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tion centers and, through time and isolation, gradually (or rapidly) is trans-
formed into a new species, a form no longer capable of successfully repro-
ducing with members of the old group. New species are generally character-
ized by new morphologies, and so it was with the ammonites, which changed
their shells rapidly by adding or subtracting ribs, by altering the spines, and
by twisting or unrolling or tightening the loops that defined their shells'
shapes.
The great breakthroughs that enabled scientists to use radiometric dat-
ing for sedimentary rocks with enclosed ash beds are not limited to Creta-
ceous rocks. Radiometric dating has now been applied to sedimentary rocks
of all ages and has made it possible to assign numerical ages to the entire ge-
ological time scale. There are still many uncertainties. But progress has been
rapid and the scientific benefits enormous. We can use this information in
the search for oil and minerals and to answer questions about rates of evolu-
tion. The use of this time machine has been a major milestone in the study
of the past.
For work with Cretaceous-aged rocks, the benefits have been enor-
mous. Here, finally, was the realization of D'Orbigny's dream: time, as eluci-
dated by an unbroken sequence of sedimentary rocks sprinkled with fossils
and diagnostic ash beds. By interleaving information from the two, geolo-
gists put together the highest-resolution scheme of ages found for
any
of the
earth's geological time scale units. Because many of the Western Interior am-
monites could be found in Texas and even Europe, this time scheme allowed
direct correlation with these faraway places. The standard for time,
Creta-
ceous
time, became centered in the middle of North America. Yet one great
problem remained: None of these fossils could be found further west, along
the Pacific coast of North America, in Japan, or elsewhere in the vast Pacific
Ocean region. To tell the time of these places, including ancient Sucia Is-
land, required an additional step—the invention of an entirely new type of
geochronometry device. The age of the Cretaceous on the western margins
of the North American continent was decoded by using the earth's magnetic
field.
