Studies of the magnetic anomalies paralleling ocean ridges have furnished partisans of continental drift with decisive arguments. To take stock of this important question, my colleague Thellier and I decided in the early summer of 1967 to make it the subject of the annual seminar on Earth physics for the school year 1967-68. Although research was still developing rapidly, the General Assembly of the International Union of Geodesy and Geophysics held in Switzerland in September, particularly some of the meetings in Zurich under the auspices of the International Committee for the Upper Mantle, appeared to confirm that we had made no important omissions. At the con clusion of the seminar, where I had been responsible for most of the lectures, I resolved to write the present volume for the non-specialized scientific reader. The project turned out to be a good deal more ambitious than I had thought. It is quite an undertaking nowadays to try to survey a rapidly growing subject, first of all because of the difficulty of gathering material; publication delays are now nearing one year, with the result that specialists communicate largely through a selective distribution of reports, as well as verbally in frequent colloquia. I warmly thank those who helped me in getting unpublished literature, especially Xavier Le Pichon. Even so, some essential work came to my knowledge only lately.