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How Geologists Tell Time:

geologic time
    One of the factors that confronted Darwin when he was formulating his ideas about natural selection was the amount of time that it would require for change to accumulate to the point that new species could evolve. The Victorian view of the age of the Earth was based on estimates derived from interpretations of scripture by Archbishop Ussher, who in 1654 proclaimed that the Earth was formed on October 23rd, 4004 B.C.  While Darwin was developing his thoughts about natural selection, geologists were refining their calculations for the age of the Earth.   How did scientists determine that the Earth was, in fact, many millions of years older than this, and that therefore there was enough time for the processes of natural selection to occur?

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Photographs courtesy of the University of California Museum of Paleontology: (

Last modified October 02 2008 03:00 PM

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