The tides extend as far as Obydos, though still 400 miles from the sea; and according to La Condamine, they are even perceptible as far as the confluence of the Madeira. But so slow is their progress upwards, that seven floods, with their intervening ebbs, roll simultaneously along upon the giant stream; and thus, four days after the tide-wave was first raised in the wide deserts of the South Sea, its last undulations expire in the solitudes of Brazil.[6] The next considerable vassal of the Amazons is the shallow Tapajos. Fancy six streams, like the Thames, strung successively together, and you have the length of this river; take the Rhine twice from its source in the glacier of Mount Adula to the sands of Katwyck, and you have the measure of the Xingu. Before the confluence of this last of its great tributaries,--for the Tocantines, though considered by some geographers as a vassal, is in reality an independent stream,-- the breadth of the Amazons appeared to Von Martius equal to that of the Lake of Constance; but soon even this enormous bed becomes too narrow for the vast volume of its waters, for below Gurupa it widens to an enormous gulf, which might justly be called the ‘Bay of the Thousand Isles.’ Nobody has ever counted their numbers; no map gives us an idea of this labyrinth. If we reckon the island of Marajo, which equals Sicily in size, to the delta of the Amazons, its extreme width on reaching the ocean is not inferior to that of the Baltic in its greatest breadth. Dangerous sand-banks guard the giant’s threshold; and no less perilous to the navigator is the famous Pororocca, or the rapid rising of the spring-tide at the shallow mouths of the chief stream and of some of its embranchments,--a phenomenon which, though taking place at the mouth of many other rivers, such as the Hooghly, the Indus, the Dordogne, and the Seine,[7] nowhere assumes such dimensions as here, where the colossal wave frequently rises suddenly along the whole width of the stream to a height of twelve or fifteen feet, and then collapses with a roar so dreadful that it is heard at the distance of more than six miles. Then the advancing flood-wave glides almost imperceptibly over the deeper parts of the river-bed, but again rises angrily as soon as a more shallow bottom arrests its triumphant career. Our knowledge of the courses of most of the tributaries of the Amazons is very imperfect, and science knows next to nothing of the natural history of their banks. Even a correct map of the main stream is still wanting, for though its general course and the most important bends are tolerably well laid down, the numerous islands and parallel channels, the great lakes and offsets, the deep bays and the varying widths of the stream are quite unknown.