432 W. M. DAVIS OBSERVATIONS IN SOUTH AFRICA 



istence of groups of transverse vertical joints or fissures in the horizontally 

 bedded basalts which here build up the plateau. The course of the river 

 near the falls is to the south-southeast. Above the falls it follows a 

 broad, open, and shallow valley inclosed by sandy uplands which ascend 

 gently one or two hundred feet above the river. A similar valley is easily 

 seen to continue below the falls, but there its floor is cut by the zigzag 

 gorge. The gradual widening of the main gorge and the increased length 

 of the lateral gorges farther and farther downstream, as described by 

 Lamphigh, leave no doubt of the normal origin of the gorge by river 

 erosion. 



The falls at present stretch across the whole breadth of the upper river. 

 The waters plunge down from a rather straight wall-like face of rock 

 into a deep transverse cleft that has been worn out on one of the groups 

 of transverse joints and fissures. The cleft is soon followed by a trans- 

 verse wall of more solid rock, through which a narrow passage leads the 

 river to the next transverse cleft, and so on down the gorge. Above the 

 falls transverse belts of smoother and deeper water interrupt the rippling 

 belts of shallower water at intervals of a few hundred feet. These deeper 

 belts probably indicate as many groups of joints and fissures and appear 

 to be the embryonic form of future transverse clefts separated by more 

 solid walls. Several local channels are forming across the wall from which 

 the falls now plunge. At times of lowest water the less worn top of the 

 wall may be partly dry, nearly all of the water being then gathered in the 

 local channels. At times of high water the great flood carries deep water 

 from bank to bank, and spray and foam then rise from the cleft in such 

 volume as almost to hide the falls. As time passes, one or another of the 

 local channels will be cut deeper and deeper, until it takes all the water 

 from the next upstream deeper transverse belt. The local channel then 

 becomes a connecting passage, the deeper belt is worn down to a deep 

 transverse cleft, to whose upstream side the falls are shifted, and the wall 

 of the present falls is left standing high and dry. It has been by essen- 

 tially a series of such changes that the existing zigzag gorge below the 

 falls has been eroded. The transverse clefts are subparallel, but the loca- 

 tion of the connecting passages is a matter of chance, as it depends on 

 the success of one local channel in being worn down a little faster than 

 another; hence the peculiar zigzag course of the narrow river below the 

 falls, in contrast to the relatively direct course of the broad river above 

 the falls. The connecting passage that now leads the waters from the 

 falls cleft through the next transverse wall is about a fourth way from 

 the eastern to the western end of the wall. The passage that cuts 

 through the second wall below the falls is at its western end ; it is around 



