So far we have talked only about rocks that are of the Cambrian Epoch or
younger—rocks that may contain fossils. Yet there are vast areas (most
of Canada, for example) that are covered with rocks older than the
Cambrian formation. Some Precambrian fossils have been found, but they
are so rare that they are useless for dating the strata containing them.
Long-range correlation of Precambrian rocks must rely on nuclear
measurements. Therefore it has been only in the last dozen or so years
that some order could be established for the Precambrian rock sequences.
The elaborate Precambrian stratigraphies (arrangements of strata in
sequence) proposed in the past, most of them based on superficial
similarities of the rocks in one place to those in another place, now
have been drastically altered and in some cases completely overturned by
nuclear measurements. We are still far from understanding the sequence
of all the events in that vast span of time we call the Precambrian.
Many thousands of nuclear age determinations will have to be made to
lighten the dark corners of our ignorance.
[Illustration: _Folded strata of Precambrian rocks, including limestones
and shales, in Glacier National Park, Montana._]