43. Tracing of PDR record, Rift Valley, Rift Mountains, High Fractured Plateau, and Upper Step 91 44. Tracing of PDR record, Western Rift Mountains 92 45. Twenty-six rift valley profiles, Mid-Atlantic Ridge 93 46. Five representative profiles, crest and western flank of Mid-Atlantic Ridge 94 47. Axial profile of the Mid-Atlantic Ridge 96 48. Profile of total magnetic intensity and topography, Mid-Atlantic Ridge 101 49. Physiographic provinces and trans-Atlantic structure 102 TABLES TABLE PAGE 1. Characteristics of continental rise, northeastern United States 27 2. Characteristics of continental rise, northwest Africa 41 3. Depths of prominent continental-shelf terraces 42 ABSTRACT The Physiographic Diagram: Atlantic Ocean, Sheet 1, which portrays the North Atlantic between 17° and 50° North Latitude, is the first of a projected series of diagrams. The diagram is based on continuous echo-sounding traverses made by research vessels. The relief shown on the profiles was sketched in perspective using the technique introduced by Lobeck. Between sounding profiles the relief is speculative, based on extrapolation of trends noted in the profiles. The area of the diagram is divided into three major physiographic regions which are in turn subdivided into the following categories of provinces. CONTINENTAL MARGIN _Category I_ Continental Shelf Epicontinental Seas Marginal Plateaus _Category II_ Continental Slope Marginal Escarpments Landward Slopes of Trenches _Category III_ Continental Rise Marginal Trench-Outer Ridge Complex Marginal Basin-Outer Ridge Complex OCEAN BASIN FLOOR _Abyssal Floor_ Abyssal Plains Abyssal Hills Abyssal Gaps and Mid-Ocean Canyons _Oceanic Rises_ _Seamount Groups_ MID-OCEANIC RIDGE _Crest Provinces_ Rift Valley Rift Mountains High Fractured Plateau _Flank Provinces_ Upper Step Middle Step Lower Step Each province is defined, briefly described, and illustrated with profiles and photographs of echo-sounding records.