

 thing about Paleon- 

 tology, a science which 

 deals with the life of 

 past geological 

 periods. Geological time includes all the time since the earth first 

 started to be formed. Geologists have estimated the age of the 

 earth by the relative age of various layers of rocks and metals. For 

 example, the presence of a form of lead has been found in a Nor- 

 wegian mineral composed chiefly of uranium. Assuming that the 

 production of uranium ceased as soon as the earth and sun sepa- 

 rated, geologists have agreed that this lead is about three billion 

 years old. This was determined by estimating the length of time 

 necessary for a small amount 

 of metal uranium to break up 

 into this form of lead. From 

 this time and amount of metal, 

 the age of the lead and, there- 

 fore, the earth was calculated. 

 The cliffs in a valley will 

 show that rocks are laid in 

 strata or layers. The succes- 

 sive layers of the rock suggest 

 that they were deposited one 

 after another from the bottom 

 upward. The top stratum is 



the mOSt recent One deposited. From drawing Museum Nat.Hist. 



TO -1- ' J* J i i P i A. colossal, hornless rhinoceros of prehistoric 



Each individual layer or rock days . 



