glaciers. When one treads on the black earth of the sponge,
though little or no water appears on the surface, it is frequently
squirted up the limbs, and gives the idea of a sponge. In the paths
that cross them, the earth readily becomes soft mud, but sinks rapidly
to the bottom again, as if of great specific gravity: the water in
them is always circulating and oozing. The places where the sponges
are met with are slightly depressed valleys without trees or bushes,
in a forest country where the grass being only a foot or fifteen
inches high, and thickly planted, often looks like a beautiful glade
in a gentleman's park in England. They are from a quarter of a mile to
a mile broad, and from two to ten or more miles long. The water of the
heavy rains soaks into the level forest lands: one never sees runnels
leading it off, unless occasionally a footpath is turned to that use.
The water, descending about eight feet, comes to a stratum of yellow
sand, beneath which there is another stratum of fine white sand, which
at its bottom cakes, so as to hold the water from sinking further.

It is exactly the same as we found in the Kalahari Desert, in digging
sucking places for water for our oxen. The water, both here and there,
is guided by the fine sand stratum into the nearest valley, and here
it oozes forth on all sides through the thick mantle of black porous
earth, which forms the sponge. There, in the desert, it appears to
damp the surface sands in certain valleys, and the Bushmen, by a
peculiar process, suck out a supply. When we had dug down to the caked
sand there years ago, the people begged us not to dig further, as the
water would all run away; and we desisted, because we saw that the
fluid poured in from the fine sand all round the well, but none came
from the bottom or cake. Two stupid Englishmen afterwards broke
through the cake in spite of the entreaties of the natives, and the
well and the whole valley dried up hopelessly. Here the water, oozing
forth from the surface of the sponge mantle, collects in the centre of
the slightly depressed valley which it occupies, and near the head of
the depression forms a sluggish stream; but further down, as it meets
with more slope, it works out for itself a deeper channel, with
perpendicular banks, with, say, a hundred or more yards of sponge on
each side, constantly oozing forth fresh supplies to augment its size.
When it reaches rocky ground it is a perennial burn, with many a