326 LIVINGSTONE'S LAST JOURNALS. [Chap. XTT, 



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 hope- 

 lessly. 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 aquatic plants 

 growing in its bottom. One peculiarity would strike any- 

 one : 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 super- 

 saturated. 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 



