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 southeast wind causes a mass of air,
equal to the diameter of the mountain, suddenly to ascend
at least
Three thousand feet; the dilatation produced 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 gliding-up and passing-over air cause the continual
formation of clouds, but the top of the vapory mass, or "table-cloth",
is level, and seemingly motionless; on the lee side, however,
the thick volumes of vapor 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 "southeaster" 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 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 inward,
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 toward the centre, they probably
guide water beneath the plains but ill supplied with moisture from the clouds.
The phenomenon 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 Links),
extends its fertilizing 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.
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