Yol. 6$.^ GEOLOGY OF THE ZAMBEZI BASIN. 199 



together by a chalcedonic cement, but is full of irregular cavities 

 from which some soluble or incoherent material has seemingly been 

 removed. Consequently,, while the rock is of flinty hardness, its 

 surface is excessively rough and irregular, and therefore very 

 damaging to foot-gear of any description. 



I found this quartzite or ' silcrete' to be very generally, though 

 sporadically, distributed throughout the country along the margins 

 of the sand-bults, often forming a broad ragged fringe between the 

 sands and the bare basalt. It does not, however, form a continuous 

 layer beneath the sand, as it was not encountered in a well sunk 

 by Mr. P. W. Sykes near the residency at Livingstone through the 

 sand to the basalt ; and is similarly wanting in some of the sand- 

 cuttings on the railway between Deka Bridge and the Yictoria 

 Palls. 



I saw some good sections of the ' silcrete ' along the eastern 

 margin of the shallow valley of the Maramba River, 2 miles above 

 Livingstone, where it occupies the surface of a little plateau 30 to 

 40 feet above the river-flat, apparently an old terrace of the 

 Maramba, and is cut through in several little kloofs. It is here 

 from 3 to 10 feet thick, resting on decomposed basalt : and has a 

 brecciated aspect, rounded lumps of the chalcedonic quartzite being 

 enclosed in a gritty siliceous matrix honeycombed with the usual 

 irregular cavities. 



It is also well-displayed, as a rugged bouldery capping to small 

 table-like kopjes, in the much-dissected country bordering the gorge 

 of the Zambezi on the west in the basin of the Masui River, 4 to 5 

 miles south of the Palls ; and in this neighbourhood it is also seen 

 along the eroded margin of the sand-bult, being there generally 

 associated with hard ferruginous sand-rock. 



The greatest thickness of this surface-rock that came under my 

 observation was in the sharp south-eastern rim of the Matetsi 

 Valley, some 500 feet above the alluvial flat at Tsheza's, 5 miles 

 from the mouth of the Matetsi. Here the beds cropped out at the 

 margin of a high sand-covered upland in a bold krantz, which 

 showed 8 to 10 feet of chalcedonic quartzite with 10 to 15 feet of 

 partly siliceous, partly calcareous, brecciated or conglomeratic 

 material below, resting on much- weathered spheroidal basalt. The 

 position of this bed in relation to the valley appeared to denote 

 its considerable antiquity. Thinner patches of similar chalcedonic 

 rock occurred, however, on the stepped slopes along the opposite 

 side of the valley at all elevations. Nor could I find in any part 

 of the region that the formation was restricted to any definite 

 position in respect to the present surface ; for although, as in the 

 above-described instance, it frequently caps the highest ground, it 

 occurs in abundance also on the lower terraces and even in the 

 bottoms of valleys. 



The general impression that I gained was, that in most cases the 

 * silcrete ' represents a progressive induration of the exposed base 

 of the sands, due to the deposition of silica on the evaporation of 

 ground- water which has slowly percolated through the sands. The 



