quatic
plants growing in its bottom. One peculiarity would strike anyone: the
water never becomes discoloured or muddy. I have seen only one stream
muddied in flood, the Choma, flowing through an alluvial plain in
Lopere. Another peculiarity is very remarkable; it is, that after the
rains have entirely ceased, these burns have their largest flow, and
cause inundations. It looks as if towards the end of the rainy season
the sponges were lifted up by the water off their beds, and the pores
and holes, being enlarged, are all employed to give off fluid. The
waters of inundation run away. When the sponges are lifted up by
superabundance of water, all the pores therein are opened: as the
earthen mantle subsides again, the pores act like natural valves, and
are partially closed, and by the weight of earth above them, the water
is thus prevented from running away altogether; time also being
required to wet all the sand through which the rains soak, the great
supply may only find its way to the sponge a month or so after the
great rains have fallen.

I travelled in Lunda, when the sponges were all supersaturated. The
grassy sward was so lifted up that it was separated into patches or
tufts, and if the foot missed the row of tufts of this wiry grass
which formed the native path, down one plumped up to the thigh in
slush. At that time we could cross the sponge only by the native
paths, and the central burn only where they had placed bridges:
elsewhere they were impassable, as they poured off the waters of
inundation: our oxen were generally bogged--all four legs went down up
to the body at once. When they saw the clear sandy bottom of the
central burn they readily went in, but usually plunged right over
head, leaving their tail up in the air to show the nervous shock they
had sustained.

These sponges are a serious matter in travelling. I crossed the
twenty-nine already mentioned at the end of the fourth month of the
dry season, and the central burns seemed then to have suffered no
diminution: they were then from calf to waist deep, and required from
fifteen to forty minutes in crossing; they had many deep holes in the
paths, and when one plumps therein every muscle in the frame receives
a painful jerk. When past the stream, and apparently on partially dry
ground, one may jog in a foot or more, and receive a squirt of black
mud up the thighs: it is only when you reach the trees and are off the
sour land that you feel secure fro