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 A T L A N T I C O C E A N 769 and Nova Scotia are Sigsbee deep, Thoulet deep, Libbey and the Sandwich group. In 40° S. lat., 40° W. long., deep, and Suhm deep, each of small area ; north-east of the the 3000-fathoms line marks an offshoot of the great Ross Bahamas Nares deep forms the largest and deepest de- deep, which runs south-eastwards to the main depression. pression in the Atlantic, in which a sounding of 4561 East of this trough the depth is, generally, over 2000 fathoms was obtained (70 miles north of Porto Rico) by fathoms, except where a bank, on which Bouvet Island the U.S. ship Blake in 1883. Immediately to the south of stands, suggests a south-easterly prolongation of the main Nares deep lies the smaller Makar off deep; and off the central ridge, which, but for this, would appear to end at coast of South America are Tizard deep and Havergal deep. Tristan da Cunha. From the south point of Africa southSouth of 40° S. lat. the form of the sea-bottom changes eastwards to the Crozet Islands and Kerguelen the depth considerably on account of the eastward trend taken by the is less than 2000 fathoms, an ill-defined ridge separating alternate elevations and depressions. Between the southern the eastern margin of the Ross deep from the main basin extremity of South America and Graham Land, in the of the Indian Ocean (q.v.) Antarctic, the depth nowhere reaches 2000 fathoms, and a The foundations of our knowledge of the relief of the Atlantic plateau of less depth extends eastward to South Georgia basin may be said to have been laid by the work of H.M.S.

shew Fahrenheit NORTH ATLANTIC CURRENTS

ulobigerina Ooze Pteropod Ooze Coral Mad Warm

At surface Beneath surface

B. V.Dazbuhir< & OJ.R.Himtarth Challenger (1873-76), of the German ship Gazelle (1874-76), the French expedition in the Travailleur (1880), and the U.S. surveying vessel Blake (1877 and later). Large numbers of additional soundings have been made in recent years by cable ships, by the expeditions of H.S.H. the Prince of Monaco, and by the German Valdivia expedition under Professor Chun (1898) ; but with the exception of the last-named none of the later workers have seriously modified the contours outlined by the Challenger and Gazelle. For the volume of the North Atlantic, Karstens gives the value 136,384,000 cubic kilometres, or 21,362,000 cubic Volume m^esj giving a mean depth of 2047 fathoms. mean ’ If we include the enclosed seas, the North Atlantic depth, and has a volume of 150,415,000 cubic kilometres, bottom or 23,560,000 cubic miles, giving a mean depth deposits. igQo fathoms. The South Atlantic has a total volume of 164,263,000 cubic kilometres, or 25,729,000 cubic miles, giving a mean depth of 2067 fathoms. The

TheNumbera shew grammes of Salt per Kilogramme of water ,

following table, due to Murray, gives details of the area of the floor of the Atlantic basin, and the volume of water resting thereon, at different levels Depth. Area. Volume. Fathoms. Square Miles. Cubic Miles. Between 0- 100 2,907,003 2,353,703 100- 500 2,404,482 8,370,611 500-1000 1,840,634 9,568,637 1000-2000 5,468,015 15,816,901 2000-3000 10,928,918 8,905,287 3000-4000 1,780,012 690,123 Over 4000 5,832 2,155 25,334,896 Mean depth, 1830 fathoms.

45,707,417

The Atlantic Ocean contains a relatively small number of islands. The only continental groups, besides some S. 1.-97