THE AGE OF THE EARTH 



379 



From drawing Museum Nat. Hist. 

 The shovel-tusked mastodon used 



constitutes a record of the time when 

 it was deposited. The thickness of 

 the stratified rocks now exposed upon 

 the earth surfaces of the continents is 

 very great. Knowing how slowly sedi- 

 ments accumulate upon the sea floor, 

 the age of the earth must measure ap- 

 proximately hundreds of millions of 

 years. Determining the arrangement 

 in which the strata were deposited is 

 difficult, because, in different areas, 

 different strata may be omitted for 

 one reason or another. Consequently, 



the sequence is not always alike. In some areas a more recent rock 

 deposit is exposed than in others. Sometimes the rock deposit 



may have been completely 

 weathered or worn away. 

 The formation of rocks differs 

 in various parts of the country. 

 Some rock was elevated from 

 the sea and forms the land of 

 to-day. This is known because 

 it contains fossils of shells of 

 animals that once lived in the 

 sea. The flatness of the beds 

 of rock seems to show that their 

 movement from the sea was so 

 uniform and gentle that their 

 original formation could not 



From drawing Museum Nat. Hist, -have been broken. 



Fossils of Titanotheres, prehistoric mammals, The oldest historical records 



have been found in both Asia and America. 



At the time they lived, Asia and America were of Egypt Or Babylon date back 

 probably one continent joined where Behrjng 



strait is now located. perhaps six or seven thousand 



