In spite of the increase of deep-sea soundings in the last few decades, they are still very irregularly distributed in the open ocean, and the isobaths (lines of equal depth) on a chart of the world include wide generalizations. Bearing this caution in mind, the existing bathymetrical charts, amongst which that of the prince of Monaco stands first, give a very fair idea of the great features of the bed of the oceans. (Carte generale bathy metrique des oceans dressee par ordre de S.A.S. le Prince Albert de Monaco, 24 sheets [Paris, 5904] ; 2nd edition, Paris, 1913 seq.) A definite terminology for the larger forms of sub-oceanic relief was put forward by the International Geographical Congress at Berlin in 1899 and adopted by that at Washington in 1904. Equiv alent terms, which are not necessarily identical or literal trans lations, were adopted for English, French and German.
The larger forms designated by special generic terms include the following : The continental shelf is the gentle slope which extends from the edge of the land to a depth usually about Soo fathoms or 200 metres, though in some cases as much as 300 fathoms or about 600 metres, and is there demarcated by an abrupt increase in the steepness of the slope to ocean depths. In the deep sea two types of feature are recognized under the general names of depression and elevation. The depression is distinguished according to form and slope as (I) a basin when of a roughly round outline, (2) a trough when wide and elongated, or (3) a trench when narrow and elongated lying along the edge of a continent. The extension of a basin or trough stretching towards the continent is termed an embayment when relatively wide and a gully when narrow. The elevation includes (I) the gently swelling rise which separates troughs and basins in the middle of the ocean, (2) the steeply sloping ridge which interposes a narrower barrier between two depressions, and (3) the plateau or wide elevation rising steeply on all sides from a depression. The deepest part of a depression is termed a deep, and the highest part of an elevation when not reaching the surface a height. In addition to these larger forms a few minor forms must be recognized. Amongst these are the dome, an isolated elevation rising steeply but not coming within Soo fathoms (200 metres) of the surface; the bank, an elevation coming nearer the surface than ioo fathoms, but not so near as 6 fathoms (io–ii metres) ; and finally the shoal or reef, which comes within 6 fathoms of the surface, and so may con stitute a danger to shipping. Similarly we may note the caldron or small steep depression of a round outline, and the furrow or long narrow groove in the continental shelf.
According to the resolutions of the International Geographical Congress the larger individual forms which have been described by generic terms shall have specific names of a purely geograph ical character; but in the case of the minor forms the names of ships and persons are considered applicable. In 1899 A. Supan published a chart of the oceans with a suggested nomenclature based on these principles; and the larger forms in the prince of Monaco's great chart also are named in accordance with the rule.
This system is not yet general. Sir John Murray considers that only deeps exceeding 3,00o fathoms in depth should be named. The Continental Shelves.—The continental shelves are those parts of the continental blocks which have been covered by the sea in comparatively recent times, and their surface consequently presents many similarities to that of the land, modified of course by the destructive and constructive work of the waters. Waves and tidal currents produce their full effects in that region, and in high latitudes the effect of transport of materials by ice is very important; while in the warm water of the tropics the reef building animals and plants (corals and calcareous algae) carry on their work most effectively there. The continental shelves include not only the oceanic border of the continents but also great areas of the enclosed seas and particularly of the fringing seas, the origin of which through secular subsidence is often very clearly apparent, as for instance in the North sea and the tract lying off the mouth of the English channel. A closer investiga tion of the numerous long narrow banks which lie off the Flemish coast and the Thames estuary shows that they are composed of fragments of rock abraded and transported by tidal currents and storms. Numerous old river valleys and furrows entrenched in the continental shelf bear witness to its land origin. Such valleys are very clearly indicated in the belts of the western Baltic, the Cape Breton deep, the Hudson Furrow, the so-called Congo Cation, the Swatch o: No Ground off the Ganges delta, and numerous similar furrows.
The seaward edge of the continental shelf of ten falls steeply to the greatest depths of the ocean, and not infrequently forms the slope of a trench, a form of depression which has usually a steep slope towards a continent or an island-bearing rise on one side and a gentler slope toward, the general level of the ocean on the other. Soundings exceeding 7,000 metres occur in trenches; there are few trenches known (on west coast of Central America) in which the maximum depth is less than 5,000 metres. Most trenches are narrow, but of considerable length, and their steeper side is believed to be due, not to a fracture, but to a folding of the earth's crust, because these trenches are only found parallel to young f old mountains like the Cordillera of North and South America. The deep trenches are distributed chiefly around the Pacific near to the shore or near the limits of former lands (Mindanao, io,800m.). Outside the Pacific we find a trench (8,526m.) north of Porto Rico, and another north of the south Sandwich Islands in the south Atlantic. In the Indian ocean there is a trench (7,450m.) west and south of Sumatra-Java, where a double-folding occurs. On a coast with old block mountains, as around Africa and the Deccan, these trenches are absent. The region of these depressions is also specially subject to great earth quakes. These submarine trenches occupy only 1% of the area of the oceans. There are also numerous "fossil" trenches, filled with various deposits, at the foot of fold mountains, as, for example, the Ganges valley.