312 G. L. COLLIE PLATEAU OF BRITISH EAST AFRICA 



in the quantity and distribution of rainfall, without calling in the agency 

 of diastrophism whatever. The writer has explained the peneplain in 

 the driftless area of Wisconsin as due to the baseleveling of the region, 

 possibly in the Cretaceous, followed by a subsequent uplift, which per- 

 mitted the streams to renew their youth and to develop a second and 

 well matured cycle within the old one. After his experience in Africa 

 the author feels that possibly this former opinion is subject to revision, 

 for the present conditions in Wisconsin may w^ell have arisen simply 

 through climatic changes. 



The Eift Valley 

 position and extent 



If the supposition of Suess is correct, the Eift Valley extends from 15 

 degrees south to the neighborhood of Antioch, Syria, 35 degrees north 

 latitude, a distance of nearly 3,500 miles. At its southern extremity, 

 almost at the outset of its course, the valley forks; one portion proceeds 

 to the northwest, while the other and main portion trends northward; 

 it is a small part of the main valley that is here described. In this par- 

 ticular region the width of the valley from crest to crest is on the average 

 about 70 miles, and that of the floor is about 40 miles. 



The depth of the valley varies greatly, also, for the bottom rises and 

 falls; there is a succession of basins separated by divides. Fault-block 

 succeeds fault-block, and, in addition, the lava accumulations on the floor 

 vary greatly in thickness. The depth of the valley ranges from 2,000 

 feet to 1,200 feet. In this region the valley lies on the very western edge 

 of the high plateau, just before the descent into the Victoria basin begins, 

 and the divide between the two is relatively very narrow. The western 

 escarpment of the Eift is generally more elevated than the eastern one 

 and it is the real summit of the whole plateau. 



ORIGIN 



The Eift Valley must be regarded as a vast graben or tectonic trough ; 

 the undulations of the floor and the absence of through trunk streams 

 forbid the supposition that it is a valley of erosion. There is very little 

 direct proof of faulting, however ; it must be inferred from the scarp, 

 terrace, and tilted block type of topography which abounds on every 

 hand, and also from an occasional repetition of lava beds above and below 

 a scarp. Although the valley is a true graben, yet its tectonic features 

 have been greatly modifled by vulcanism and in a lesser degree by normal 

 erosion. Great floods of lava have swept over the floor, the sides, and the 



