114 THE ENVIRONMENT 



TYPE OF GEOLOGICAL EVIDENCE DRAWN FROM FOSSILS AND 

 FROM THE ENVIRONMENT 



If we now compare our findings from the last two chapters, we 

 observe a wide variation between the kinds of evidence geology is 

 able to supply. We have distinguished between three groups: real 

 fossils, biogenic deposits, and evidence drawn from the environment, 

 from contemporary exogenic processes. 



Fossils tell us not only that there w^as life on earth at that time, 

 but also what kind of life. 



Biogenic deposits, either the limestone crusts described by 

 MacGregor, or the graphitic slates of Lepp and Goldich; either the 

 microscopical biogenic spots in pyrite grains pictured by Ramdohr, 

 or the big reefs of lime-secreting Algae of the type of Collenia, tell 

 us only in a very general way that there was life on earth during 

 that period. 



But neither the fossils nor the biogenic deposits tell us anything 

 about the metabolism of life of their time; that is, whether this was 

 oxygenic or anoxygenic. An answer to the latter question can only 

 be drawn from the study of the environment of early life. Only from 

 certain special types of ancient deposits, formed by contemporaneous 

 exogenic processes, can conclusions be drawn about the character of 

 the ancient atmosphere, i.e. if this was oxygenic or anoxygenic. 



