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the stable elements in the earth's crust, but relatively speaking, it is ten thousand times more abundant in meteorites, and presumably, in all other bolides. The reason for this anomaly is well understood: when the bodies in the solar system condensed out of the original "solar nebula," the earth had the same elemental composition as the asteroids, meteorites and comets. But when the earth heated up, in its earliest stages, the molten iron preferentially scrubbed out platinum, iridium and the other siderophiles (iron lovers), and carried them down into the core. So the earth's crust is now severely depleted in iridium, compared to the smaller and therefore uncored objects such as meteorites, comets and earth-orbit-crossing asteroids.

Figure 2 shows time, or more accurately, stratigraphic height in the sedimentary rock increasing vertically, and iridium concentration increasing to the right. An unusual feature of the vertical scale is that it is linear for roughly the central third, which represents 30 centimeters. The lower third is logarithmic, going through 1 meter, 10 meters, 100 meters, down to 1 kilometer, while the upper third is also logarithmic, and goes up to 100 meters. These levels were measured in the Bottaccione Gorge, behind the medieval town of Gubbio, in northern Italy, where Walter had been doing field work on paleomagnetism for the past several years. We were all surprised to see the iridium concentration spurt up by a factor of 30, at the bottom of the clay layer, and then fall back more slowly, to its background value, which was the same in the limestones above and below the clay layer. When we measure it these days, it goes up by a factor of 300, because we no longer dissolve away the calcium carbonate before doing the assay.

In the summer of 1979, when Walter was collecting more rock samples in Italy, I set myself the task of identifying the origin of the iridium, under the "ground rules" that whatever was the source of the iridium must also provide a plausible basis for what I

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