South of Newfoundland the continental rise merges with the abyssal plain with no scarp or line of hills. Here the boundary is taken at the point where the gradient is gentler than 1:1000. This is not an entirely arbitrary definition since an appreciable change in gradient generally occurs near this point. South of the boundary the gradient gradually decreases to 1:3000 at 37° N., beyond which it decreases even more gradually and reaches 1:5000 at the southern edge of the plain. HATTERAS ABYSSAL PLAIN: Northeast of the Bermuda Rise on a line between Long Island (N. Y.) and Bermuda a small area of abyssal hills lies between the continental rise on the northwest, the Bermuda Rise on the southeast, the Sohm Abyssal Plain on the northeast, and the Hatteras Abyssal Plain on the southwest. Although this area has been traversed by more tracks than any area of similar size in the Atlantic, the relationships here are still somewhat obscure. This area of some 6000 square miles is one of irregular relief, but it is not known if it is of the character of an irregular sill or a complex abyssal gap, although the latter seems more probable. The northwest margin of the Hatteras Abyssal Plain is abruptly formed by the lower continental rise hills. The eastern boundary is generally distinct and in places is formed by a 50- or 100-fathom scarp, but generally it is not so impressive as the east and west boundaries of the Sohm Abyssal Plain. South of about 34° N., the western boundary of the Hatteras Abyssal Plain is formed by the outer ridge. In some sectors the eastern flank of the outer ridge is rather gentle, and the abyssal plain gently laps the ridge without a sharp break. This boundary is generally sharp north of 27° N. but to the south is less definite. The Hatteras Plain slopes southeast to about 31° N. where the direction of slope changes to south. A gradient of about 1:3000 and depths of about 2900 fathoms are reached at 25° N. Within about 60 miles of Vema Gap the plain is broken by numerous mid-ocean canyons which converge on the gap (Pl. 14, fig. 3). The Hatteras Abyssal Plain has no known seamounts or sea knolls, in contrast to the Sohm Abyssal Plain and Nares Abyssal Plain. The sediments of the Hatteras Abyssal Plain resemble the northern abyssal plains in having many beds of quartz silts (Pl. 28) and contrast sharply with the Blake-Bahama Abyssal Plain where calcareous sands are abundant. On the west the outer ridge forms a continuous barrier to turbidity-current sediments coming from the near-by Bahamas so that the only source of turbidity-current sediments is from the Cape Hatteras region to the north or possibly from the Hudson Canyon via the suspected abyssal gap mentioned above. The type of sediment is entirely consistent with the gradient of the plain which slopes southward from the Hatteras region. [Illustration: Heezen _et al._, PL. 14