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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 of the pure white clay which 

 occurs in many parts of the Amazon, and which helps to give 

 the waters their peculiar whiteness. The Madeira and Purus 

 have also white waters in the wet season, when their powerful 

 currents bring down the alluvial soil from their banks ; but ill 

 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 Tocantms, the Xingu, and the Tapaj 6z, 

 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. 



