The entire scientific party of each of the more than 50 expeditions represented in the data of this paper took turns marking and adjusting the echo sounder, and all navigational officers on these expeditions took the fixes and kept the logs. To these sea-going scientists and mariners too numerous to list the authors are extremely grateful. Soundings in the northeast Atlantic compiled by the British Admiralty Hydrographic Department were kindly provided by Cmdr. J. S. N. Pryor of that organization. Dr. M. N. Hill of Cambridge and Dr. G. E. R. Deacon of the (British) National Institute of Oceanography were instrumental in obtaining many of these valuable deep-sea soundings. Original sounding sheets of many areas were provided by the Coast and Geodetic Survey through the courtesy of Admiral A. Karo and Mr. G. F. Jordan. B. Luskin's development of the Precision Depth Recorder and his continued research and development in echo-sounding equipment made it possible to obtain many of the detailed data of this paper. The following expedition chief scientists conducted sounding surveys which have been incorporated in this paper: M. Ewing, J. L. Worzel, J. E. Nafe, I. Tolstoy, R. S. Edwards, G. R. Hamilton, C. L. Drake, B. Luskin, W. C. Beckmann, F. Press, J. Northrop, J. Hirshman, M. J. Davidson, R. J. Menzies, F. C. Fuglister, E. T. Miller, and B. C. Heezen. The writers are grateful to the great number of scientists who encouraged them in this work and especially to those who offered suggestions and discussed the data and conclusions. We are particularly indebted to W. H. Bucher for discussions relative to tectonics, to David B. Ericson for problems relating to sediment distribution and analysis, and to C. O'D. Iselin and C. H. Elmendorf for general encouragement and support during the several years of this study. CONTENTS PAGE ABSTRACT 1 PART 1. PREPARATION OF THE PHYSIOGRAPHIC DIAGRAM 3