96 
ABSENCE OF EAIN — VEGETATION. 
ClL4P. 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. 
Tins is the same phenomenon, on a gigantic scale, as that 
wliich 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 de 2 :)osit 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 Bisa 
grandiflora, 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 wliich 
is composed of the oldest Silurian rocks. This basin has been 
burst through and filled up m many parts by eruptive traps 
and breccias, which often bear in their 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 
