298 G. L. COLLIE PLATEAU OF BRITISH EAST AFRICA 



Somewhat extended journeys on foot were made in each of the physio- 

 graphic provinces of the area. Xo long trips were taken away from the 

 railroad, however, nor were they possible except by organizing a safari, 

 a method of travel beyond the author's means. Longer trips into more 

 remote regions would have brought out many added details and doubtless 

 some modification of statement, but it is not btjieved that they would 

 have changed materially the conclusions of this paper. 



The area to be discussed in the following pagis is a portion of the 

 great plateau of central Africa. A brief summary of the conditions ob- 

 taining in this relatively unknown region should be given at the outset, 

 that there may be a clear understanding of the plateau and of the prob- 

 lems connected with its development. 



The foundation of the whole area under considerat'on is an ancient, 

 gnarled, and foliated gneiss which forms the surface rock over wide 

 stretches of the central portions of the ^jlateau. The region has been 

 peneplained and exhibits two cycles of erosion. The surface of the pla- 

 teau is strongly graded and slopes to the eastward. The average rise 

 from the Indian Ocean to the summit of the plateau west of the Eift 

 Valley is about 20 feet to the mile. The eastern portion of the region 

 has been depressed below sealevel, and upon it a series of elastics has been 

 deposited, these rocks being mainly Mesozoic in age. The coast has 

 passed through a number of minor oscillations since that time. The 

 western part of the plateau has been covered by floods of lava, which have 

 come from the present site of the Eift Valley, the earliest of these lavas 

 being probably of early Tertiary age. 



Either during or following the extrusion of the lavas there occurred 

 the trough faulting which resulted in the formation of the great graben 

 known as the Eift Valley, the most remarkable feature of its type in the 

 world. The Xile basin has been excavated out of the plateau, and ero- 

 sion is still attacking the western face and causing a marked eastward 

 retreat, which will ultimately result in the obliteration of the rift and 

 the addition of its present area to the drainage system of the Nile. 



In this paper it is proposed to describe the plateau and to consider its 

 problems under three heads, as follows : 



A. A description of the country as an example of the relation of geo- 

 logical structure to topography. 



B. A discussion of the possibility of peneplanation at high altitude 

 without any stage of high relief. This question at present must have a 

 theoretical aspect, yet it deserves careful study and thought. The con- 

 ditions on the high plateau of Africa indicate the possibility of reducing 

 the level of a country from a high altitude to a low without any stage of 

 marked relief. 



