PART II 



HISTORICAL GEOLOGY 



CHAPTER IX 



Geologic and human history compared. By the word 

 history we usually understand the story of men, nations, or 

 civilizations. In reality this is only one kind of history, 

 only a phase of a much larger history, which deals not only 

 with the human race, but with all other animals, with plants, 

 and with the earth itself back to the time of its beginning. 

 The history of the earth and its inhabitants represents a 

 vastly greater length of time than human annals, but much 

 less is known about it. It is impossible to say just how long 

 the earth has existed ; but if we measure the existence of the 

 human race in thousands of years, the duration of the earth 

 must be reckoned at least by tens or hundreds of millions of 

 years. While such enormous figures are quite too vast for 

 comprehension, we may gain some idea of the length of time 

 involved when we learn that although the great mountain 

 ranges of to-day have remained almost unchanged since the 

 dawn of human history, others like them have been built up 

 and then entirely worn away, time after time, even during the 

 later portions of geologic history. The stupendous canon 

 of the Colorado River in Arizona seems to our eyes one of 

 the fixed and everlasting features of the earth ; yet it has all 

 been made in a very recent period of the earth's existence. 



How geologic history is worked out. The threads of 

 human history have been gradually pieced together from 



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