In the Indian Ocean the Antarctic Drift divides off Cape Leeuwin, one branch holding on to the east, along the south coast of Australia, and meeting off Victoria the East Australian current ; the other branch proceeds in a northerly direction nearly parallel to the coast of West Australia, as far as North-West Cape, where it is deflected to the west by the great South Equatorial current of the Indian Ocean. This vast constant current sets into the Indian Ocean from the Pacific through the numerous seas and channels of the Malay Archipelago, south of the equator, and sweeps across towards the African coast. Off the Mauritius and the adjoining islands, it bifurcates, the larger portion flowing south along the east coast of Madagascar, while the other pushes through the channels between Cape Amber, the Amirante Islands, and the Seychelles, and, uniting with a drift setting south along the African coast, forms the Mozambique current, flowing through the channel of that name between Madagascar and the main land.' Debouching out of this channel with increased power and a velocity of not less than 70 or 80 miles a day, it curves round the coast of South Africa as the well known Agulhas current. The continuity of this current is broken by the Agulhas Bank ; a minor branch probably doubles the Cape of Good Hope and flows into the Atlantic, but the main 'portion is deflected due east as the great Southern Connecting turrent of the Indian Ocean, which sets towards Australia with an initial velocity of 50 miles a day, and, aided by the prevalent westerly winds, enables vessels to make a quick passage from the Cape to Australia and New Zealand. This current flows in the same general direction as the Antarctic drift-current, of which it forms indeed the northern limits, and with which it is carried along to the east, one portion being deflected off Cape Leeuwin to the north, while the other portion continues its easterly flow south of Australia, and is finally deflected north along the west coast of New Zealand.
The greatest depth hitherto discovered in the Indian Ocean is in the deep basin bounded by Java, Flores, and Timor on the north, and the Australian coast on the south. Here the Gazelle took two soundings of 3,010 and 3,020 fathoms respectively. South of latitude 35° the general depth varies from 2,000 to 1,500 fathoms, while the main basin has an average depth of upwards of 2,500 fathoms, deepening to wards the east to 3,000 fathoms. South of Australia the general depth is not fax short of 2,000 fathoms ; which is also the depth of the Bay of Bengal, and the basin between Mauritius and Madagascar. The Mozambique Channel, Red Sea, and Persian Gulf are each under 1,000 fathoms in depth.
As regards the form of its bed, the various soundings taken in connection with submarine telegraphs, and the results of the Gazelle and Challenger expeditions, enable us to form a general but pretty accurate idea of the bottom of the Indian Ocean. The islands and archipelagos are of course but the
summits of extensive submarine plateaux. A series of lines connecting the various islands, however, do not mark accu rately the direction and limits of the elevated parts of the sea-bottom, and require to be supplemented by several series of soundings. Thus the Chagos Archipelago might at first sight be taken to indicate the existence of a continuous ridge or plateaux under water, from the Maldives to the Mauritius and adjoining islands and banks ; whereas the fact is that the Chagos are situate in the middle of the only deep-sea channel communicating between the Arabian Sea and the main basin of the Indian Ocean. The latter lies north and east of the Kerguelen plateau, extending from Marion and Prince Edward Islands to St. Paul and Amsterdam Islands ; a line through Cape Agulhas, Madagascar, Rodrigue, Chagos Islands, to Cape Comorin, limits it on the west ; the Bay of Bengal is its most northerly, and the Arafura Sea its most easterly, extension. This basin is also extended south of Australia and Tasmania, and probably opens into the southern basin of the Pacific by a depression south of New Zealand. The average depth of the main basin, in a line from South Africa to Java, is over 2,500 fathoms, deepening in the angle south-east of the latter to 3,000 fathoms. What may be called the North-western Basin of the Indian Ocean is separated from the main basin by the line of islands extend ing from the south-western coast of India, by the Maldives, Chagos, Seychelles, Madagascar, to Mozambique. The Red Sea and Persian Gulf on the north, and the Mozambique Channel on the south, of this basin, are all under 1,000 fathoms in depth. From the equator to 10° N. lat, the average depth is about 2,500 fathoms; further north it sinks rapidly to 1,000 fathoms and under. The narrow but deep depressions between the Maldives and Chagos, and the latter and the Saya de Malha Bank, connects this basin with the main basin on the east. A smaller basin, having an extreme depth of 2,000 fathoms, lies between the east coast of Madagascar and the line of islands and banks extending from Cape Amber, by the Amirante Islands, Seychelles, Saya de Malha Bank, Alba tross, Mauritius, and Bourbon. This minor basin might be called the "Sea of Mascarhenas," from the discoverer of the principal islands limiting it on the east.
254. The South-western Basin of the Indian Ocean is entirely included in the so-called Southern Ocean, and extends from the Antarctic plateau on the south to the Kerguelen plateau (of which Marion, Prince Edward, Crozet, Kerguelen, Amsterdam, and St. Paul's islands are the highest points) on the north. Both these plateaux have a general elevation of about 1,500 fathoms below the sea-level ; the deeper portions of the intermediate depression between them have an average depth of about 2,000 fathoms.