Abstract This paper presents primarily the soundings obtained by the Research Vessel ATLANTIS during the summer field season of 1947. It also utilizes some of the data obtained by later expeditions of the ATLANTIS and by several other vessels. Xhe ATLANTIS is equipped with a continuously recording fathometer with a range up to 4000 fathoms. The outstanding features of the area investigated in 1947 (ATLANTIS cruise #150) are: (1) A conspicuously flat plain at a depth of 2900 fathoms occupies the deeper sections of tlfe North Atlantic basin between, Bermuda and the Azores. More recent data indicate that this horizontal stretch of sea floor extends at least as far south as Lat. 29°N. and possibly as far north as Lat. 40°N., between Long. 50°W. and 56°W., and also that at the-same depth a similar plain is found east of the Ridge in the Northern Canary basin. They are dotted with small sea mounts of variable height whose exact shapes have not yet been determined. The size and number of these appear to increase toward the south, and the southern extremities of the plains consjjgi of smooth horizontal stretches interrupted by pronounced elevations and depressions. (2) The Mid-Atlantic Ridge is characterized by two strongly contrasting types of topography: (a) The central backbone of the Ridge, or Main which is shoaler than 1600 fathoms, consists of a series of parallel ranges trending NE.-SW. Several oPthege rise to less than 800 fathoms. Their trend folloWtpughly tbilgfchf Main Range. The portion of the Main Rang which the most attention wis devoted is between lat 30° and 34°N. In this area its width is of the order of 150 miles, and it is of the order of Several hundred miles in.the area of the Azores. (b) The flanks, between the 1600- and 2500- fathorn isobaths, consist of a succession of smooth shelves, each from 1 to 50 nautical miles or more in width. This zone is 200-300 miles in width. (c) The region between the limit of the 2900- fathom plain and the foot of the first of the series of shelves characterizing the flanks of the Ridge, at 2500 fathoms, in soirte localities stands out as a distinct physiographic province. (3) The folio wing local features are noteworthy: (a) Close to 31°N.Lat. a deep east-west trench extends from about 41° to 43°WLong. and cuts deep irito the Maip Range. Its deepest point is at. 2800 fathoms. Crushed and metamorphosed ultra- basics were broiqfct to the surface by dredging its flanks. (b) At 30°15′W., 34°N., a flat-topped sea mount rises to 180 fathoms, about 100 miles southeast of the Main Range. The dredge brought up calcareous discs of probably Cenozoic age off its top. They were about 15 cm in diameter and about 4,cm thick. (c) Another sea mount was found at 33°43′N., 62°30′W., 150 miles NE. of Bermuda, shoaling to 780 fathoms, having a NW.-SE. elongation, an unknown total length, and a width of 4 miles across its flat top. (d) Another flat suboceanic plain occupies the bottom of the basin separating Beriftuda from the Northeastern United States, at a depth of 2650 fathoms.

Abstract In the North Atlantic one may distinguish Several types of topography, each of which'is characteristic of certain portions of the ocean floor. The continental slopes are now known to show in many cajpes a steplike succession ofliorizontal or im-perceptibly sloping shelves or terraces. They are dissected by systems of submarine canyons, the pattern of which is such as to suggest to many a subaerial origin. A recent investigation of the Hudson submarine valley by the ATLANTIS has shown that this valley extends at least as far as the 2500 fathom curve, 300 miles out to sea, and shows features suggesting a subaerial origin. The broad basins are chkracterized by smooth floors or plains covering areas ofrmore than 200,000 square miles qut of which there rise large sea mounts, isolated or in groups. Thus plains at a-depth of 2900 fathoms occupy,the floors of the North American and the North Canary basins. The floor of the northwestern part of the North American basin, north of Bermuda, is characterized by a plain at 2650 fathoms. Sea mounts of various bizes and shapes rise out of these otherwise smooth plains. There is a conspicuous group of flat-topped sea mounts rising from the 2650-fathom plain of the western North American basin. They rise approximately to 800 fathoms and string out toward thk southeast, roughly from the direction of Cape Cod. They show evidence of terracing on their flanks. The central part of the Atlantic Ocean is characterised by along submarine mountain range. Extending from Iceland to a point southwest of the Cape of Good Hope, known as the Mid-Atlantic Ridge. Numerous fathograms across the Ridge have shown that it is characterized by a high central zone, or Main Range, consisting of parallel ridges following the general trend of the Ridge and rising'in many places-to less than 800 fathoms below the Surface pf the ocean. Between 1600 and 2500 fathoms the flanks of the Ridge are characterized by a succession of flats which for the lack of any better tprm have been called terraces. Rtflection-shgoting studies by Ewing and Press and others .have shown that these terraces are or have been areas of greater deposition. Finally, between this Terraced Zone and the 2900-fathom plain, one ftiay usually recognize a mountainous area standing out as a distinct physiographic province.

Abstrac Seismic-refraction measurements in the western basins of the North Atlantic Ocean show that an average crustal section consists of ½–1kmkm of low-velocity sediments and 4-6 km of oceanic crustal rock in which the seismic velocity is about of 6.5km/sec. There is good evidence from sub-bottom reflections andllshear waves that in many places there is a layer with a velocity between 4.5 and 5.5 km/sec and a thickness of 1-2 km betwleten the Jow-velocity Pediments and-the 6.5 km/sec layer, although it is not usually detected by refracted arrivals. These layers are underlain by the mantle which has an average seismic velocity near 8 km/sec. Measurements in the eastern basins show a similar crustal section, but the velocity below the deep discontinuity appears to be lower (7.7–7.8 km/sec). Measurements in the Mediterranean Sea show only low-velocity sediments underlain by a refracting layer in which the average velocity is about 4.5 km/sec. On the Mid-Atlantic Ridge the sediments are underlain by two refracting layers with velocities averaging 5.6 and 7.4 km/sec respectively. The results indicate that the ridge has been built blithe upwelling of great amounts of basalt magma along a tenslonal fracture zone. Presumably the extcnsional forces and the supply of basalt magma come from convection currents deep in the mantle. Measurements in the Norwegian and Greenland seas show results very similar to those on the Mid-Atlantic Ridge, and, from this and the extension of the belt of active seismicity, ft appears that the ridge structure continues through the Norwegian and Greenland seas into the Arctic Ocean. The results of a few stations on the continental shelf of North America, Britain, and Norway are presented and compared with previously published results in these areas