Page:EB1911 - Volume 19.djvu/1010

 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 100 fathoms of the surface; the bank, an elevation coming nearer the surface than 100 fathoms, but not so near as 6 fathoms; and finally the shoal or reef, which comes within 6 fathoms of the surface, and so may constitute 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 geographical 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. Although put forward by the highest international authority recognized by geographers the system of nomenclature has not been adopted universally. In particular Sir John Murray considers that only deeps exceeding 3000 fathoms in depth should be named, and in his charts he has named these deeps after persons whether the individuals thus honoured had themselves discovered or explored the deeps in question or not. Some of the “deeps” to which names have been given disappear or are divided into two or three smaller deeps when the contour lines representing hundreds of fathoms are translated into contour lines representing hundreds of metres. A similar change in the contour lines may result from the substitution of lines in fathoms for those originally drawn in metres, and hence it is extremely desirable that specific names should only be given to such features as are pronounced enough to appear on maps drawn with either unit. For the sake of uniformity it is to be hoped that the system of nomenclature recommended by the International Geographical Congress will ultimately be adopted.

The continental shelves are parts of the great 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 investigation 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 in the same way that the chalk and limestone worn off from the eastern continuation of the island of Heligoland during the last two centuries has been reduced to the coarse gravel of the off-lying Düne. 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 by furrows a thousand yards wide and twenty to thirty fathoms deeper than the neighbouring sea-bed. Amongst the best known of the furrows of the continental shelf are the Cape Breton Deep, in the Bay of Biscay, the Hudson Furrow, southward of New York, the so-called Congo Canon, the Swatch of No Ground off the Ganges delta, the

Bottomless Pit off the Niger delta, and numerous similar furrows on the west coast of North America and outside the fjords of Norway, Iceland and the west of Scotland, as well as in the Firth of Forth and Moray Firth.

The seaward edge of the continental shelf often 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 towards the general level of the ocean on the other. All the greatest depths of ocean, i.e. all soundings exceeding 4000 fathoms, occur in trenches, and there are only a few small trenches known (on the west coast of Central America) in which the maximum depth is less than 3000 fathoms. Most trenches are narrow, but of considerable length, and their steeper side is believed to be due in every case to a great fracture of the earth’s crust. Strong evidence of this is afforded by the association of some of the depressions, notably the Japan Trench and the Atacama Trench, with the origin of frequent submarine earthquakes. Troughs and rises are features of more frequent occurrence and are best described as they occur in the particular oceans.

In the Atlantic the prevailing meridianal direction of the shore lines extends to the submarine features also. Captain Sherard Osborn in 1870 was the first to recognize that the North Atlantic Basin was divided by a central rise running generally from north to south into two parallel depressions. In 1876 the “Challenger” expedition found that a similar configuration exists in the South Atlantic also. As the result of all the deep-sea surveys now available we know that the central rise of the Atlantic starts from Iceland as the Reykjanes Ridge with less than 1000 fathoms of water over it in most parts and runs south-westward until in 51° N. it widens into what was called by Maury the Telegraph Plateau. Continuing southwards the rise joins the Azores Plateau, which has in parts a very marked relief, and runs thence southward almost exactly in the middle of the ocean, becoming gradually lower as it goes. As far as 29° N. the depth over it is less than 1500 fathoms, thence to 12° N. the depths are between 1500 and 2000 fathoms, and then it rises again to about 1500 fathoms and runs eastward under the name of the Equatorial Ridge. Crossing the equator in 13° W. the rise resumes a southerly direction and from Ascension to Tristan d’Acunha, the depth is in many places less than 1500 fathoms. The soundings of Bruce’s Antarctic expedition in the “Scotia” showed that the rise cannot be traced beyond 55° S. where the depths increase rapidly to over 2000 fathoms. The whole length of the rise which divides the Atlantic into an eastern and a western basin may be taken as 7500 nautical miles. Between 30°and 40° S. two lateral ridges diverge from the great Atlantic rise, the Rio Grande ridge towards the north-west and the Walfisch ridge towards the north-east. The existence of the latter, which extends to the African continent, was announced by Sir Wyville Thomson in 1876 as a result of his discussion of the deep-sea temperature observations of the “Challenger” expedition, though the fact was not confirmed by soundings until many years later.

The West Atlantic Trough lying on the western side of the Central Rise widens in the north into the North American Basin, and its greatest depths appears to be in the Porto Rico Trench, where in 1882 Capt. W. H. Brownson, U.S.N., obtained a sounding of 4561 fathoms in 19° 36′ N., 66° 26′ W. The Brazilian Basin has also a large area lying at a depth greater than 2500 fathoms and culminates in the Romanche Deep close to the Equatorial Ridge in 0° 11′ S., 18° 15′ W. with a depth of 4030 fathoms. The Eastern Atlantic Trough cannot boast of such great depths though the Peake Deep with 3284 fathoms sinks abruptly from the Azores Plateau in 43° 9′ N., 19° 45′ W., and several soundings exceeding 2700 fathoms have been obtained in the Bay of Biscay east of the meridian of 5° E. The North African Basin has several deeps with more than 3300 fathoms to the north-west and the south-west of the Cape Verde Islands, but the South African Basin is less deep. In the South Atlantic there is no connexion between the Central Rise and the Antarctic Shelf,