Chapter VII: Introduction (4)

_Azores-Gibraltar Ridge._--An ill-defined irregular ridge runs from the eastern end of the Azores Plateau to the Straits of Gibraltar. Largely on the basis of its seismicity we infer that this ridge is structurally and topographically similar to the Mid-Atlantic Ridge. The few existing topographic profiles across this feature suggest that the earthquake belt is associated with a rift valley of the same general type as the central Rift Valley of the Mid-Atlantic Ridge. Depths in this rift appear to reach 2300-2800 fathoms, and the depth of the tops of the adjoining mountains range from 1600 to 2000 fathoms. The flank provinces are even less well developed.

* * * * *

_Atlantis-Plato-Cruiser-Great Meteor Seamount Chain._--South of the Azores a chain of great seamounts branches off from the High Fractured Plateau and crosses the Upper and Middle steps in a nearly north-south direction. These seamounts--Atlantis, Plato, Cruiser, and Great Meteor--in general have broad, nearly flat summits at depths of 100-250 fathoms. The largest one, Great Meteor, was discovered by workers on the METEOR in 1937. This seamount, 60 miles across at its base, rises majestically more than 2600 fathoms above the floor of the ocean. Sands and calcareous rocks have been dredged from the summits, and Tertiary sediments have been obtained from the flanks of the seamounts. Photographs of the tops and of the flanks to a depth of 1600 fathoms show ripple marks. This group is described in a paper by Heezen, Ewing, Ericson, and Bentley (in press).

GEOLOGY AND GEOPHYSICS OF MID-ATLANTIC RIDGE PHYSIOGRAPHIC PROVINCES

_Seismicity of the Mid-Atlantic Ridge._--The earthquake epicenters instrumentally determined for the North Atlantic up to 1956 are shown in Plate 29. Nearly all earthquakes fall in the crest zone. Considering that determination of shocks is accurate only to within -½° to 1° +, it is quite surprising that the plotted epicenters form such a narrow belt. Investigation of the problem of the physiographic province most seismically active reveals that many epicenters actually plot in the Rift Valley and that virtually all that do not are within about 1° of the Rift Valley. All seismic activity therefore is limited to the crest provinces, and probably virtually all the activity is concentrated within the Rift-Valley Province. A line of epicenters runs from the Rift Valley near Flores Island of the Azores toward the Straits of Gibraltar.

* * * * *

_Sediments and physiographic provinces of the Mid-Atlantic Ridge._--The Mid-Atlantic Ridge is the major site of undisturbed pelagic sedimentation of the Atlantic because of its isolation from down-slope movements starting on the continental margin. However, turbidity currents must form near the shores of oceanic islands and the edges of shallow banks and probably contribute sediments to the intermontane valleys. Photographs taken on the Rift Mountains and on the sides of major seamounts show scour marks and ripple marks, indicating considerable winnowing and scour by deep-ocean currents (Pl. 19). This sediment carried from the tops of peaks and deposited on the steep mountain sides probably slumps occasionally and forms turbidity currents which flow to the adjacent valley floors (Pl. 28). For this reason cores taken in intermontane valleys and near the higher peaks will have considerable interlayering of turbidity-current deposits, and much of the sides and crests of individual high mountains is bare rock. In the Rift Mountains true pelagic sediments are only occasionally found. It is striking to note that in coring and bottom photography bare-rock slopes are found most commonly in the Rift Mountains and High Fractured Plateau, but flat-floored intermontane basins are absent in these provinces. This must indicate either that the topography of the Rift Mountains is very new or that the sediment eroded from the crest provinces is carried all the way to the Upper Step Province where it is deposited in the intermontane basins.

* * * * *

_Rocks of the Mid-Atlantic Ridge._--Our knowledge of the lithology of the Mid-Atlantic Ridge comes from three sources: (1) rocks dredged from the sea floor, (2) detrital rock fragments found in sediment cores, and (3) rocks exposed on the islands of the Ridge.

Some of the earliest rock dredging on the Mid-Atlantic Ridge was done in 1885 by the TALISMAN expedition. In 1949 Furon (1949) reported the occurrence of fossil trilobites in dredge samples which had been stored for more than half a century in a French Museum. One dredging was made in the High Fractured Plateau of the eastern Atlantic at 42° 21´N., 17° 12´W., in 4255 meters depth (2330 fathoms). Furon believes that the material was _in situ_ and therefore proof of early Paleozoic outcrops on the Mid-Atlantic Ridge. The abundant evidence of glacially rafted rocks even as far south as 30° N. casts serious doubt on this conclusion, but nevertheless the possibility that the material might have been _in situ_ must be considered.

The Mid-Atlantic Ridge Expedition of 1947 led by Ewing made a number of successful rock-dredge hauls on the Mid-Atlantic Ridge. The most successful hauls were made in the Rift Valley and on the adjacent Rift Mountains at about 30° N. Lat. The specimens have been described by Shand (1949), who reported olivine gabbro, serpentine, basalt, and diabase. One limestone of probably Tertiary age was collected the same year but has not been described. The suite of crystalline rocks obtained is similar to those found on oceanic islands elsewhere on the Mid-Oceanic Ridge.

* * * * *

_Crustal structure and the Mid-Atlantic Ridge provinces._--The crustal structure of the Mid-Atlantic Ridge provinces has been determined at about 20 places by the seismic-refraction technique (Fig. 35f). These studies, conducted by John I. Ewing and W. M. Ewing (in press), have shown that the average crustal structure of the crest provinces and Upper Step consists of 0.4 km of low-velocity sediment and 2.8 km of rock with a velocity of 5.1 km/sec overlying a substratum in which the velocity is 7.3 km/sec. The thickness of the layer of low-velocity sediment varies considerably from place to place. In the crest provinces the 5.1 km/sec layer is commonly exposed. In the flank provinces appreciable thicknesses (to 1 km) of sediment have been measured. The sediment seems to thicken between major scarp zones and ridges as if the sediment were collecting in longitudinal basins parallel to the axis of the ridge. An insufficient number of measurements have been made to determine whether these accumulations correlate with the boundaries of individual intermontane basins or with the limits of individual step provinces.

The structure of the abyssal floor, crest provinces, and flank provinces is compared in Figure 35. The two higher-velocity layers shown in the abyssal-floor sections (Nares and Sohm Abyssal Plain, Fig. 35), have been observed in all measurements made in these provinces. The 6.7 km/sec layer is generally considered to be gabbroic, and the 8.1 km/sec layer is by definition the earth's mantle. In the Mid-Atlantic Ridge section the upper high-velocity material has an average velocity of 5.1 km/sec and is generally identified as basaltic rock. The velocity of the underlying material (7.3 km/sec) is intermediate between the velocity of oceanic crustal rocks (6.7 km/sec) and that of mantle rocks (8.1 km/sec), as observed both beneath the continents and beneath the abyssal floor.

Ewing and Ewing (in press) suggest that this intermediate velocity is the result of a physical mixture of oceanic crustal rocks and mantle rocks. To explain such large-scale mixing they propose that extensive vulcanism and intrusion along the Mid-Atlantic Ridge have produced an intermingling of the crustal and mantle rocks, and that this was associated with convection cells in the deep mantle which supply large quantities of basaltic magma and produce extensional forces on the crust and upper mantle.

Nearly 20 crossings of the crest of the ridge have been made with the total-intensity magnetometer towed behind research vessels employing continuously recording echo sounders. A characteristic anomaly pattern has been noted by Ewing, Heezen, and Hirshman (1957). The Rift Valley is characterized by a large positive anomaly, while the adjoining Rift Mountains show negative anomalies of 300 to 500 gammas (Fig. 48).

Free-air gravity anomalies over the crest provinces and Upper Step are usually 30-50 mg positive, while the Rift Valley as measured in two places gave free-air anomalies of -3 and -20 mg.

Soundings made with PDR. Magnetic measurements made with fluxgate total-intensity magnetometer. Magnetic values in gammas relative to an arbitrary zero.]

Based on scattered seismic-refraction measurements in the North Atlantic which have been projected along province boundaries. The topographic profile was pieced together from continuously recorded echo-sounding profiles from New York to Spanish Sahara.

In the continental margin the upper layer represents the sedimentary rock. The dashed symbol indicates the continental crustal rocks. The lower layer represents oceanic crustal rocks. The mantle lies below the lowest layer.]

A heat-flow measurement by E. C. Bullard in the Rift Valley province in the North Atlantic indicated a value of about 7 × 10⁻⁶ cal./cm²/sec. which is about 6 times the average value of 1.2 × 10⁻⁶ cal./cm²/sec observed in the Lower Step and abyssal floor of the eastern Atlantic (Bullard, 1954; Bullard _et al._, 1956).

High heat-flow values have also been observed on the Easter Island Ridge of the Southeast Pacific, suggesting that the entire Mid-Oceanic Ridge rift system may be so characterized.

An adequate synthesis and explanation of all these converging lines of evidence has not yet been formulated. However, the correlation of so many types of geophysical and geological data speaks favorably for the validity and tectonic significance of the physiographic provinces described here.

On the basis of the observed correspondence of crustal structure and physiographic provinces a hypothetical trans-Atlantic structure section was prepared (Fig. 49). Seismic-refraction measurements were projected along province boundaries and plotted beneath an echo-sounding profile from New York to Spanish Sahara. The black splotched areas represent the 7.3 km/sec layer. This velocity intermediate between 8.1 km/sec of normal mantle and 6.7 km/sec of normal oceanic crust is considered (1) a mixture of the two normal layers; (2) a low-velocity part of the mantle, or (3) a distinct crustal layer characteristic of mid-oceanic ridges. The structure shown for the continental margin of Africa is based on analogy with the structure of the continental margin of northeastern United States. This procedure seems justified by the close similarity of the continental-margin physiographic provinces of the two areas.

* * * * *

_Origin of the Mid-Atlantic Ridge._--Of the many theories which have been proposed for the origin of the Mid-Atlantic Ridge almost all have been extremely speculative, and none has been based on any very detailed knowledge of the feature. We are still a long way from having a comprehensive knowledge of the Ridge. The various theories of origin and their factual basis have been briefly reviewed by Tolstoy and Ewing, who conclude that it is impossible to say if the feature is primarily of folded or faulted origin. In a paper in press Heezen and Ewing compare in detail the topography and seismicity of the African rift valleys and the Rift Valley of the Mid-Atlantic Ridge. Their conclusion is that the two areas are of basically the same structure, and in fact both form parts of the same continuous structural feature. Since the African rift valleys seem clearly to be the result of normal faulting resulting from extension of the crust, Heezen and Ewing conclude that the topography of the Mid-Atlantic Ridge is largely the result of normal faulting. Whether the forces are the result of horizontal extension or vertical uplift remains the most important unsolved problem in connection with the origin of the continental as well as the suboceanic rift-valley systems. Hess (1954) has proposed a mechanism relating suboceanic uplift to expansion due to serpentization of the upper mantle.

SUB-BOTTOM REFLECTIONS RECORDED ON PRECISION DEPTH RECORDER RECORDS AND PHYSIOGRAPHIC PROVINCES

In some areas of the ocean PDR records show a reflecting surface a few fathoms below the bottom. Such horizons are observed only when the sounder is operated with a short (5-millisecond) ping length (in echo sounding the transmitted sound is called the ping, and its duration is called the ping length). When a long ping is used the first returning echo masks any subsequent echoes occurring less than about 10 fathoms after the first echo. To establish continuity of the lower horizon it is necessary to run the recorder without interruption, sending pings once a second. Since a faulty pinging circuit or some accident of geometry could conceivably send out two closely spaced pings, the supposed sub-bottom echoes must be carefully checked to make sure that they are not both bottom echoes from two closely spaced pings. If two pings were being sent out the second echo would always symmetrically underlie the bottom surface. If, however, the two surfaces show local variations, it can be safely concluded that the deeper one is a true sub-bottom echo. In order to observe sub-bottom echoes the sea floor should be reasonably smooth since in rugged relief side echoes and crossing "highlight" hyperbolas obscure any sub-bottom echoes which might occur. Sub-bottom echoes in the Gulf of Maine have been well described by Murray (1947). In local inshore areas prominent sub-bottom echoes recorded by unmodified or slightly modified standard echo sounders have been used to map basement rocks (Smith _et al._, 1952).

In the deep sea, sub-bottom echoes or "penetration" are observed most frequently in the continental rise, oceanic rises, and the far edges of the abyssal plains. Penetration is rare on the open continental shelf and on the continental slope. As the depth increases, echoes are more difficult to obtain, so that records from different depths cannot be directly compared in reference to ease of penetration. It nevertheless seems to be true that sub-bottom echoes are rare or absent on PDR records from the parts of the abyssal plains closest to the continental margin. Penetration in the continental rise is common but frequently irregular and intermittent. One of the most persistent and uniform sub-bottom reflecting horizons observed occurs on the outer ridge east of the Bahamas (south of 30° N.) (Pl. 6).

Records from the abyssal plain immediately adjacent to the abyssal hills (Pl. 13 Fig. 4) and from the flat-floored tongues in the abyssal hills (Pl. 10) reveal some of the deepest and strongest sub-bottom echoes. Good sub-bottom echoes are common in the Bermuda Plateau.

The sub-bottom reflecting layers frequently crop out, and the overlying sediments thicken and thin, revealing apparently noticeable variations in the rate of accumulation of sediments. Outcropping of sub-bottom layers on the steeper slopes indicate slumping, while the deepening of the sub-bottom reflecting horizon in valleys indicates a greater rate of deposition. High-frequency sound is normally strongly attenuated by transmission through sediments. The observation of sub-bottom reflections with high-frequency sound pulses (12 kc) indicates (1) that the surface sediment is uniform and is of low density, and (2) that a fairly sharp density change occurs beneath this surface layer of low-density material. In areas such as the outer ridge from 22° to 29° N. Lat. and the southern Bermuda Rise, it can be safely assumed that the upper layer consists of deep-sea red clay. Density measurements on red clay have indicated values of 1.25 to 1.45. The lack of sub-bottom reflections over the parts of the abyssal plains close to the continents is attributed to the numerous sand and silt layers found in the cores which reflect most of the sound. The occurrence of good reflections beneath the outer edges of the abyssal plains could be explained by either assuming that for a long geologic time no sand-or silt-carrying turbidity current has reached this area, or that red clay is deposited here much faster than elsewhere.

An extremely prominent sub-bottom reflector observed over a vast area of the east tropical Pacific has been identified by coring with a 10-cm thick bed of white, vitreous ash. This suggests that sub-bottom reflections found elsewhere may, in general, represent ash horizons. This, of course, would presuppose ash falls so vast that some record should have been preserved on land. There is no reason to assume that there is but a single cause of deep-sea sub-bottom echoes.

The widespread occurrence of the sub-bottom interface on the deeper isolated rises may be of great importance if it be interpreted as evidence of a sudden change in sedimentation resulting in a change from higher- to lower-density sediment. It is just conceivable, however, that some unstable diagenetic process may cause a sudden increase in compaction at a depth corresponding to the sub-bottom reflection.

The sub-bottom reflections in depths of 2600 fathoms on the southern Bermuda Rise and the outer ridge is about .02 second after the bottom echo, and this indicates a layer about 10 fathoms thick. At a rate of deposition of 1 cm/1000 years this change in sediment type would have occurred 20 million years ago.

In a remotely situated oceanic area the factors controlling whether red clay or _Globigerina_ ooze is laid down are largely related to depth and temperature of the bottom water. These two factors are related to those which control the solubility of the carbonate and thus the type of bottom deposit. Emiliani and Edwards (1953), from a study of oxygen isotopes in benthic Foraminifera in Tertiary deep-sea sediments from the eastern Pacific, concluded that the temperature of Pacific bottom water decreased 8° C. from the Eocene to the present. This should have caused a great increase in the solution of carbonate assuming other factors unchanged. Sub-bottom reflections then may also be interpreted as the result of a change in the temperature or the circulation of bottom water in the deep basin. Extensive, basin-wide sub-bottom reflectors, whether the result of vast beds of ash or widespread changes in pelagic sedimentation, imply events of global importance. The further investigation and identification of these reflectors should produce data of far-reaching application in geology, climatology, and paleo-oceanography.

SUMMARY OF PROVINCE CHARACTERISTICS

The Atlantic Ocean floor consists of three major morphological divisions: (1) continental margin, (2) ocean-basin floor, and (3) Mid-Oceanic Ridge. The continental margin is formed by three categories of provinces which represent (1) the submerged continental platform, (2) the steep edge of the continental block, and (3) the raised or depressed edge of the ocean floor. The topographic detail of the continental margin is predominantly smooth except for the submarine canyons and minor irregularities of the upper continental rise. A close correspondence of topography and distribution of recent sediments is apparent. For example, deep-sea sands are found in the submarine canyons and on the canyon deltas of the lower continental rise. The continental slope appears to be a thinly veneered or bare outcrop of Tertiary and Mesozoic sediments. Individual topographic benches can be traced for many miles along the strike. On the basis of published descriptions and dating of dredged rock, certain prominent benches are identified as the outcrop pattern of various Cretaceous and Tertiary formations. The lower continental rise can be directly traced into the outer ridge at Cape Hatteras. The upper continental-rise and the marginal-trench provinces lie between the abrupt continental slope and the outer ridge. Seismic-refraction measurements in the continental margin indicate the greatest thickness of sedimentary rocks under the upper continental rise. Thus if we consider the initial form as an unfilled depression it would have been remarkably similar to the form of the present marginal trenches.

The ocean-basin floor lies between the continental margin and the Mid-Oceanic Ridge and consists of the deeper abyssal floor and the elevated oceanic rises. On the abyssal floor adjacent to the continental margins are found the flattest surfaces of the earth. These abyssal plains apparently were built by turbidity-current deposits. The unburied abyssal floor is represented by the abyssal hills. The oceanic rises are broad uplifts which rise from the abyssal floor through a series of scarps. Oceanic rises are covered with pelagic sediments except locally near islands and seamounts. The crustal structure of oceanic rises differs significantly from the typical abyssal floor in having lower velocities and generally thicker crustal layers.

The Mid-Oceanic Ridge is a broad fractured arch whose axis follows the median line of the ocean. It generally covers the center third of the ocean. The ridge provinces are divided into crest provinces and flank provinces. The crest provinces include (1) the Rift Valley, a long axial cleft 15-30 miles wide and 500-1500 fathoms deep; (2) the Rift Mountains which form the sides of the Rift Valley; and (3) the High Fractured Plateau, a rugged plateau which borders the Rift Mountains. The flank provinces consist, on each side, of roughly three steps separated by large scarps. A seismic belt accurately follows the Rift Valley. The topography of the Mid-Oceanic Ridge seems best explained by extensive normal faulting. The mid-oceanic Rift Valley connects with and is probably of the same origin as the African rift valleys.

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Sci., v. 250, p. 849-873

---- 1955, Orleansville earthquake and turbidity currents:
Amer. Assoc. Petrol. Geol. Bull., v. 39, p. 2505-2514

---- In press, Seismicity of the Mid-Atlantic Ridge: Amer.
Geophys. Union Trans.

HEEZEN, B. C., ERICSON, D. B., AND EWING, M., 1954, Further
evidence for a turbidity current following the 1929 Grand
Banks earthquake: Deep Sea Research, v. 1, p. 193-202

HEEZEN, B. C., EWING, M., ERICSON, D. B., 1951, Submarine
topography in the North Atlantic: Geol. Soc. Am. Bull., v.
62, p. 1407-1409

---- 1954, Reconnaissance survey of the abyssal plain south of
Newfoundland: Deep Sea Research, v. 2, p. 122-133

HEEZEN, B. C., EWING, M., AND MENZIES, R. J., 1955, The
influence of submarine turbidity currents on abyssal
productivity: Oikos, v. 6, p. 170-182

---- 1958, Submarine turbidity flows: Priroda, Feb. 1958, p.
100-104 (in Russian)

HEEZEN, B. C., EWING, M., AND MILLER, E. T., 1953,
Trans-Atlantic profile of total magnetic intensity and
topography, Dakar to Barbados: Deep Sea Research, v. 1, p.
25-33

HEEZEN, B. C., EWING, M., ERICSON, D. B., AND BENTLEY, C. R.,
in press, The flat-topped Atlantis, Cruiser and Great
Meteor seamounts: Geol. Soc. Am. Bull.

HEEZEN, B. C., and others, in preparation, Submarine structural
benches and drowned beaches of the Atlantic

HESS, H. H., 1954, Geological hypotheses and the earth's crust
under the ocean: Roy. Soc. Proc., A. v. 222, p. 341-348

---- 1933, Interpretation of geological and geophysical
observations, Navy Princeton gravity expedition to West
Indies in 1932: Naval Observatory Publ. no. 19, p. 27-54

HILL, M. N., 1954, Topography of the Mid-Atlantic Ridge
(Abstract): Abstracts of Rome meeting of U.G.G.I.

---- 1956, Notes on the bathymetric chart of the northeast
Atlantic: Deep Sea Research, v. 3, p. 229-231

---- 1957, Recent geophysical exploration of the ocean floor,
p. 129-163 _in_ Ahrens, L. H., _et al._, _Editors_, Physics
and chemistry of the earth: Pergamon Press, London, 259 p.

JOHNSON, DOUGLAS, 1939, Origin of submarine canyons: Columbia
Univ. Press, New York, 216 p.

KATZ, S., AND EWING, M., 1955, Seismic-refraction measurements
in the Atlantic Ocean, Part VII, Atlantic Ocean basin west
of Bermuda: Geol. Soc. Am. Bull., v. 67, p. 475-510

KAY, M., 1951, North American geosynclines: Geol. Soc. Am.
Memoir 48, 143 p.

KELLER, F. J., MEUSCHKE, J. L., AND ALLDREDGE, L. R., 1954,
Aeromagnetic surveys in the Aleutian, Marshall, and Bermuda
islands: Amer. Geophys. Union Trans., v. 35, p. 558-572

KNOTT, S. T., AND HERSEY, J. B., 1956, Interpretation of
high resolution echo-sounding techniques and their use
in bathymetry, marine geophysics, and geology: Deep Sea
Research, v. 4, p. 36-44

KOCZY, F. F., 1954, A survey on deep sea features taken during
the Swedish Deep Sea Expedition: Deep Sea Research, v. 1,
p. 170-184

---- 1956, Echo soundings, Reports of Swedish Deep Sea
Expeditions, v. IV, Bottom investigations: Fasc. II, p.
99-158, Göteborgs Kungl. Vetenskaps-och vitterhetssamhälle

KUENEN, PH. H., 1950, Marine geology: Wiley, New York, 568 p.

LEE, C. S., 1951, Geophysical surveys on the Bahama Banks:
Inst. Petrol. Jour. London, v. 37, no. 334, p. 633-657

LOBECK, A. K., 1924, Block diagrams: Wiley, New York, 206 p.

---- 1939, Geomorphology: McGraw-Hill, New York, 731 p.

---- 1948, Physiographic diagram of the United States: Geog.
Press, N. Y.

LUSKIN, B., AND ISRAEL, H. G., 1956, Precision depth recorder
MK-V: Lamont Geological Observatory and Time Facsimile
Corp., Tech. Rept. no. 12, 31 p.

LUSKIN, B., HEEZEN, B. C., EWING, M., AND LANDISMAN, M., 1954,
Precision measurements of ocean depth: Deep Sea Research,
v. 1, p. 131-140

MATTHEWS, D. J., 1939, Tables of the velocity of sound in pure
water and sea water for use in echo sounding and echo
ranging: Admiralty Hydrographic Dept., London, 52 p.

MAURY, M. F., 1855, Physical geography of the sea: Harpers, New
York, 389 p.

MENARD, H. W., 1955, Deep sea channels, topography and
sedimentation, Amer. Assoc. Petrol. Geol. Bull., v. 39, p.
236-255

MILLER, E. T., AND EWING, M., 1956, Geomagnetic measurements
in the Gulf of Mexico and in the vicinity of Caryn Peak:
Geophysics, v. XXI, p. 406-432

MURRAY, H. W., 1947, Topography of the Gulf of Maine: Geol.
Soc. Am. Bull., v. 58, p. 153-196

MURRAY, J., AND HJORT, J., 1912, Depths of the ocean: John
Murray, London, 821 p.

NAFE, J., AND DRAKE, C. L., 1957, Variation with depth in
shallow and deep water marine sediments of porosity,
density and the velocities of compressional and shear
waves: Geophysics, v. XXII, p. 523-552

NEWELL, N. D., 1955, Bahamian platforms, p. 303-316 in
Poldervaart. Arie, _Editor_, Crust of the earth: Geol. Soc.
Am. Special Paper 62, 762 p.

NORTHROP, J., AND FROSCH, R., 1954, Seamounts in the North
America basin: Deep Sea Research, v. 1, p. 252-257

NORTHROP, J., AND HEEZEN, B. C., 1951, An outcrop of Eocene
sediment on the continental slope: Jour. Geol., v. 59, p.
369-399

OFFICER, C. B., AND EWING, M., 1954, Geophysical investigations
in the emerged and submerged Atlantic coastal plain, Part
VII, Continental shelf, continental slope, and continental
rise south of Nova Scotia: Geol. Soc. Am. Bull., v. 65, p.
653-670

OFFICER, C. B., EWING, M., AND WUENSCHEL, P. C., 1952, Seismic
refraction measurements in the Atlantic Ocean basin, Part
IV, Bermuda, Bermuda Rise, and Nares Basin: Geol. Soc. Am.
Bull., v. 63, p. 777-809

OFFICER, C. B., EWING, J. I., EDWARDS, R. S., AND JOHNSON,
H. R., 1957, Geophysical investigations in the eastern
Caribbean. Venezuelan Basin, Antilles Island Arc, and
Puerto Rico Trench: Geol. Soc. Am. Bull., v. 68, p. 359-378

PETTIJOHN, F. J., 1957, Sedimentary rocks: Harpers, New York,
718 p.

PRESS, F., AND BECKMANN, W. C., 1954, Geophysical
investigations in the emerged and submerged Atlantic
coastal plain, Part VIII, Grand Banks and adjacent shelves:
Geol. Soc. America Bull., v. 65, p. 299-314

RAISZ, E., 1952, Landforms of the United States: Harvard Univ.,
Cambridge, Mass.

RAITT, R. W., 1956, Seismic-refraction studies of the Pacific
Ocean basin, Part I, Crustal thickness of the equatorial
Pacific: Geol. Soc. Am. Bull., v. 67, p. 1623-1640

RICHARDS, H. J., AND RUHLE, J. L., 1955, Mollusks from a
sediment core from the Hudson submarine canyon: Pa. Acad.
Sci. Proc., v. XXIX, p. 186-190

SCHOTT, G., 1942, Geographie des Atlantischen Ozeans: C.
Boysen, Hamburg, 438 p.

SCHÜLER, F., 1952, On the accuracy of configuration of sea
bottom profiles with high frequency echo sounders: Intl.
Hydro. Review, v. VII, p. 126-135

SHAND, S. J., 1949, Rocks of the Mid-Atlantic Ridge: Jour.
Geol., v. 57, p. 89-92

SHEPARD, F. P., 1931, Saint Lawrence (Cabot Strait) submarine
trough: Geol. Soc. Am. Bull., v. 42, p. 853-864

---- 1948, Submarine geology: Harpers, New York, 348 p.

SHEPARD, F. P., AND COHEE, G. V., 1936, Continental shelf
sediments off the Mid-Atlantic states, Geol. Soc. Am.
Bull., v. 47, p. 441-457

SHEPARD, F. P., TREFFENDEN, J. M., AND COHEE, G. V., 1934,
Origin of Georges Bank: Geol. Soc. Am. Bull., v. 45, p.
281-302

SHURBET, G. L., AND WORZEL, J. L., 1957, Gravity anomalies and
structure of the West Indies, Part III: Geol. Soc. Am.
Bull., v. 68, p. 263-266

SMITH, E. H., SOULE, F. M., AND MOSBY, O., 1937, MARION and
GENERAL GREENE expeditions to Davis Strait and Labrador
Sea under the direction of the U. S. Coast Guard,
1928-1931-1933-1934-1935: U. S. Coast Guard Bull., v. 19,
p. 1-259

SMITH, W. O., UPSON, J. E., and others, 1952, Preliminary
report on the Passamaquoddy bedrock survey, July-August
1951: U. S. Geol. Survey Water Resources Division, 49 p.

SPANGLER, W. B., 1950, Subsurface geology of Atlantic coastal
plain of North Carolina: Amer. Assoc. Petrol. Geol. Bull.,
v. 34, p. 100-132

SPANGLER, W. B., AND PETERSON, J. J., 1950, Geology of Atlantic
coastal plain in New Jersey, Delaware, Maryland, and
Virginia: Amer. Assoc. Petrol. Geol. Bull., v. 34, p. 1-99

STETSON, H. C., 1936, Geology and paleontology of Georges Bank
canyons: Geol. Soc. Am. Bull., v. 47, p. 339-366

---- 1938, Sediments of the continental shelves off the eastern
coast of the United States. Woods Hole Oceanogr. Inst.
Papers in Physical Oceanography and Meteorology, v. V, no.
4, p. 5-48

---- 1949, The sediments and stratigraphy of the east coast
continental margin; Georges Bank to Norfolk Canyon: Woods
Hole Oceanogr. Inst. Papers in Physical Oceanography and
Meteorology, v. XI, no. 2, p. 1-60

STOCKS, T., AND WÜST, G., 1935, Die Tiefenverhältnisse des
offenen Atlantischen Ozeans: Deutsche Atlantischen Exped.
METEOR, 1925-29, Wiss. Erg. Bd., III, Teil 1, Lief 1, 31 p.

STOMMEL, H., 1957, A survey of ocean current theory: Deep Sea
Research, v. 4, p. 149-184

SUTTON, G. H., BERCKHEMER, H., AND NAFE, J. E., 1957, Physical
analysis of deep sea sediments: Geophysics, v. XXII, p.
779-812

SWAIN, F. M., 1947, Two recent wells in coastal plain of North
Carolina: Amer. Assoc. Petrol. Geol. Bull., v. 31, p.
2054-2060

SWALLOW, J. C., AND WORTHINGTON, L. V., 1957, Measurements of
deep currents in the western North Atlantic: Nature, v.
179, p. 1183

TOLSTOY, I., 1951, Submarine topography in the North Atlantic:
Geol. Soc. Am. Bull., v. 62, p. 441-450

TOLSTOY, I., AND EWING, M., 1949, North Atlantic hydrography
and the Mid-Atlantic Ridge: Geol. Soc. Am. Bull., v. 60, p.
1527-1540

TORPHY, S. R., AND ZEIGLER, J. M., 1957, Submarine topography
of Eastern Channel Gulf of Maine, Jour. Geol., v. 65, p.
433-441

UDINTSEV, G. B., 1955, Topography of the Kurile-Kamchatka
Trench: Trans. Inst. of Oceanology, v. XII, p. 16-61 (in
Russian)

UPHAM, W., 1894, The fishing banks between Cape Cod and
Newfoundland: Am. Jour. Sci., 3d ser., v. 47, p. 123-129

VEATCH, A. C., AND SMITH, P. A., 1939, Atlantic submarine
valleys of the United States and the Congo Submarine
Valley: Geol. Soc. Am. Special Paper 7, 101 p.

WASHINGTON, H. S., 1930, Origin of the Mid-Atlantic Ridge: Md.
Acad. Sci. Jour., v. 1, p. 20-29

WISEMAN, J. H. D., AND OVEY, C. D., 1950, Recent investigations
on the deep sea floor: Geol. Assoc. Proc., v. 61, p. 28-84

---- 1953, Definitions of features on the deep sea floor: Deep
Sea Research, v. 1, p. 11-16

---- 1955, Proposed names of features on the deep sea floor 2;
General principles governing the allocation of names: Deep
Sea Research, v. 2., p. 261-263

WORZEL, J. L., AND SHURBET, G. L., 1955a, Gravity
interpretations from crustal sections, p. 87-100 _in_
Poldervaart, Arie, _Editor_, Crust of the earth, Geol. Soc.
Am. Special Paper 62, 762 p.

---- 1955b, The Toro Seamount: Geol. Soc. Am. Bull., v. 66, p.
465-466

WORZEL, J. L., AND SHURBET, G. L., 1955c, Gravity anomalies
at continental margins: Nat. Acad. Sci., Proc., v. 41, p.
458-469

WÜST, G., 1935, Die Stratosphāre: Deutsche Atlantischen Exped.
METEOR, 1925-29, Wiss. Erg., Bd. VI, Teil 1, Lief. 1-2., p.
109-288

WÜST, G., 1940a, Das relief des Azoren Sockels und des
Meeresbodens Nördlich und Nordwestlich der Azoren: Ann. d.
Hydrogr. u. Marit. Meterol., August-Beiheft, Lief 2, p. 1-19

---- 1940b, Zur Nomenklatur der Grossformen der Ozeanboden:
Assoc. Oceanog. Union Geod. Geophys. Internat., Publ. Sci.
no. 8, p. 12-124

---- 1943, Der subarktische Bodenstrom in der westatlantischen
Mulde: Ann. d. Hydrogr. u. Marit. Meterol., H. IV/VI, p.
244-255

---- 1957, Quantitative Untersuchungen zur Statik und Dynamik
des Atlantischen Ozeans; Stromgeschwindigneiten und
Strommengen in den Tiefen des Atlantischen Ozeans: Deutsche
Atlantischen Exped. METEOR, 1925-29, Wiss. Erg., Bd. VI,
Teil 2, 420 p.

ZEIGLER, J., ATHEARN, W. D., AND SMALL, H., 1957, Profiles
across the Peru-Chile trench: Deep Sea Research, v. 4, p.
238-249

AUTHOR INDEX

Agostinho, J., 98, 109

Alldredge, L. R., 51, 111

Andrade, C. F. de, 5, 21, 37, 109

Armstrong, J. C., 36, 109

Ashton, W., 55, 110

Athearn, W. D., 60, 113

Atwood, W. W., 11, 109

Bartholomew, J. C., 5, 109

Beckmann, W. C., viii, 23, 66, 109, 112

Behm, A., 8

Bentley, C., 98, 109, 111

Berckhemer, H., 79, 112

Bourcart, J., 37, 49, 109

Bucher, W. H., viii

Bullard, E. C., 49, 52, 103, 109

Cloos, H., 98, 109

Cohee, G. V., 53, 112

Crary, A. P., 43, 110

Dahl, W. H., 43, 109

Davidson, M. J., viii, 52, 109

Day, A. A., 49, 109

Deacon, G. E. R., viii

de Smitt, V. P., 67, 109

Dietrich, G., 5, 109

Dietz, R. S., 8, 66, 109

Dorman, J., 66, 70, 74, 110

Drake, C. L., vii, 50, 52, 53, 109, 112

Edwards, G., 106, 110

Edwards, R. S., viii, 110, 112

Elmendorf, C. H., viii, 50, 85, 109

Emery, K. O., 5, 109

Emiliani, C., 106, 110

Ericson, D. B., vii, 21, 27, 33, 35, 47, 53, 55, 56, 57, 59, 60, 66,
70, 74, 75, 80, 95, 98, 110, 111

Ewing, J. I., 36, 81, 100, 110, 112

Ewing, W. M., vii, 21, 23, 25, 27, 33, 35, 36, 43, 47, 49, 51, 52, 55,
56, 57, 59, 60, 66, 70, 74, 75, 76, 78, 80, 83, 84, 95, 98, 100,
103, 109, 110, 111, 113

Fenneman, N. M., 11, 110

Fessenden, R., 8

Fisher, R. L., 60, 110

Florisson, 8

Frassetto, R., 36, 110

Frosch, J., 76, 112

Fuglister, F. C., vii

Furon, R., 99, 110

Gaskell, T. F., 49, 52, 55, 109, 110

Gibson, W. M., 66, 110

Gould, Capt. D., vii

Grousson, R., 39, 110

Gutenberg, B., 80, 110

Hamilton, E. L., 78, 110

Hamilton, G. R., vii

Heezen, B. C., viii, 5, 21, 23, 27, 33, 35, 36, 41, 46, 47, 50, 51,
53, 55, 56, 57, 59, 60, 66, 70, 74, 75, 80, 83, 85, 98, 100, 103,
109, 110, 111, 112

Hersey, J. B., vii, 111

Hess, H. H., 35, 103, 111

Hill, M. N., viii, 37, 49, 52, 80, 109, 111

Hirshman, J., viii, 100, 110

Hjort, J., 55, 56, 57, 83, 112

Hubbard, C., vii

Iselin, C. O'D., vii

Israel, H. G., 8, 111

Johnson, D., 21, 111

Johnson, G. L., III, vii

Johnson, H. R., vii, 110, 112

Jordan, G. F., viii

Karlson, Capt. A., vii

Karo, Admiral A., viii

Katz, S., 47, 80, 81, 111

Kay, M., 54, 111

Keller, F. J., 51, 111

Knott, S. T., 111

Koczy, F. F., 55, 111

Kohler, Capt. H., vii

Kuenen, P. H., 21, 111

Landisman, M., vii, 3, 8, 49, 56, 111

Lane, Capt. A. K., vii

Langevin, 8

Langseth, M., vii

Laughton, A. S., 49, 109

Lee, C. S., 36, 111

Lobeck, A. K., v, 1, 11, 111

Luskin, B., vii, 3, 8, 56, 111

Marie, P., 49, 109

Matthews, D. J., 13, 111

Maury, M. F., iv, 5, 111

Maxwell, A. E., 103, 109

Menard, H. W., 66, 74, 109, 111

Menzies, R. J., vii, 55, 56, 111

Meuschke, J. L., 51, 111

Miller, E. T., vii, 51, 52, 78, 109, 111

Mosby, O., 67, 112

Murray, H. W., 25, 105, 111

Murray, J., 55, 56, 57, 83, 112

Nafe, J. E., viii, 49, 50, 79, 112

Nelson, A. L., Cmdr. R.N. (R.), vii

Newell, N. D., 53, 112

Northrop, J., vii, 36, 46, 66, 70, 74, 76, 110, 112

Officer, C. B., 80, 81, 110, 112

Ovey, C. D., 12, 38, 113

Peterson, J. J., 46, 112

Pettijohn, F. J., 53, 112

Pike, Capt. J., vii

Press, F., viii, 23, 112

Pryor, J. S. N., Cmdr. R.N., viii

Raisz, E., v, 112

Raitt, R. W., 80, 112

Revelle, R. R., 103, 109

Richards, H. J., 112

Richter, C. F., 80, 110

Roberts, A., vii

Ruhle, J. L., 112

Rutherford, H. M., 43, 110

Schott, G., 112

Schüler, F., 112

Shand, S. J., 99, 112

Shepard, F. P., 19, 20, 23, 25, 53, 83, 112

Shurbet, G. L., 36, 52, 76, 112, 113

Simonson, K., vii

Sinclair, V., Captain U.S.N. (Ret.), vii

Small, H., 60, 113

Smith, D., vii

Smith, E. H., 67, 112

Smith, P. A., 5, 21, 25, 113

Smith, W. O., 105, 112

Soule, F. M., 67, 112

Spangler, W. B., 46, 112

Stetson, H. C., 25, 43-46, 50, 53, 112

Stocks, T., 112

Stommel, H., 50, 112

Sutton, G. H., vii, 37, 52, 53, 79, 80, 109,
110, 112

Swain, F. M., 46, 112

Swallow, J. C., 49, 50, 109, 113

Talwani, M., following page 122

Tolstoy, I., vii, 5, 22, 27, 57, 66, 76, 83, 84, 95, 98, 103, 113

Torphy, S. R., 25, 113

Treffenden, J. M., 112

Udintsev, G. B., 60, 113

Upham, W., 43, 113

Upson, J. E., 105, 112

Usher, Capt. F. S., vii

Van Santford, H., vii

Veatch, A. C., 5, 21, 25, 113

Vine, A. C., 43, 110

Washington, H. S., 113

Wirshup, M., vii

Wiseman, J. H. D., 12, 38, 113

Woollard, G. P., 43, 110

Worthington, L. V., 50, 113

Worzel, J. L., vii, 36, 43, 52, 65, 76, 109, 110, 112, 113

Wuenschel, P. C., 80, 81, 112

Wüst, G., 5, 12, 22, 50, 57, 98, 112, 113

Zeigler, J. M., 25, 60, 113

SUBJECT INDEX

Abyssal floor, 1, 15, 55
abyssal gaps, 1, 15, 66
abyssal hills, 1, 15, 55, 61
abyssal plains, 1, 15, 55
mid-ocean canyons, 1, 15, 66

Abyssal-floor topography, origin of, 74

Abyssal gaps
definition, 66
general, 1, 66
Theta Gap, 66, 72, 73, 74
Vema Gap 58, 66, 72, 74

Abyssal hills, 1, 61
definition, 61
distribution of, 61, 63
origin of, 65
PDR records of, 38
sub-bottom echos, 105

Abyssal plains
Balearic Abyssal Plain, 57
Bay of Bengal, abyssal plain in, 55
Biscay Abyssal Plain, 56, 57, 60
Blake-Bahama Abyssal Plain, 56, 57, 58, 59
Canary Abyssal Plain, 56, 61
Cape Verde Abyssal Plain, 57, 61
Cayman (Trench) Abyssal Plain, 56, 60
definition, 61
discovery of, 55
distribution of, 61, 63
general, 1, 15, 53
Hatteras Abyssal Plain, 56, 57, 58
Hispaniola-Caicos Abyssal Plain, 60, 65
Horseshoe Abyssal Plain, 57, 61
Iberia Abyssal Plain, 56, 57, 61
Madeira Abyssal Plain, 56, 61
Nares Abyssal Plain, 56, 57, 59
Newfoundland Abyssal Plain, 56
nomenclature, 55
Old Bahama Abyssal Plain, 60
Puerto Rico (Trench) Abyssal Plain, 56, 60
Sohm Abyssal Plain, 55, 57, 71, 72, 74
Tagus Abyssal Plain, 56, 57
Weddell Sea, abyssal plain in, 55

African rift valleys, 103, 107

ALBATROSS III, Research Vessel, vii

Ampere Seamount, 59, 78

Andros well, 47, 48

Anegada Passage, 36

Antilles Outer Ridge, 32, 33, 36, 52, 53, 59, 60

Army Map Service, 5

Atlantis-Plato-Great Meteor Seamount Group, 98

ATLANTIS Research Vessel, vii, 7, 8, 79

Atlantis Seamount, 98

Axis of maximum depth, North Atlantic Ocean, 41, 63, 96

Azores Plateau, 97

Azores-Gibraltar Ridge, 98

Bahamas, 33-36, 47

Banquereau Bank, 24

Basalt, 75, 79, 99

"Basins and deeps", 11

Basins
Cape Verde, 12
Guiana, 12
Iberia, 12
Labrador, 12
Nares, 56
Newfoundland, 12, 56
North America, 12
North Canary, 12
South Canary, 12
West Europe, 12

Bay of Biscay, 17, 56, 60
Tertiary outcrops, 49

Beaches, ancient submerged, 41, 42

Bell Telephone Laboratories, vii

Benches, 1
Bahamas, 34, 35, 49
Bay of Biscay, 49
Blake Plateau, 33, 47-49
Cape Hatteras, 46-48
Georges Bank, 43, 46
Gibraltar, 37, 38
Puerto Rico, 36

Bermuda Apron, 75, 77, 81

Bermuda Pedestal, 75, 77

Bermuda Plateau, 75, 81

Bermuda Rise, 74-77
Bermuda Apron, 75, 77, 81
Bermuda Pedestal, 75, 77
Bermuda Plateau, 75, 81
Bermuda Scarp Zone, 76, 79
Crescent Peaks, 75
crustal structure, 80, 81, 102
Muir Seamount Group, 76
PDR records of, 79
sediments on, 76

Bermuda Scarp Zone, 76, 79

Bioclastic debris, 59

Biscay Abyssal Plain, 39, 56, 57, 60, 65

Blake-Bahama Abyssal Plain, 32, 33, 34, 56, 57, 58, 59

Blake-Bahama Basin, 33, 34
abyssal plain in, 59
sediments in, 59

Blake Escarpment, 17, 19, 47
benches on, 32, 47
outcrops on, 47
PDR record of, 34
seismic investigations, 47, 48

Blake Plateau, 17, 32, 47, 48, 59
description of, 32-33
PDR record of, 35, 38

Browns Bank, 24

Cable failures, submarine, 23, 67

Cable ships, 79

Campeche Escarpment, 33

Canary Abyssal Plain, 56

Cape Breton Submarine Canyon, 37

Cape Hatteras, 25, 46, 47, 48, 50
bottom currents, 50
Esso No. 1 Test, 46
geologic section at, 46, 48

Cape Verde Abyssal Plain, 57, 61

Cape Verde Basin, 12

Carte Générale Bathymétrique des Océans, 5, 11, 12

CARYN, Research Vessel, vii, 78

Caryn Seamount, 28, 72, 78

Cayman Trench (Abyssal) Plain, 56, 60

Classification of deep-sea relief
bathymetric system, 11, 12, 19
textural system, 11

Coast and Geodetic Survey, viii, 25, 43, 44, 45,48

Columbia University, vii, 3, 6, 8, 21

Compaction of sediments, 50, 106

Continent and ocean, 16, 107

Continental margin
benches and terraces, 38, 41-51, 107
categories of, 15, 17
definition, 17
magnetic anomalies, 51
past, present, and future, 53
photographs of, 39, 50
regional description of, 21-41
seismicity of, 51

Continental margin, development of, 53
youth, maturity, and old age, 53
geosynclines, 53, 54

Continental margin Europe and Africa, description of, 36-41
Anglo-French sector, 37
Gibraltar sector, 37-38
Iberian sector, 37-38
North African sector, 38-41

Continental margin North America, description of, 21-36
Anegada Passage, 36
Antilles Outer Ridge, 33
Bahamas sector, 33
Blake Escarpment, 33
Blake Plateau, 32
Flemish Cap, 21
Gulf of Maine, 24-25
Laurentian Channel, 23
Northeastern United States sector, 25
Northern Grand Banks sector, 21
Puerto Rico sector, 36
Scotian Shelf sector, 23
Southeast Newfoundland Ridge, 22
southern Grand Banks sector, 22

Continental margin, provinces
category I, 1, 17, 53, 107
category II, 1, 18, 53, 107
category III, 1, 19, 53, 107

Continental rise, 1, 19, 20, 25, 26, 27, 37, 38, 40, 41
definition, 20
PDR records of, 32
tables of characteristics, 27, 41

Continental shelf, 15-28, 32-42, 107
definition, 18
structure of, 49, 51-54
submerged beaches, 42

Continental slope, 1, 15-48, 107
benches on, 41, 42-51
currents on, 50
definition, 18
geologic map of, 51
photographs of, 39
profiles of, 28, 29

Convection currents, 103

Coral, photograph of, 39

Corner Rise, 63, 64, 77

Corner Seamount, 77

Cretaceous outcrops
Bahamas, 47
Blake Escarpment, 47
Caryn Seamount, 78
Georges Bank, 43

Cruiser Seamount, 98

Crustal convection currents, 103

Crustal structure
continental margin, 52, 53, 102, 107
Mid-Atlantic Ridge, 100, 102, 107
ocean-basin floor, 80, 81, 102, 107
trans-Atlantic structure section, 102

Currents, deep-sea bottom
photographed evidence of, 39, 59, 79
scour, 50, 51, 59, 79
velocities of, 50

Deeps
Nares, 55
Sohm, 55

Deep-sea channels, 66

Deep-sea sands, 28, 35, 36, 53, 58, 59, 60, 61, 65, 74, 75, 80, 99,
106, 107

Diabase, 99

DISCOVERY II, Royal Research Ship, vii

Easter Island Ridge, 103

Echo sounders
early sounding machines, 8
invention of, 8
Lamont-Facsimile PDR, 8, 9
NMC, 7, 8, 22, 23, 24, 31, 68
UQN-1B, 7, 8, 69

Echo soundings
accuracy of, 3, 9
sources of, 3, 5, 6, 7
units employed, 12

"Echo-time" depth, nominal fathoms, 12

Emerald Bank, 24

Eocene outcrops, 46, 47, 48, 76

Epicontinental seas, 1, 15, 18

Erosion, submarine, 23, 39, 50, 51, 74, 79, 80, 99, 105, 106, 107

Esso Hatteras Light Test, 43, 46, 47

Eugeosyncline, 54

Exaggerated profiles, plotting of, 5

Exuma Sound, 35, 49

Faulting, 42, 51, 76, 103, 107

Fecal pellets, photographs of, 39, 79

Flemish Cap, 21

Foraminifera, displaced, 60

French Hydrographic Service, 38

Georges Bank, 25, 43-45

Georges Bank canyons
dredging, 44-45
identification of benches, 44
outcrops on, 43-44

Geosynclines, 53-54, 107

Gibraltar, Straits of, 38

GLACIER, U.S.S., 55

_Globigerina_ ooze, 33, 53, 106

Gradients, conversion tables, 12, 15

Gran Canary Island, profile near, 39

Grand Banks earthquake, 22

Grand Banks sector, 21

Gravel, 59, 79

Gravity anomalies, 2, 65, 100

Graywacke suite, 53

Great Meteor Bank, 98

Guiana Basin, 12

Gulf of Maine, 17, 24, 25
sub-bottom echoes, 105

Gulf of Mexico escarpments, 18, 33

Gulf of St. Lawrence, 17

"Gully," the, 24

Guyots, 78

_Halimeda_, 60

Hatteras Abyssal Plain, 56, 57, 58

Heat flow, measurement of, 103

High Fractured Plateau, 1, 15, 90, 91, 94, 95, 99

Hispaniola-Caicos Abyssal Plain, 56, 60

Hispaniola-Caicos Channel, 36

Holothurians, photographs of, 39

Horseshoe Abyssal Plain, 57, 61

Horseshoe Seamount Group, 59, 78

Hudson Submarine Canyon, 8, 27-28, 31, 58, 72

Hudson Submarine Channel, 8

Hudson Submarine Delta, 28, 78

Hydrographic Department, British Admiralty, vii, 3, 6

Hydrographer Canyon, 43

Iberia Abyssal Plain, 56, 60

Iberia Basin, 12

Intermontane basins, 95, 99

International Hydrographic Bureau (Monaco), vii, 3, 5, 11, 12

Josephine Seamount, 78

KEVIN MORAN, Tug, 70

Kelvin Seamount Group, 72, 77, 78

Labrador Basin, 12

Lahave Bank, 24

Lamont Geological Observatory, 3, 6, 8, 21

Landward slopes of trenches, 1, 19, 36

Laurentian Channel, 23

Lisbon Submarine Canyon, 37

Lower Continental Rise Hills, 56

Lower Step, Mid-Atlantic Ridge, 1, 15, 90, 91, 94, 95, 96

Madeira Abyssal Plain, 56, 58, 61

Madeira Island, profile near, 28, 29

Madeira Rise, 58, 61

Magnetic anomalies
continental margin, 51, 52
Mid-Atlantic Ridge, 51, 100, 101
ocean-basin floor, 51, 65, 72
seamounts, 51

Magnetic surveys
airborne, 51
ship-towed, 51

Manganese nodules, 39, 78

Mantle, 80, 102

Marginal basin-outer ridge complex, 20, 36

Marginal escarpments
Bay of Biscay, 19, 37
Blake, 19, 32, 35
Gulf of Mexico, 19, 33
New Zealand, 19