PREVALENCE OP THE PHENOMENON. 
325 
tion by filtration tbrougb a thiol mass of grasses. I suggest 
these ideas only in the form of a doubt. The colouring 
principle seems to be in little abundance ; for I observed 
that the waters of the Guaina or Bio Hegro, when subjected 
to ebullition, do not become brown like other fluids charged 
with carburets of hydrogen. 
It is also very remarkable, that this phenomenon of black 
waters, which might be supposed to belong only to the low 
regions of the torrid zone, is found also, though rarely, on 
the table-lands of the Andes. The town of Cuenca in the 
kingdom of Quito, is surrounded by three small rivers, the 
Machangara, the Eio del Matadero, and the Yanuncai ; of 
which the two former are white, and the waters of the last 
are black (aguas negras). These waters, like those of the 
Atabapo, are of a coffee-colour by reflection, and pale yellow 
by transmission.. They are very clear, and the inhabitants of 
Cuenca, who drink them in preference to any other, attri- 
bute their colour to the sarsaparilla., which it is said grows 
abundantly on the banks of the Eio Yanmnjai. 
We left the mouth of the Zarna at five in the morning of 
tbe 23rd of April. The river continued to be skirted on 
both sides by a thick forest. The mountains on the east 
seemed gradually to retire farther back. We passed first 
the mouth of the Eio Mataveni, and afterward an islet of a 
very singular form ; a square granitic rock that rises in the 
middle of the water. It is called by the missionaries El 
Castillito, or the Little Castle. Black bands seem to indi- 
cate. that the highest swellings of the Orinoco do not rise 
at this place above eight feet ; and that the great swellings 
observed lower down are owing to the tributary streams 
' r bich flow into it north of the raudales of Atures and May- 
Pures. We passed the night on the right bank opposite the 
mouth of the Eio Siucurivapu, near a rock called Aricagua. 
Curing the night an innumerable quantity of bats issued 
b’Om the clefts of the rock, and hovered around our ham- 
mocks. 
On the 24th a violent rain obliged us early to return to our 
boat. "We departed at two o’clock, after having lost some 
books, which we could not find in the darkness of the night, 
0,1 the rock of Aricagua. The river runs straight from 
^uth to north ; its banks are low, and shaded on both sides 
