MUEEAY— DEPTH AND MAEINE DEPOSITS. 361 



In the central portion of the ocean the lines of soundings across the ocean from north, 

 to south and from east to west intersect each other, and in most cases little is known 

 regarding the depth of the numerous areas between these lines. There is one such area 

 between lat. 0° and 10° N. and long. 58° and 72° E., bounded on the west by the line of 

 soundings taken by H.M.S. ' Stork ' in 1897 between Socotra and Seychelles, on the east 

 by the Maldive Archipelago, on the north by the 'Egeria' line of soundings of 189i 

 between Socotra and Ceylon, and on the south by that run by U.S.S. 'Enterprise' in 

 1883 between the Maldives and the Seychelles. Another such area lies to the south and 

 south-east of the last one, i. e. between lat. 0° and 20° S. and long. 65° and 95° E., the 

 northern boundary being formed by the ' Enterprise ' line of soundings of 1883 along 

 the equatorial belt, tlie southern boundary by the 'Egeria' soundings of 1887 from 

 Cocos-Keeling to Mauritius, the eastern boundary by part of the series taken by 

 S.S. ' Valdivia ' in 1899 between Cocos-Keeling and Sumatra, and the western boundary 

 by Stanley Gardiner's soundings in 1905 between the Maldives, the Seychelles, and 

 Mauritius. A small portion of this larger area is cut off by the line of soundings 

 made by S.S. ' Sherard Osborn ' in 1900 between Cocos-Keeling and Rodriguez. A 

 large part of what is shown as Wharton Deep to the east of the ' Sherard Osborn ' line 

 from Cocos-Keeling to Fremantle also calls for further investigation. 



Between lat. 20° and 49° S. is a large area enclosed by the soundings run by H.M.S. 

 'Egeria' in 1887 from Cocos-Keeling to Mauritius, then to a position in lat. 37° S. and 

 long. 52° E., and then to the south-west point of Australia, crossed by the 'Valdivia ' 

 line of 1899 from Amsterdam Island to Cocos-KeeUng and by the ' Gauss ' line of 1905 

 from Amsterdam Island towards Mauritius, comjiaratively little being known of the 

 depth in the intervening portions. 



South of lat. 50° S. soundings are sparsely distributed, being limited to those taken 

 by H.M.S. ' Challenger,' and by the German ships ' Gazelle,' 'Valdivia,' and ' Gauss.' 



Before proceeding to describe the deposits which cover the floor of the Indian Ocean 

 it is desirable to cast a glance at the winds, currents, temperature, and salinity of the 

 waters of this ocean, for it has been found that these physical factors largely determine 

 the nature of the deep-sea deposits. 



Winds. 



The Indian Ocean differs essentially from the other great oceans in being completely 

 land-locked on its northern side, and this distinguishing feature plays an important part 

 in determining the prevailing winds and oceanic currents. The place of the north-east 

 trade-winds of the Atlantic and Pacific Oceans is taken by the monsoons, which blow 

 from north-east or south-west accordins: to the season. About the month of October an 

 area of high pressure begins to form over Central Asia and becomes more and more 

 pronounced towards January. Erom this high-pressure area winds blow outwards in all 

 directions, bringing, north-easterly winds over the Bay of Bengal and the Arabian Sea 

 as far south as the Equator — the winter monsoon. In summer the conditions are 

 practically reversed ; the pressure is now low over the continent of Asia, and the 

 resulting indraught of air from the south-west is known as the summer monsoon. 



