96 ABSENCE OF RAIN — VEGETATION. Chap. V. 



there, meeting with the rarefied air of that hot dry surface, the 

 ascending heat gives it greater capacity for retaining all its re- 

 maining humidity, and few showers can be given to the middle 

 and western lands in consequence of the increased hygrometric 

 power. 



This is the same phenomenon, on a gigantic scale, as that 

 which takes place on Table Mountain, at the Cape, in what is 

 called the spreading of " the table-cloth." The south-east wind 

 causes a mass of air, equal to the diameter of the mountain, sud- 

 denly to ascend at least three thousand feet ; the dilatation pro- 

 duced by altitude, with its attendant cold, causes the immediate 

 formation of a cloud on the summit ; the water in the atmosphere 

 becomes visible ; successive masses of air gliding up and passing- 

 over cause the continual formation of clouds, but the top of the 

 vapoury mass, or " table-cloth," is level, and seemingly motion- 

 less ; on the lee side, however, the thick 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 

 grandijiora, gladiolus, rushes, and lichens, which now appear on 

 Table Mountain, we should have only the hardy vegetation of 

 the Kalahari. 



Why there should be so much vegetation on the Kalahari 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 winch 

 is composed of the oldest silurian rocks. This basin has been 

 burst through and filled up in many parts by eruptive traps 

 and breccias, winch often bear in then substances angular frag- 

 ments 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 



