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.