CHAPTER NINETEEN 



THE HISTORY OF LIFE 



Absolute I. IT is well known that life has existed on the earth 



and relative f many millions of years. The evidence for this is 



age of fossils f. 11111. 



found in the fossil remains scattered through the sedi- 

 mentary rocks. The relative age of nearly all the rock 

 formations is known, since they have been found in 

 various, places one upon the other. Thus if in one 

 locality B is above A, we know that it is later, except in 

 the case of a complete overturn, which can occur only 

 in a limited area. At another place we find B with a 

 third formation C above it, though A may here be 

 absent. We have, then, A, B, C, in proper sequence. 

 Somewhere else D will be found over C, and so on, until 

 we are able to construct a geological column, such as 

 may be found in textbooks. Of course all the forma- 

 tions cannot be found actually forming such a column, 

 if only because each new deposit is necessarily made up 

 of materials derived from older ones. The absolute 

 age of the rocks is very much more difficult to deter- 

 mine, but some estimates have been made from the 

 consideration of various factors, such as the probable 

 rates of deposition and denudation, and the changes 

 taking place (at approximately known rates) in radio- 

 active minerals. 



Type fossils 2. The correctness of the method just described 

 necessarily depends on our ability to recognize various 

 formations when we find them. How are we to know 

 that the B which lies beneath C is the same B which 

 elsewhere rested above A ? It is not likely to be con- 

 tinuous from one place to the other. The character 

 of the rock itself is no certain guide ; rocks of entirely 

 different periods may present the same appearance and 



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