>In Grand Canyon, there is a syncline in the eastern limb where the
>Precambrian sediments are in apparent conformable contact with the Cambrian
>Tapeats, from which they are indistinguishable lithologically. However, as
>one moves laterally, the beds of the Precambrian rise up to cut the
>Tapeats, and the unconformity becomes obvious. Also there are lots of
>formations in the west (California, Nevada, Utah, etc,) which are
>designated Precambrian/Cambrian or Precambrian? because until a trilobite
>or some other metazoan fossil is located, the rocks are considered
>Precambrian. This "logic" would be tautologic but for the occurrence
>nearly everywhere of the boundary unconformity.
Thanks for the info on the conformable relation at the Grand Canyon. I
would say that there are ways to get out of the tautology you mention.
Radioactive dating of something will get you out of the problem at some
localities.
glenn
Adam, Apes, and Anthropology: Finding the Soul of Fossil Man
and
Foundation, Fall and Flood
http://www.isource.net/~grmorton/dmd.htm