282 PHYSICAL GEOGRAPHY OF
to the mouth of the Uaycali, when they become blue or trans- parent, and the white waters are extended up that branch.
This has been taken as an evidence of the Uaycali being the main stream of the Amazon ; but I cannot consider that it has anything to do with the question. It is evident that if equal quantities of clear and muddy water are mixed together, the result will differ very little from the latter in colour, and if the clear water is considerably more in quantity the resulting mixture will still be muddy. But the difference of colour between the white- 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 yellow or olive-coloured streams. A river may therefore rise in a rocky district, and after some time flow through an alluvial basin, where the water will of course change its colour, quite independently of any tributaries which may enter it near the junctions of the two formations.
The Iga and Japura have waters very similar in colour to the Amazon. The Rio Branco, a branch of the Rio Negro from the north, is remarkable for its peculiar colour : till I saw it, I had not believed it so well deserved its name. The Indians and traders had always told me that it was really white, much more so than the Amazon ; and on descending the Rio Negro in 1852 I passed its mouth, and found that its waters were of a" milky colour mixed with olive. It seemed as if it had a quan- tity of chalk in solution, and I have little doubt of there being on its banks considerable beds__o£_the pure wh ite clay which occurs in many parts of the Amazon, and which helps to give the waters their peculiar whiteness. The Madeira and Puriis have also white waters in the wet season, when their powerful currents bring down the alluvial soil from their banks ; but in the dry season they are a dark transparent brownish-olive.
All the rivers that rise in the mountains of Brazil have blue or clear water. The Tocantins, the Xingu, and the Tapajoz, are the chief of this class. The Tocantins runs over volcanic and crystalline rocks in the lower parts of its course, and its waters are beautifully transparent ; the tide, however, enters for some miles, and renders it turbid, as also the Xingu. The Tapajoz, which enters the Amazon about five hundred miles above Para, is clear to its mouth, and forms a striking contrast to the yellow flood of that river.
