Vol. 6^.~] GEOLOGY OF THE ZAMBEZI BASIN. 179 



than does the unaltered basalt. Thus there has sprung iuto 

 existence a range of high kopjes marking the course of the 

 indurated sandstone, with lower ground on the one side due to the 

 more rapid disintegration of the unaltered sandstone, and on the 

 other side due to the decomposition of the basalts. At the same 

 time, as a whole, the basalts are less enduring under the African 

 climate than even the unaltered sandstones, so that there has been 

 relatively a general lowering of the country on the basalt side of the 

 fault. The presence of a few high flat-topped kopjes of approxi- 

 mately equal elevation on the opposite sides seems, however, to 

 indicate the former existence of a plateau, before the initiation of 

 the present drainage-system, on which there was no conspicuous 

 feature to mark the fault ; and this condition should be borne in 

 mind in any attempt to trace the fault south-westward into the 

 unbroken plateau beyond the head of the Deka basin. 



The effect of the fault upon the course of the Deka River now 

 requires notice. As previously mentioned (p. 173), the upper 

 portion of this river, so far as it came under my observation, lies 

 ■wholly within the basalt-country, the two main branches of its head- 

 waters, which unite a short distance below Mr. Geise's ranch at Deka, 

 having shallow channels on the surface of the plateau, while below 

 their confluence the valley is still sunk but little below the general 

 level of the hummocky plain of basalt. This open valley is 

 continued for many miles ; but afterwards gives place to a steep-sided 

 gorge, the change setting in somewhere between 25 and 35 miles 

 east-north-east of Deka, in a portion of the river-course which I was 

 not able to examine. This steep trench where we struck into it near 

 'Ngoni's Kraal was still entirely within the basalts, though the 

 high kopjes of sandstone marking the great fault ran almost 

 parallel with the canon within 2 or 3 miles to the south. About 

 a mile above the place where the railway crosses the Deka, the 

 river swings up to the foot of the sandstone-kopjes, but rebounds 

 from them in a sharp curve ; and in this neighbourhood the valley 

 has lost for a time its canon-shape, through the disappearance of its 

 north-western or basaltic wall. Immediately below the railway- 

 bridge, the river runs for nearly a mile almost exactly along 

 the junction between the two rocks, as shown in fig. 3 (p. 177), 

 once or twice impinging upon the hardened sandstone, but 

 swinging off sharply again as if rebounding from the impact, and 

 finally sweeping away northward into the basalt-country in a bold 

 curve. 



These phenomena are repeated almost exactly near the Rondultr 

 confluence, 7 or 8 miles farther down stream, where the river returns 

 with a south-easterly course from its incursion among the basalts, 

 and again impinges upon the fault, managing to carry one sharp 

 loop of its bed just within the sandstone-boundary before being 

 driven off again into the basalts (see fig. 2, p. 177). 



The presence of several low streamless ' poorts ' or cols gapping 

 the high ridge on the sandstone side of the fault suggests that the 

 river may at some past time have broken through the belt ; but 



