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.