Yol. 6$.~] GEOLoar of the Zambezi basin. 16$ 



not too steep, the ground is covered more or less thickly by scrub 

 and low trees, interspersed with a scanty growth of tall harsh 

 grasses. At the time of our traverse, in the long drought, the 

 streams were dry or merely trickling, the trees mostly leafless, and 

 everything was parched and still ; but during the rains (November 

 to March), when floods are roaring through all its ravines, this must 

 indeed be a turbulent region. 



On the outer fringe of this broken country the strips of original 

 plateau between the ravines become broader, and frequently include 

 truncated segments of shallow valleys, showing where the youthful 

 drainage is destroying the older system. The present streams 

 possess similar shallow troughs farther back, where the plateau is 

 as yet intact ; and most of these break off suddenly at a deep waterfall 

 or a series of cascades, below which the stream is at once engorged, 

 like the Zambezi itself at the great Palls. Thus the valleys of the 

 upland stand in the relation of ' hanging- valleys ' to the low-level 

 drainage. 



These shallow troughs of the plateau-streams are generally 

 bordered by low slopes of greatly decomposed basalt, and their 

 broad floors are covered with dark stiff loam or earthy clay, not 

 often exceeding 3 or 4 feet in thickness, apparently derived partly 

 from the rotting rock upon which it rests and partly from the dense 

 growth of tall rushy grasses which it nourishes. The stream- 

 channels along these flats are somewhat canal-like, showing an 

 alternation of long narrow pools where the basalt i3 most readily 

 decomposed, with low bars of bare rock where weathering is 

 less effective. During the season of drought, water is retained in 

 the pools long after the stream has ceased to flow ; and further aid 

 is thus lent to the decomposition of the rock beneath them, which 

 leads to the rapid excavation of deep crevice-like gorges (Pis. X & 

 XV & fig. 8, p. 191), often curiously angular, along the rotted belts 

 as the rejuvenated drainage breaks back into the plateau. 



The grassy flats are generally bordered by a fringe of well-grown 

 trees, while the low stony slopes of the valleys are covered with the 

 bushy growth and scanty grasses that overspread the neighbouring 

 plateau. But, towards the head of most of the streams, the grassy 

 flats of dark earth expand into shallow basins ranging up to a mile 

 or two in width and several miles in length. These sometimes 

 become confluent and lose altogether their relation to any definite 

 valley, the loam then thickening and spreading over gentle slopes 

 in a manner somewhat similar to the peat of humid climates. The 

 dark soil of such tracts is indeed called ' torf ' by the Boer farmers, 

 but in composition it is quite different from peat. In the wet 

 season it is a very tenacious clay, which retains the moisture and 

 works up into a pasty mud ; and during the drought it contracts so 

 greatly in drying that the surface is reticulated by gaping cracks 

 which are very troublesome to the traveller. Of such tracts I saw 

 the most striking examples in the upper basins of the Lukunguli, 

 the Matetsi, and the Deka, but they are prevalent throughout the 



Q. J. G. S. No. 250. b- 



