A year ago Nobel Laureate Luis W. Alvarez suggested that supernovae may have been responsible for the great geoextinctions, particularly the most recent one, which marked the demise of both orders of dinosaurs, marine and flying reptiles, and many large mammals. In fact, all members of the animal kingdom heavier than 50 lb are suddenly absent in the geologic record at the base of the Tertiary period, and marine invertebrates are affected also. This sort of suggestion (Emanuel Velikovsky aside) generally does not gain much acclaim from earth scientists. But more recently, in a presentation to the American Chemical Society in Houston (Chemical & Engineering News, 58, 15, 25–26, 1980), Alvarez, a physicist, and his son Walter, a geologist, made a geochemical analysis, based on modern planetology, of clays sampled from the Cretaceous-Tertiary boundary in Italy, Denmark, and Spain. The clay layer corresponds to the most recent geoextinction. Trace element analyses of the clays indicate that metals of the platinum group, such as iridium, are enriched relative to the over- and underlying limestones by approximately a factor of 20,at the parts per billion level.