Yol. 63.'] GEOLOGY OF THE ZAMBEZI BASES'. 1205 



great; boulders only partly waterworn, interrupted here and there 

 by miniature beaches of glaring-white quartz-sand, loudly ' musical ' 

 when trodden on ; but the very scanty material intermediate in size 

 between the boulders and the sand consists almost entirely of agate 

 and chalcedonic pebbles derived from the amygdules. 



The same peculiarity is observable in the case of the smaller 

 streams draining the basalt-country; so that neither in their present 

 beds nor on the higher ledges which they have once occupied is 

 there any noticeable accumulation of fluviatile detritus ; and even 

 their flood-bars, like those of the Zambezi itself on its emergence 

 from the great gorge, are built up of partly-worn basalt-blocks, 

 occasionally faced with patches of white sand. 



In the north-eastern district, however, where the streams have 

 derived part of their detritus from the older rocks (p. 172), medium- 

 sized pebbles of these rocks and nut-like pebbles of pink quartz are 

 abundant, not only in the present river-beds but also on low flats 

 away from the streams, indicating a former drainage-system differing 

 in detail from that of to-day. But even here the pebbles, where 

 thickest, seem only to form a surface -layer, and I saw no instance of 

 a gravel-terrace of construction such as one is accustomed to find in 

 the home-country. 



I could not detect any of these extraneous pebbles on the flats 

 of the Zambezi at Makwa, below the confluence of the above- 

 mentioned streams ; but the abundant flakes of white mica con- 

 tained in the river-sands at this place, and absent in the higher 

 reaches, have no doubt been carried by these streams. I can 

 therefore confirm Chapman's shrewd guess 1 that this mica was 

 brought down to the river from the north. 



The smoothly-worn basalt along the stream-beds, especially on 

 the flood-platforms and other places slightly above ordinary water- 

 level, is generally glazed with a thin black surface-film bearing a 

 high polish, which gives no foothold and renders passage along the 

 floors of the gorges very difficult. Glazing of this kind appears to 

 be of very common occurrence on similarly-situated hard rocks in 

 hot climates, and its origin has been frequently discussed. In 

 Egypt, Mr. A. Lucas, who has carefully studied its occurrence at the 

 Nile Cataracts, leans to the opinion that the film is deposited by 

 water evaporating from the rock 2 ; and my experience of the 

 phenomenon in the Zambezi valley led me to the same opinion. 



I have elsewhere 3 remarked on the singular absence of recent 

 animal-remains in the region, but the point will bear repetition. 

 Prom the hunters' records we know how abundant formerly 

 were the bigger mammals, including the elephant, rhinoceros, 

 hippopotamus, buffalo, zebra, lion, etc., and indeed, though sadly 

 diminished, they still inhabit the country. In the district south of 

 the Batoka Gorge they were killed in great numbers during the last 

 century; yet I failed to find anywhere either bone or tooth to 



1 ' Travels in the Interior of South Africa ' vol. ii (1868) p. 213. 



2 'The Blackened Bocks of the Nile Cataracts & of the Egyptian Deserts 

 Govt. Bep. : Ministry of Finanee, Cairo, 1905. 



3 Bep. Brit. Assoc. 1905 (South Africa) p. 299. 



