Yol. 6$.~] GEOLOGY Of THE ZAMBEZI BASIN. 201 



The Kalahari Sand. 



The mode of occurrence and the distribution of the surface- 

 sands have been indicated in the first part of this paper, and are 

 further illustrated by the section, fig. 1 (p. 170), and by the map 

 (PL XVII). 



These red sands must attain a great depth in the broad smooth 

 bults of the plateau, but are rarely exposed in section. In the 

 well-sinking near Livingstone previously mentioned (p. 199) they 

 were 20 feet thick ; in railway-cuttings south of the Victoria Falls 

 they are exposed up to a depth of 15 feet: and these are the 

 principal sections that came under my observation. 



Where untrodden, these sands are firm and compact at the 

 surface, but they rapidly disintegrate and become quite friable 

 under traffic. So far as I could find, they show no sign of bedding 

 or other evidence of sedimentation ; and their position with regard 

 to the present surface debars the possibility that they can have been 

 accumulated as aqueous sediments where they now occur. For 

 example, besides occupying the higher parts of the plateau, they 

 enwrap the slopes of the shallow outer valley of the Zambezi in 

 several places, and reach close up to the river on both sides above 

 the Falls ; and I noticed similar conditions in several of the smaller 

 open valleys. This gives some clue to the period of their accumu- 

 lation, since it is evident that they have attained such positions at 

 some time subsequent to the erosion of these particular valleys. 

 But it is noteworthy that the sands on the opposite sides of the 

 Zambezi are identical in appearance ; and it appears, therefore, 

 that the river, if still existing at the time, did not form a barrier 

 to their accumulation. 



Although the sands are at present always heavily bush-clad and 

 firmly fixed in position, the hypothesis that they have been wind- 

 borne under conditions different from those which now prevail 

 agrees best with their general characters. I did not anywhere, 

 however, see any dune-shaped masses — always the sand lay in 

 broad smooth swells or bults, with gentle slopes leading to an 

 even summit ; and the flatness of the sandy surface over wide 

 tracts was very striking. 



The identity of these sands with the ' Kalahari Sand ' of Dr. 

 Passarge has already been mentioned, and was indeed first recognized 

 by that observer himself. 1 His explanation of their origin is that 

 they have been, in the first place, brought down by large rivers into 

 the interior basin during a period of heavy rainfall ; and then partly 

 redistributed by wind and other local agencies during a subsequent 

 epoch of drought : and this idea finds no contradiction in any 

 evidence that came under my own observation. Very little of the 

 sand can have been derived from the basalts, in which the quartz- 

 filled amygdules alone could supply this material ; so that, unless 

 indeed the sand represents the destruction of supra-basaltic arena- 

 ceous deposits of wide extent, for which there is little evidence, 



1 'Die Kalahari ' 1904, pp. 541 & 561. 

 Q.J.G.S. No. 2.50. p 



