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Re: Geological Time Lines - History



Allan Edels wrote:

>         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.

In the late 19th century, many geologists were still trying to square the
geologic record with Lord Kelvin's calculations that allowed no more than
eighty million years for the whole age of the Earth.  The relative lengths of
the eras were estimated at 12:3:1 -- that is, the Palaeozoic was estimated at
12 times as long as the Cenozoic, and the Mesozoic 3 times as long as the
Cenozoic.  Archaean time was thought to be nearly as long as all the others put
together.  If you put the start of the Paleozoic at 48MYA to fit Kelvin's
limits, the Mesozoic started 12 MYA, and the Cenozoic 3 MYA.

The ideas on the length of the geologic timeline started changing in the 1920s
and 1930s with the advent of radio-dating.  By 1939, the total had been
extended to 1.5 billion years, and all the individual period-ages ratcheted
upward to match.  They kept testing older and older rocks, and using better and
better methods.  Now the oldest Earth rocks known are about 3.8 billion, and
dates of meteors converge on 4.5 to 4.7 billion years.

The oldest geology book I have, James Dana's textbook from 1874, doesn't give
any geologic timeline after the fashion we all know and love. <g>  It just
gives the relative proportions of the geologic ages, and three or four
different possible ways of assigning absolute ages to the relative timeline.

-- Jon W.