408 
PHYSICAL GEOGRAPHY OF 
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. 
It is above the Madeira that we first meet with the 
curious phenomenon of great rivers of black water. The 
Rio Negro is the largest and most celebrated of these. 
It rises in about 2° 30' N. lat., and its waters are there 
much blacker than in the lower part of its course. All 
its upper tributaries, the smaller ones especially, are 
very dark, and, where they run over white sand, give it 
the appearance of gold, from the rich colour of the 
water, which, when deep, appears inky black. The 
small streams which rise in the same district, and flow 
into the Orinooko, are of the same dark colour. The 
Cassiquiare first pours in some white or olive-coloured 
water. Lower down, the Cababuris, Maraviha, and 
some smaller white-water streams help to dilute it, and 
then the Rio Branco adds its flood of milky water. 
Notwithstanding all this, the Rio Negro at its mouth 
still appears as black as ink ; only in shallow water 
it is seen to be paler than it is up above, and the sands 
