308 G. L. COLLIE PLATEAU OF BRITISH EAST AFRICA 



portions of the plateau to the west, receive more rain than the plains 

 below. 



To the west of this arid belt, toward the Eift on the high plateau, the 

 rainfall is about 42 inches per year. The Kift forms a narrow but arid 

 belt, depressed below the plateau from 1,300 to 2,000 feet. 



Here the rainfall is scanty, and this belt is, on the whole, the driest of 

 the whole region. Passing over the divide that separates the Eift from 

 the basin of Victoria Xyanza, an annual precipitation of 60 inches is 

 found. There are, generally speaking, two rainy seasons; the greater 

 rains fall from March to June, the lesser rains in Xovember and Decem- 

 ber. The two wet seasons are thus separated by months of dry weather, 

 during which the soil becomes parched, vegetation dries up, and arid 

 conditions prevail. Thus the climatic conditions are such that there is 

 a broad semi-arid belt, flanked on the east and wTst by narrower belts of 

 rainfall. The east and west sides of the plateau, as a result, are seats of 

 erosion, while the middle j)ortion is a region of aggradation, on the 

 whole. When the seasonal rainfall has ceased, the ground water near the 

 surface is rapidly evaporated in tlie dry, hot climate which succeeds. 



OTHER FACTORS 



The water table lies at a great depth over much of the plateau in all 

 probability. In a few instances, where wells have been drilled, it is found 

 at a depth of 600 or 700 feet. 



The rocks of the plateau in general contain a great many structural 

 planes. Many of the rocks are porous and coarse-grained, and these 

 conditions naturally facilitate the withdrawal of water to a depth. 



There are no ranges of high mountains on the plateau which might 

 furnish snowfields and glaciers for the constant supply of water through 

 the dry seasons. One or two of the large volcanoes on the plateau — 

 Kenia, for instance — are of such great heigbt and size that they are able 

 to maintain living rivers throughout the year — rivers which have volume 

 enough to reach the sea. The Tana Eiver flows from the Kenia region 

 and reaches the ocean, but its course lies far outside the area described 

 in this paper, and this exceptional stream can have little bearing upon 

 the development of the plateau as a whole. The absence of mountain 

 ranges precludes the existence of trunk streams, and hence it is that in 

 this portion of Africa little of the interior drainage is able to make its 

 way to the sea, or it does so only in an intermittent fashion, because it 

 has no backing. 



The various factors mentioned above all combine to form a type of 

 pene})lanation in which stages of great relief are lacking, because stream 



