Iridium And Mass Extinctions
Alvarez and his colleagues at the University of California, while chemically analyzing a series of sedimentary strata from Italy, discovered that one layer had 25 times the concentration of iridium residing in adjacent strata. The iridium-rich layer forms the boundary between the Cretaceous and Tertiary periods, 65 million years ago. During that death-filled interval, 50% of the earth's genera were wiped out. Such are the two correlated facts: iridium increase and mass extinction. But do they have the same cause? Alvarez et al point out that iridium is rare on earth but much more common out in space. The anomalous concentration of iridium could have been injected by a massive solar flare, a big meteor impact, or come other extraterrestrial catastrophe. Thus is catastrophism being resurrected.
(Anonymous; "An Iridium Clue to the Dinosaur's Demise," New Scientist, 82: 798, 1979.)