GEOLOGY OF THE KALAHARI. 85 



volumes of vapour curl over and descend, but when they 

 reach the point below, where greater density and higher 

 temperature impart enlarged capacity for carrying water, 

 they entirely disappear. 



Now if, instead of a hollow on the lee side of Table 

 Mountain, we had an elevated heated plain, the clouds 

 which curl over that side, and disappear as they do at 

 present when a " south-easter " is blowing, might deposit 

 some moisture on the windward ascent and top ; but 

 the heat would then impart the increased capacity the 

 air now receives at the lower level in its descent to leeward, 

 and instead of an extended country with a flora of the Disa 

 grandiflora, gladiolus, rushes, and lichens, which now appear 

 on Table Mountain, we should have only the hardy vegeta- 

 tion of the Kalahari. 



Why there should be so much vegetation on the Kala- 

 hari may be explained by the geological formation of the 

 country. There is a rim or fringe of ancient rocks round 

 a great central valley, which, dipping inwards, form a 

 basin, the bottom of which is composed of the oldest 

 silurian rocks. This basin has been burst through and 

 filled up in many parts by eruptive traps and breccias, 

 which often bear in their substances angular fragments 

 of the more ancient rocks, as shown in the fossils they 

 contain. Now, though large areas have been so dislocated 

 that but little trace of the original valley formation 

 appears, it is highly probable that the basin shape prevails 

 over large tracts of the country ; and as the strata on the 

 slopes, where most of the rain falls, dip in towards the 

 centre, they probably guide water beneath the plains but 

 ill supplied with moisture from the clouds. The phe- 

 nomenon of stagnant fountains becoming by a new and 

 deeper outlet never-failing streams may be confirmatory 

 of the view that water is conveyed from the sides of the 

 country into the bottom of the central valley ; and it is 

 not beyond the bounds of possibility that the wonderful 

 river system in the north, which, if native information 

 be correct, causes a considerable increase of water in the 

 springs called Matlomagan-yana (the Iyinks), extends its 

 fertilising influence beneath the plains of the Kalahari. 



The peculiar formation of the country may explain 

 why there is such a difference in the vegetation between 

 the 20th and 30th parallels of latitude in South Africa 

 and the same latitudes in Central Australia. The want 



