DI8TK1BOTI0N OF BAlNPAiL. 271 



then a veritable deluge ensues. In a few moments cataracts rush 

 from the mountain heights, the smallest and most thread-like rivulets 

 are transformed into toirents, and the rivers overflowing their banks 

 cover the plains : this sometimes lasts for days together (Casalis). 

 It is from the accumulation of these waters that the Lekoa, the 

 Caledon, and many other tributaries of the great Orange River, which 

 with slow and nlajestic course flows to the westward across the vast 

 plains of the centre of South Africa, take their rise. As the moun- 

 tains, however, merge into the plains, and these again into the 

 KAlahiri, we are reminded by the gradual diminishing rivers of the 

 continual aridity of the soil, till we reach Great Namaqualand, where 

 the occurrences of periodically filled water-courses again testifies to 

 the descent of rain. 



" In this latter district, however, as well as in the desert, rain falls 

 only from thunder-clouds. These rise from the northeast, and are 

 always hailed with delight by the inhabitants of those parched and 

 burning regions ; but they are partial in the distribution of their 

 precious treasure, the storms frequently passing over with tremendous 

 violence, striking both European and native with awe at their terrific 

 grandeur, while not a particle of rain descends to cool and fructify 

 the barren waste.'' 



Thus do we find in Asia, in America, and in Africa alike, a general 

 accordance between the distribution of the rainfall and of trees. 



Sect. II. — On the DistrihiUion of the Rainfall dependent on Geogra- 

 phical Position, being determined hy the contour of a country. 



Of two existing phenomena it is sometimes difficult to determine 

 what is the order of sequence in which they have appeared — which 

 is cause and which is effect — or whether both be not consequences 

 of the same cause ; and this we may experience in view of the cor- 

 respondence between the distribution of the rainfall and that of 

 forests. 



Whatever may be the effect of a copious rainfall on a mountain 

 side in promoting a growth of forest trees — and whatever may be the 

 effect of a forest there, in producing and maintaining an abundant 

 rainfall — ^it is the case that the configuration of a country, irrespective 

 of forests, exercises a considerable influence on the territorial distri- 

 bution of the rainfaU. 



The effect which may be produced by altitude alone on the dis- 

 tribution of the rainfall receives an interesting illustration from 



