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to be primary and to record an instantaneous and singular event. For
this reason, sharp iridium peaks corroborate the Alvarez theory to a
far stronger degree than the negative or indeterminate evidence of
iridium hills at other locations detracts from the theory. One peak is
sufficiently likely to be primary as to be worth several hills.
In 1996, A. D. Anbar and his co-workers at Caltech used ultra-
sensitive techniques to measure the minute amounts of iridium in
rivers and the sea. 2 4 They found the K-T boundary clay to contain
1,000 times as much iridium as all the world's oceans put together,
confirming that the iridium did not precipitate from normal sea-
water. They also determined that iridium, once present in the oceans,
remains there for some 10,000 to 100,000 years before it is removed
by sedimentation, providing yet another way to explain the iridium
hills: They could merely be the result of the long residence time of
iridium once it had been injected into the oceans by meteorite
impact.
S HOCKED
M INERALS
Bruce Bohor's 1981 discovery of shocked quartz (previously found
only at known impact craters and at the sites of nuclear explosions,
in the K-T boundary clay in the Hell Creek area of Montana, home
of T. rex) convinced many geologists that impact was a reality. Glenn
Izett of the U.S. Geological Survey, who wrote the definitive paper
on the K-T section in the Raton Basin, spoke for them: "I started off
as a nonbeliever. What got me was the appearance of these shocked
minerals at the K-T. In the impact bed, you see grains everywhere
that have these features in them. Just a millimeter or two below,
you'll never see any of those features." 2 5
Unshocked quartz has no fracture planes; quartz deformed in
other geologic settings than impact sites sometimes has single sets.
The multiple sets of crisscrossing planes illustrated in Figure 12,
however, are diagnostic of great shock. The planes actually are close-
ly set layers of glassy material precisely oriented to the crystal struc-
ture of quartz. Officer and Drake took exception to the claim that
shocked quartz was diagnostic of impact: "The presence of lamellar
quartz features [the parallel planes] does not in and of itself demon-
strate a meteor impact origin." 2 6 As evidence, they stated: "Lamel-
lar features . . . are also a characteristic of both normal tectonic
[mountain building] metamorphism and shock metamorphism,
although the normal tectonic features are quite different from the
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