The concept of plate tectonics has developed during the past four years from the hypotheses of continental drift and sea-floor spreading, supported by a variety of evidence from paleomagnetism, geochronology, and marine geology and geophysics. A series of four contiguous papers in the March 1968 issue of the Journal of Geophysical Research correlated on a global scale the linear magnetic anomalies, which are parallel to and bilaterally symmetrical about the oceanic ridge system, with the polarity reversals of the earth's magnetic field imprinted on new oceanic crust as it was generated at the oceanic ridge crests [Pitman et al., 1968; Dickson et al., 1968; Le Pichon and Heirtzler, 1968; Heirtzler et al., 1968]. In 1967 and 1968, four major papers introduced plate tectonics: the earth's surface is considered to be made up of a few rigid crustal plates or blocks in motion relative to each other [McKenzie and Parker, 1967; Morgan, 1968; Le Pichon, 1968; Isacks et al., 1968].