1868.] DIFFICULT NATURE OF THE GROUND. 327 



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 oyer 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 from mud and leeches. As one has to 

 strip the lower part of the person in order to ford them, I 

 found that often four were as many as we could cross in a 

 day. Looking up these sponges a bird's-eye view would 

 closely resemble the lichen-like vegetation of frost on 

 window panes ; or that vegetation in Canada-balsam which 

 mad philosophical instrument makers will put between the 

 lenses of the object-glasses of our telescopes. The flat, or 

 nearly flat, tops of the subtending and transverse ridges of 

 this central country give rise to a great many : I crossed 

 twenty-nine, a few of the feeders of Bangweolo, in thirty 

 miles of latitude in one direction. Burns are literally 

 innumerable : rising on the ridges, or as I formerly 

 termed them mounds, they are undoubtedly the primary 

 or ultimate sources of the Zambezi, Congo, and Nile : by 



