William Glen's documentation of the evidence for continental drift--properly called plate tectonics--is an extraordinarily complete case history of a scientific revolution. A Berkeley Fellow in the History of Science and professor of geology at the College of San Mateo, Glen has made extensive use of the personal accounts of the principal participants, setting their narratives against the historical and technical background. In particular, he highlights the overwhelming anti-drift consensus of the 1950s and early '60s. Then, piecing together the route to the clinching bit of evidence--the Jaramillo event, a 900,000-year-ago reversal in the earth's magnetic field--Glen dextrously follows three separate threads: the development of radiometric dating; studies of magnetic rocks; and theories on the formation of the ocean crust. Thus, we follow the history of potassium-argon dating with its own pivotal events--such as the development of the Reynolds' static mass spectrometer and the application of the dating method to ""young"" rocks. Next, we follow research on remanent magnetism, learning that those who believed the earth's magnetic field undergoes periodic polarity reversals were once a fringe minority. Finally, we hear of a case of scientific coincidence-simultaneous hypotheses put forth by Morley, in Canada, and Vine and Matthews, in England, concerning seafloor spreading. The theory: upwelling of matter from the earth's mantle occurs at midocean ridges with consequent seafloor spreading on either side of the ridge; as the outflow cools, it should retain a record of the earth's magnetic field at the time. To make the revolution, all it took was to correlate these activities: use potassium-argon dating to get an absolute age for magnetic rocks, and develop a paleomagnetic time scale in accord with the fossil stratigraphic record; then, correlate dates and ages with the magnetic ""stripes"" (field reversals) that showed up as geological vessels traversed midocean ridges and profiled the seafloor. The result was convincing evidence that the crust moves, carrying the continents with it. Glen's story climaxes in a burst of eurekas, masterful papers, and jubilant scientists. Read as technical history, the events in themselves are exciting. And, from Glen's philosophical interests, comes an added dimension: how chance, coincidence, prejudice, personality, and rivalry stir the action. Untutored readers will need to do some skipping; regardless, this is a solid, stimulating account of what is clearly the highpoint in the history of geology.