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 I warmly thank those who helped me in getting unpublished literature, especially Xavier Le Pichon.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.

I/Seismic Geography. Bathymetry of the Oceans.- General remarks.- tertiary foldings.- transcurrent faults.- island arcs.- the ridge crests.- bathymetry of the oceans.- the focal mechanism of earthquakes and the direction of tectonic stress.- II/Results of Magnetic Measurements at Sea. Transverse Fractures.- The magnetic anomalies of the ridges.- linear anomalies in the neighborhood of island arcs and coasts.- the east pacific rise and the great transcurrent fractures.- active and fossil fractures.- return to the east pacific rise.- III/Reversals of the Earth's Magnetic Field. The Vinematthews Hypothesis.- Remanent magnetization of lavas and the magnetic field in the past.- periods of polarity and intervening events.- magnetization of ocean sediments and various phenomena connected with reversals.- theoretical examination of the origin Of reversals.- vine-Matthews hypothesis.- transform faults.- focal mechanisms of earthquakes occurring on ridges.- IV/Ocean Spreading.- From ridges to trenches.- simple examples of expansion.- expansions without ridges.- local changes in the rate of expansion.- worldwide variations of expansion.- overall pattern of the anomalies.- their enumeration and dating.- V/Continental Drift.- Floor spreading, drift, and paleomagnetism.- reconstruction of continental blocks.- the hypothesis of rigid plates.- kinematic considerations and the first applications of the plate hypothesis.- the synthesis of le pichon.- ancient spreadings.- VI/Study of the Oceanic Crust by Seismic Refraction and Gravimetry.- Seismic refraction. crustal structure, seismic study of the ridges.- dredging of igneous rocks.- transverse fractures and the seismic anisotropy due to sea floor spreading; gravimetric study of ridges and grabens.- continental margins.- complete studyof trenches.- VII/Heat Flow Through the Ocean Floor.- World heat flow distribution.- heat flow on the ridges.- interpretation of the results.- VIII/The Nature of the Upper Mantle, and Theories of Convection.- Thermal convection, cause of ocean spreading.- properties of the upper mantle given by seismology.- temperature and composition of the upper mantle.- rheology of the mantle.- convection by vertical instability.- convection by gorizontal instability.- IX/Oceans and Continents. Conclusion.- Birth of the oceanic crust.- death of the oceanic crust.- the ocean floor as a conveyor belt conclusion.- X/Recent Progress.- magnetic anomalies and the sea floor.- ridges andrifts, transform faults.- compressive boundaries.- kinematics of plate motions.- dynamics and thermodynamics of plate motions.- terrestrial heat flow.- epicontinental seas.- gravity anomalies and plate theory.- orogenesis on the plate theory.- application of plate tectonics to ancient continental movements.- References.