AGE OF THE ANOXYGENIC ATMOSPHERE 113 



(Gussow, 1959). The other instance is from the Labrador geosyncline 

 in central Canada, where Gastil and others quite recently reported 

 the occurrence of red beds, older than the so-called Grenville orogeny, 

 which is dated at around 1 billion years (Gastil et al., 1960, p. 25). 

 These particular red beds, however, occur outside the area of the 

 Grenville orogeny, which makes correlation extremely difficult. 



Just as with the tentative dates given for the sediments formed 

 under an anoxygenic atmosphere, we can consequently also give only 

 a tentative age for the oldest red beds. In a very general way this 

 is one billion years. 



PPOVISIONAL DATING OF TRANSITION BETWEEN THE 

 TWO ATMOSPHERES 



The tentative age of 1 biUion years for the oldest red beds gives us 

 a provisional minimum age for the present oxygenic atmosphere. 

 On the other hand, a tentative age of 2 billion years could be 

 established for the youngest sediments known to be deposited under 

 an anoxygenic atmosphere. This is the lowest age we can assign up 

 to now for such an atmosphere still to be present on earth. 



Concluding this chapter, we may thus for the present state pro- 

 visionally that from a study of sediments formed in the early history 

 of the earth, geology supplies the following evidence. 



Before 2 billion years ago, and at least up to that time, the 

 atmosphere of the earth was anoxygenic and did not contain free 

 oxygen. Life on earth and exogenic processes in geology were pre- 

 actualistic, at least up to that time. 



From 1 billion years ago at least the atmosphere was oxygenic. 

 Life on earth and exogenic processes in geology were actualistic, at 

 least from that date onward. 



Between 2 billion years and 1 billion years ago the transition 

 between the early anoxygenic to the present oxygenic atmosphere 

 took place. We do not know when this transition actually occurred, 

 but we may confidently expect that further studies will define this 

 period more narrowly. Neither do we know how long this transition 

 itself took in absolute time, say in years or millions of years. We 

 suppose, however, that it was a gradual transition, a slow change, 

 which might have taken a period of considerable length, even when 

 compared with the length of time other major processes in geology 

 take. 



