impalpable substance held in a
state of irreducible solution. This is proved from the fact, that even
when these waters enter a reservoir, and the earthy matter is allowed to
settle, they still retain the same tinge of yellowish olive. There are
some white rivers, as the Rio Branco, whose waters are almost as white
as milk itself!

The _blue_ rivers of the Amazon valley are those with clear transparent
waters, and the courses of these lie through rocky countries where there
is little or no alluvium to render them turbid.

The _black_ streams are the most remarkable of all. These, when deep,
look like rivers of ink; and when the bottom can be seen, which is
usually a sandy one, the sand has the appearance of gold. Even when
lifted in a vessel, the water retains its inky tinge, and resembles that
which may be found in the pools of peat-bogs. It is a general
supposition in South America that the black-water rivers get their
colour from the extract of sarsaparilla roots growing on their banks.
It is possible the sarsaparilla roots may have something to do with it,
in common with both the roots and leaves of many other vegetables. No
other explanation has yet been found to account for the dark colour of
these rivers, except the decay of vegetable substances carried in their
current; and it is a fact that all the black-water streams run through
the most thickly wooded regions.

A curious fact may be mentioned of the black rivers; that is, that
mosquitoes--the plague of tropical America--are not found on their
banks. This is not only a curious, but an important fact, and might be
sufficient to determine any one on the choice of a settlement. You may
deem a mosquito a very small thing, and its presence a trifling
annoyance. Let me tell you that settlements have been broken up and
deserted on account of the persecution experienced from these little
insects! They are the real "wild beasts" of South America, far more to
be dreaded than pumas, or crocodiles, or snakes, or even the fierce
jaguar himself.

Day after day our travellers kept on their course, meeting with many
incidents and adventures--too many to be recorded in this little volume.
After passing the mouth of the Rio Negro, they began to get a peep now
and then of high land, and even mountains, in the distance; for the
valley of the Amazon, on approaching its mouth, assumes a different
character from what it has farther up-stream. These mountains bend
tow