The numerous tributary streams of the Amazons differ remarkably in the colour of their waters and may be divided into three groups--the white or pale yellowish-water rivers, the blue-water rivers and the black-water rivers. The difference of colour between the white-water and blue-water rivers is evidently owing to the nature of the country they flow through; a rocky and sandy district will always have clear-water rivers; an alluvial or clayey one will have troubled streams. The Rio Negro is the largest and most celebrated of the black-water rivers. All its upper tributaries, the smaller ones especially, are very dark, and, when they run over white sand, give it the appearance of gold, from the rich colour of the water, which, when deep, appears inky black. In the rainy season, when the dark clouds above cause the water to appear of a yet more funereal blackness and the rising waves break in white foam over the vast expanse, the scene, as may well be imagined, is gloomy in the extreme. The peculiar colour of the black-water rivers appears to be produced by the solution of decaying leaves, roots and other vegetable matter. In the virgin forests in which most of these streams have their source the little brooks and rivulets are half choked up with dead leaves and rotten branches giving various brown tinges to the water. When these rivulets meet together and accumulate into a river, they of course have a deep brown hue very similar to that of our bog or peat water, if there are no other circumstances to modify it. But if the stream flows through a district of soft alluvial clay, the colour will of course be modified and the brown completely overpowered. A peculiarity of the black waters is the absence of mosquitos along their banks, which thus afford agreeable places of refuge to the persecuted traveller. No inducement will make an Indian boatman paddle so hard as the probability of reaching one of these privileged spots before midnight and being enabled to enjoy the comforts of sleep till morning. The basin of the Amazons extending over an area of 2,330,000 English square miles surpasses in dimensions that of any other river in the world. All western Europe could be placed in it without touching its boundaries and it would even contain our whole Indian empire. It is entirely situated in the Tropics, on both sides of the Equator, and receives over its whole extent the most abundant rains. The body of fresh water which it empties into the ocean is therefore far greater than that of any other river; not only absolutely but probably also relatively to its area, for as it is almost entirely covered by dense virgin forests, the heavy rains which penetrate them do not suffer so much evaporation as when they fall on the scorched Llanos of the Orinoco or the treeless Pampas of La Plata.