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Re: Geological Time Lines - History



 Allan Edels asked"

>    All:
>
>        After reading an old article (by Barnum Brown, in National
>Geographic 1919), I discovered that at that time, the Cretaceous was
>believed to have been 3 mya, not 65 mya.  (Also, based on additional info.,
>the end of the Permian was believed to be around 12 mya).
>
>    It seems that in the 1920's the Geological Time Line was revised to
>become close to what we know today.  Does anyone know for sure when the
>change occured - and what information sparked the change?  If anyone has a
>listing of the OLD time line, I'd like to see it.

I have a 1928 edition of W. J. Miller's Introduction to Historical Geology
which mentions radiometric dating of the changes of uranium "through
successive stages of radium into a certain type of lead." It gives "an age
of considerably more than a billion yeras has been assigned to Archaeozoic
rocks, the Paleozoic era opened more than 500 million years ago; the
Mesozoic era nearly 200 million years ago, and the Cenozoic era at least 50
million years ago."

All in all, not bad for what were clearly initial estimates.
-- Jeff Hecht