Vol. 63.~] GEOLOGY OF THE ZAMBEZI BASIN. 197 



occurred along the northern slope of the sandy plateau between the 

 head-waters of the Matetsi and the Deka Rivers — an upland known 

 to Chapman as ' Boomka,' although this name appears to be no longer 

 current. Here, though poorly exposed, the flaggy rock seemed to 

 underlie a loamy flat of considerable extent, and was visible along 

 our route at intervals for 3 miles or more, the surface of several 

 shallow depressions or dry vleys being littered with its debris. I 

 thought at first that it might be only an abnormal silicified surface- 

 bed of recent origin, pertaining to these vleys ; but on crossing for 

 4 or 5 miles the intervening sand-plain, I found similar rock crop- 

 ping out along the edge of a shallow valley on the Deka side of the 

 bult, under conditions implying that it might be the relics of a 

 formation distinct from and older than the other surface-deposits. It 

 differs from any of the surface-quartzites that I examined in its 

 closely-knit fine-grained texture, flaggy bedding, colour, and general 

 aspect ; and I saw nothing like it in any other part of our traverses. 

 The surface of the slabs occasionally showed flattened cylindrical 

 markings recalling the ' worm-tracks ' of some of our ancient 

 greywackes. Microscropically, it is described by Mr. Thomas as 



' a fine-grained quartzose sediment, iron-stained, and made up of very angular 

 grains .... certainly not wind-blown ' (see Appendix I, F 1038, p. 211). 



Unfortunately, I saw no clear section in this deposit; but I think 

 that its thickness where I crossed it can only have been slight — 

 probably not more than 10 or 15 feet — as the basalts emerged in 

 several places in the immediate neighbourhood. In the above- 

 mentioned exposure at the margin of the little valley, the flaggy 

 fragments frequently protruded edgewise at the surface, as if the 

 beds possessed a steep dip ; but this tilting appeared to be merely 

 a superficial condition, confined to the loose slabs. I mention this, 

 because it is just possible that an exposure of similar character may 

 have supplied the basis of Chapman's description of ' vertical strata 

 of sandy schist (?) " x in and around the head of the Deka Valley, 

 which was my main incentive in making this particular journey. 



These flaggy Boomka strata may conceivably be a feeble equi- 

 valent of some portion of the similarly fine-grained ' Forest Sand- 

 stones ' of the Bulawayo country ; or they may belong to the 

 earlier of the superficial formations, like the ' Pfannensandstein' of 

 Dr. Passarge ; but, without further knowledge, it is useless to 

 attempt their correlation. Once more the absence of fossils — that 

 perpetual hindrance to conclusions in South African geology — brings 

 to mind Livingstone's striking apophthegm, l In Africa ; the very 

 rocks are illiterate.' 2 



Another exposure deserves notice here, as possibly indicating the 

 presence of beds of a different character overlying the basalts. On 

 the march from Deka to Wankie, about a mile before reaching 

 'Mtoro's Kraal, at the south-eastern side of the Deka basin some 

 15 miles distant from Wankie Station, a small steep hillock or 



1 ' Travels in the Interior of South Africa ' vol. ii (1868) pp. Ill & 158. 



2 ' Missionary Travels & Researches in South Africa ' 1857, chap, xii, p. 214. 



