11. Ocean bottom photographs of the continental margin and ocean basin floor 38 12. PDR record of Biscay Abyssal Plain 52 13. PDR records of Madeira Abyssal Plain and the Madeira Rise 52 14. PDR records of Mid-Ocean Canyon No. 2 and canyons at Vema Gap 58 15. Ocean-bottom photographs of seamounts 58 16. Representative PDR records from Bermuda Rise 78 17. PDR record, Bermuda Scarp Zone 78 18. PDR record, Upper Step Mid-Atlantic Ridge 78 19. Ocean-bottom photographs; Mid-Atlantic Ridge 78 20. Physiographic provinces, Atlantic Ocean 122 21. Control chart 122 22. Six trans-Atlantic topographic profiles 122 23. Index chart showing locations of profiles of Plates 24, 25, 26 and Figure 45 122 24. Thirty-four profiles of the continental margin: western North Atlantic 122 25. Twenty-three profiles of the continental margin of Europe and Africa 122 26. Crustal structure and continental-margin provinces 122 27. Five topographic profiles, western North Atlantic 122 28. Distribution of deep-sea sands in relation to physiographic provinces 122 29. Earthquake epicenters, North Atlantic 122 30. Location of PDR records and bottom photographs reproduced as illustrations. 122 FIGURES PAGE FIGURE 1. Method of preparation of physiographic diagram 4 2. Precision depth recorder (PDR) sounding lines obtained by research vessels 6 3. Good, but nonprecision sounding lines obtained by research vessels 7 4. Major basins of the North Atlantic, after Wüst (1940) 12 5. Sound-velocity corrections for echo soundings 13 6. Conversion diagram for degrees, per cent grade, feet per statute mile, and gradient 14 7. Gradients from 1:5 to 1:8000 shown at 40:1 vertical exaggeration 15 8. Outline of submarine topography 15 9. Major morphologic divisions: North Atlantic Ocean 16 10. Three categories of continental-margin provinces 17