534 



Era 7 Psychozoic 



Era 6 Cenozoic 

 Era 5 Mesozoic 

 Era 4 Paleozoic 



The Earth mid Its inhabitajits Change uni r x 



Began about 1,000,000 years ago 



Began about 550,000,000 years ago 



Began about 1,200,000,000 years ago 



Began about 2,000,000,000 years ago 



Began about 3,000,000,000 years ago 



Fig. 483 (above) A diagram 

 of the 7 eras of time. Read 

 from the bottom up. The 

 rocks of the first era were 

 the first to be laid down OJid 

 are generally deepest down 

 in the earth. Note that about 

 five sixths of the eartlfs his- 

 tory was over when the 

 fourth {Paleozoic) era be- 

 gan. But that was scarcely 

 recent. It was ^^0,000,000 

 years ago! 



Fig. 484 (left) After long 

 years of careftd study of the 

 rocks, geologists think this 

 ?nap represents North 

 America during an early pe- 

 riod of the Paleozoic era. 

 Seas are shown in white. 

 Was yotir ho7netown loca- 

 tion under or out of water 

 at this time? 



The earth in the first three eras. The 

 first rocks, it is estimated, were formed 

 three billion (3,000,000,000) years ago 

 as the earth cooled. Geologists think it 

 probable that no life existed during the 



first billion years, the first era (Azoic). 

 After about a billion years great changes 

 occurred that have left their mark on the 

 eaith. Geologists therefore say that a 

 second era began. In the rocks laid down 



