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pattern of reversals above and below the clay might bracket its age
of formation and allow an upper limit to be placed on how long it
could have taken to deposit the layer. Alas, during this period of
geologic history the reversals had not happened often enough: All
that could be told is that the clay layer fell within a 6-m section
of limestone deposited during a single period of magnetism, called
29 R (for reversed), that was known to have lasted for about
750,000 years. Six meters in 750,000 years is equivalent to 0.8 cm
of sediment deposited every 1,000 years. Since the boundary clay
is about 1 cm thick, at that rate it would have taken a little more
than 1,000 years to form. This appeared to be an improvement
over Walter's rough estimate, but since the clay is quite different
from the limestone, there really was no basis for assuming the
same sedimentation rate for both. The attempt to determine the
time interval using the magnetic chronology thus failed, but in
another way the effort succeeded, for the mind of Luis Alvarez
was now locked in.
What was needed, he reasoned, was a geologic clock that had
been operating at the time the clay layer formed but that could be
read today. Because no one knew how much time the clay layer
might represent, the clock might have to measure small differences.
None of the standard geologic clocks—the ones based on radio-
active parent-daughter pairs of atoms that are used to calculate
exact ages—had enough sensitivity or would work on the chemical
elements in the clay layer. Therefore, as he had done so many times
in his career, Luis Alvarez invented a new technique. To do so, he
looked not down to the earth but up to the heavens, postulating
that the amount of a rare metallic element called iridium might
provide the clock.
When the earth formed, iridium, like other elements of the
platinum group (which includes osmium, palladium, rhodium, and
ruthenium), accompanied iron into the molten core, leaving these
elements so rare in the earth's crust that we call some of them pre-
cious. Their abundance in meteorites and in average material of the
solar system is many times higher than in the earth's crust. The
iridium found in sedimentary rocks (and often it is too scarce to be
detected) appears to have settled from space in a steady rain of
microscopic fragments—a kind of cosmic dust—worn from tiny
meteorites that form the shooting stars that flame out high above
the earth. Such meteorites are believed to reach the upper atmo-
sphere at a constant rate, so that the metallic rain falls steadily to
earth, where it joins with terrestrial material—dust eroded from
the continents and the skeletons of microscopic marine animals—
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