Not all the dates can have been adjusted this way, because the
first radiometric dates had nothing to be compared to. They were much
older than expected. In a more modern example, moving the
Precambrian-Cambrian boundary from 570 to 540 million [someone found
datable deposits closer to the boundary than anyone had previously] was not
what was expected by evolutionary evidence-that makes the changes within
the Cambrian occur much more rapidly.
It should be kept in mind that there are several lines of evidence
that a given rock is unlikely to yield good dates. Magma can melt older
rock which gets mixed in and causes an older apparent date; however, often
some chunks of the older rock do not get totally melted (xenoliths).
Metamorphism or alteration by groundwater will have chemical effects as
well as possibly affecting the relative amount of various isotopes.
Sedimentary rocks generally do not form in environments where fresh
radioisotopes are deposited and are not readily dated by such means.
Non-radiometric data can be used to check them as well. Counting
orbital cycles backwards through the fossil record confirms that
radiometric dating works well back though the Pliocene; comparison of the
growth lines in corals with the rate of decrease in the earth's spin agrees
with the estimated age for the mid-Paleozoic; the amount of dust on the
moon is roughly in agreement with what one would expect for it to have been
out there 4 billion years, etc. In addition, there are several lines of
evidence that, though not yielding exact numbers, suggest that the earth is
old. Observation of these produced a widespread consensus that the earth
was much older than 10,000 years before Darwin published any of his ideas
on evolution.
David Campbell