THE AMAZON VALLEY. 
407 
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 con- 
siderably 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 dear-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 inde- 
pendently of any tributaries which may enter it near the 
junction of the two formations. 
The 19 a and Japura have waters very similar in colour 
to the Amazon. The Bio Branco, a branch of the Bio 
Negro from the north, is remarkable for its peculiar 
colour : till I saw it, I had not believed it so well de- 
served 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 Bio 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 quantity 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 white- 
ness. The Madeira and Purus have also white waters 
