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continent, from the primitive beginning to the birth of the 

 Appalachian range, the oldest mountains of the continent, and 

 during the succeeding millions of years came the western 

 ranges of mountains, or the youngest, completing the great 

 interior valley and its drainage. Then the ice age, with its two 

 advances, leaving its characteristic markings, our ranges of 

 hills, highlands and valleys. These changes have been slow 

 and imperceptible, and are occurring as rapidly to-day as during 

 the primeval past, and will continue through untold ages, until 

 every part of the earth's surface shall have been alternately 

 dry land and sea, times without limit. 



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