2. Envisaging the Past and Future 



Jlqw things are more fascinating than speculation 

 about happenings in the remote future — or in an equally 

 remote past. For instance, what did the earth look like 

 two billion or 500 million or even a paltry 100 milUon 

 years ago? We have strong reasons to believe that our 

 Tellus really did exist as long ago as two billion years. 

 Assuming that the radioactive transformations known to 

 us proceeded then at the same rate as today, we can 

 ascribe an age of over two billion years to the oldest 

 rocks hitherto submitted to radioactive analysis. 



However, we know next to nothing about the division 

 between land and sea then prevailing, and whether water 

 in large quantities existed on the primeval earth is 

 subject to discussion. Arguments have been advanced 

 to prove that the total volume of ocean water was at the 

 beginning only a fraction of the present total. 



In a recent brilliant paper William W. Rubey has 

 attempted to show that the water masses of the oceans 

 were derived largely from the earth's crust rather than, 

 as has been assumed, from a cataclysmic condensation 

 of water vapor present in a very dense primitive 

 atmosphere.^ For representatives of the two opposing 



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