68 



Figure IV-6.— The giant red spot of Jupiter and the complex cloud patterns 

 suggest a natural laboratory for chemical evolutionary processes. 



probably hydrothermal solutions as well. Some metamorphosed sedi- 

 mentary units at Isua approach the composition of normal present- 

 day sedimentary rocks, which bear conclusive evidence of an atmo- 

 sphere that contained sizable quantities of carbon dioxide. Little can 

 be said about the oxygen content of the atmosphere at the time of 

 deposition of the Isua sediments. There is a fair amount of evidence 

 that the oxygen content of the atmosphere more than 2 b.y. ago was 

 substantially less than today. Therefore, there must have been less 

 oxygen at the time of deposition of the sediments at Isua. The 



