_Origin of abyssal-floor topography._--The explanation that abyssal plains represent portions of the abyssal floor buried beneath sediments transported by turbidity currents, and that the abyssal hills represent this unburied surface, has been offered by Heezen _et al._ (1951; 1954) and by Menard (1955). Abyssal gaps are pictured as passages through which turbidity currents flow from a higher plain to a lower one. Sediment must have filled in the Hatteras Plain to the present depth of the western end of Vema Gap before the Nares Abyssal Plain could begin to form. Ewing _et al._ (1953) suggest that the Northwest Atlantic Mid-Ocean Canyon was probably formed by turbidity currents which flow from the vicinity of Greenland to the Sohm Abyssal Plain. Whether the canyon-forming process was largely erosional or depositional remains to be seen, but the narrow, deep abyssal gap in the Southeast Newfoundland Ridge suggests erosion. The parallelism of the Mid-Ocean Canyon with the Mid-Atlantic Ridge even suggests a tectonic origin. The deep-sea sands of the canyon floor and its continuous gradient argue as strongly for a turbidity-current origin. The features of the smooth parts of the abyssal floor seem clearly the result of deposition by turbidity currents (Ericson _et al._, 1955; in press). OCEANIC RISES _Definition and distribution._--An oceanic rise is a large area (measured in hundreds of miles), not connected to or included in a mid-oceanic ridge or connected to a continental rise, which rises a few hundred fathoms above the surrounding abyssal floor. The topography of an oceanic rise ranges from gentle to extremely rugged. Included in this classification is the Bermuda Rise and Corner Rise of the North Atlantic, the Rio Grande Rise of the South Atlantic, and a number of similar but unnamed features in the Indian and Pacific oceans. * * * * * _Regional description._--Only three oceanic rises are situated in the North Atlantic. They are: Bermuda Rise, Corner Rise, and Rockall Rise. Of these only the Bermuda Rise has been crossed by many echo-sounding traverses. BERMUDA RISE: In the center of the North America Basin and surrounding the island of Bermuda is an oval asymmetrical arch about 300 by 600 miles with the long axis oriented northeast. The topography of the Bermuda Rise is relatively gentle as compared to the Mid-Atlantic Ridge but considerably more rugged than the continental rise (Pl. 27). The rise is usually distinctly marked on the west by a 10- to 100-fathom decrease in depth and a change from the monotonously smooth abyssal plain on the west to gently rolling hills, 20-50 fathoms high and 2-10 miles wide (Pl. 16). In contrast the eastern edge of the rise is marked by a series of scarps 300-900 fathoms high (Pl. 17), and the adjacent rise topography is relatively rugged. West of the scarp zone the topography is quite smooth to the western margin of the rise (Pl. 16).