Animals Before Man 



is situated. Similarly, to say that remains of 

 the great dinosaur Triceratops occur in the 

 Laramie sandstone is to convey the information 

 that Triceratops is one of a group of extinct 

 reptiles and tell at just what period it lived. 



One thing must be borne In mind, and that 

 is that periods of geological time have no exact 

 equivalent In years. Our own standard for the 

 measurement of time is the period required for 

 the earth to make a complete revolution about 

 the sun ; but divisions of geological time have 

 no such fixed standard, being records of the 

 changes that have taken place among plants 

 and animals rather than actual measurement 

 of the lapse of years ; hence these divisions 

 may be, and are, of very unequal length. 



The number of years represented by any 

 given group of rocks is computed by estimating 

 the time that would be required to wear away 

 and deposit in the shape of mud or sand suf- 

 ficient of the earth's surface to form the beds 

 under consideration. But as the rate of wear 

 varies greatly, not to say enormously, according 

 to the material, rainfall, and elevation of the 



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