(a) Values of gradient from 1:10-1:1500; (b) Values of gradient from 1:100-1:5000] The inclination of the bottom is given as the tangent of the angle between the sloping plane and the horizontal expressed as a ratio of integers. These ratios are referred to as gradients. In Figure 6 slope values expressed in degrees, per cent grade, feet per statute mile, and gradient are compared. With a few exceptions all profiles are represented with a 40:1 vertical exaggeration. To facilitate judging the magnitude of slopes on these profiles, Figure 7 shows various gradient ratios at a 40:1 vertical exaggeration. Slope corrections have not been made to the soundings. Except in special cases such corrections would make insignificant changes in the 40:1 profile. All distances are given in nautical or geographical miles (1 nautical mile = 6080 feet). [Illustration: FIGURE 7.--_Gradients from 1:5 to 1:8000 shown at a 40:1 vertical exaggeration_ Most profiles reproduced in this paper are at 40:1 vertical exaggeration. This template is provided to aid the reader in judging slopes on these exaggerated profiles.] [Illustration: FIGURE 8.--_Outline of submarine topography_ Line 1, first-order features of the crust; line 2, major topographic features of the ocean; line 3, categories of provinces and super-provinces; line 4, provinces; line 5, sub-provinces and other important features.] CONTINENT AND OCEAN The two first-order morphologic divisions of the earth's crust are continent and ocean. The oceans can be divided into a few major divisions which are in turn subdivided into categories of physiographic provinces and then into individual provinces. The area of the present study is composed of the three major divisions shown in Figure 9: continental margin, ocean-basin floor, and mid-oceanic ridge. The discussion and description of the physiographic provinces of the North Atlantic will follow the schematic outline shown in Figure 8. [Illustration: FIGURE 9.--_Major morphologic divisions: North Atlantic Ocean_ The profile is a representative profile from New England to the Sahara Coast.] CONTINENTAL MARGIN DEFINITION AND GENERAL CATEGORIES The continental margin includes those provinces of the continents and of the oceans which are associated with the boundary between these two first-order features of the earth.