By this central plateau, the Atlantic Ocean is divided into two longitudinal areas of depression or channels, one following the shores of North and South America, the other the shores of Europe and Africa. The depths vary from 2,000 to nearly 4,000 fathoms, the average depth being about three miles. The deepest portion of the eastern channel is situated to the westward of the Cape de Verde Islands, and forms an area of depression of over 3,000 fathoms. In the western channel there are two such depressions, one placed between the Antilles, Bermudas, and the Azores, the other between Cape St. Roque, Ascension, and Trinidad. They are divided from each other by a submarine elevation, which apparently connects the central plateau with the South American continent. The soundings taken in the Indian Ocean prove the existence of a submerged plateau on the limit between the Indian Ocean and the Southern Ocean. It rises in many parts to within 1,500 fathoms of the sea-surface, and forms the common foundation of all the islands situated in this part of the world--viz., Prince Edward Island, the Crozet Islands, the Kerguelen group, the Heard Islands, and the islands of St. Paul and Amsterdam. The origin of all these islands is probably volcanic. The main basin of the Indian Ocean with an average depth of over 2,000 fathoms, stretches from the meridian of the Cape of Good Hope toward the angle between Java and northwestern Australia, where it attains its greatest depths, forming a depression of over 3,000 fathoms. It communicates with the Arabian Sea by two narrow channels situated north and south of the Chagos Archipelago, being nearly cut off from that sea by a line of islands and shallow soundings which connect Africa, Madagascar, Bourbon, and Mauritius, the Chagos Islands and the Maldive Islands with the Asiatic continent. The 2,500-fathom area of the Indian Ocean crosses the parallel of lat. 40° S. between St. Paul and Amsterdam Islands and Cape Leeuwin in Australia, and forms, between the south coast of Australia and the forty-fifth parallel, an area of depression which extends beyond the southern end of Tasmania, includes the deepest portion of the basin between New South Wales and New Zealand, and probably communicates with the depths of the Pacific by a channel situated off the southern extremity of New Zealand.