What killed the dinosaurs? At last the great mystery has been solved."" Coming from an esteemed geologist, a former college president, and currently the director of the Los Angeles County Museum of Natural History, Powell's claim cannot be dismissed as the ravings of a crank, but its certitude is, to say the least, unusual. Then the qualifier: ""A theory is never proven,"" which settles Powell square in the Popper/Kuhn nexus and gives him room to move. The answer to what killed the dinosaurs, Powell believes, has been found in the Alvarez Theory, elucidated by a Nobel Prize--winning physicist and his geologist son, which suggests a random catastrophe--a large meteorite striking the earth--raised clouds of dust, lowered temperatures and halted photosynthesis and devastated the food chain, thus spelling the great lizards' doom. This is long at odds with the gradualist, deep-time approach governing much geologic thought, and provoked much score. Powell endeavors to make the AIvarez idea accessible, but he can't help but wade through thickets of vertebrate paleontology and rare-metal chemistry, pick his way among impact markers like shatter cones and shocked quartz grains, painstakingly dissect the iridium anomaly found in Cretaceous-Tertiary boundary clays. Even so, Powell rarely loses his readers, and all but the most geochronologically, microisotopically, paleobotanically challenged will be able to follow his drift (and appreciate the fact that he gives rival theories their day in his people's court, as well as admitting to the more outlandish conjectures of the pre-impact theorists). Although the evidence Powell submits on behalf of the impact theory is compelling, perhaps more so are his comments on the politics of scientific enquiry: the power plays and back stabbings, the ugly career-ending insults, the absurd effort involved in querying entrenched, if suspect, theories. Powell's overriding notion is undebatable: Chance happenings surely help shape our world, and serendipity--in available tools, say, or disciplinary cross-fertilization--fuels scientific advancement.