3^ PRKSIDENTIAL ADDRESS— SECTION B. 



siliceous. As a consequence the overflowing lava sheets con- 

 verted them into vitreous quartzites containing agates and 

 chalcedony in geodes. Pipes and kernels are numerous under 

 the effusions, and at Tabas Induna a root-like silicification, some 

 14 inches long, now in the Bulawayo Museum, suggests tree life 

 and a land surface. The frequency of silicified wood-branches 

 and tree-trunks among unaltered sandstones points to hydro- 

 thermal action originated by the lava flows, for the vegetable 

 tissue would be decomposed and replaced by heated siliceous 

 waters. Flinty fossil casts of the shells of limnea and other 

 land gastropods have been found among sandstones, but whether 

 these were buried at the time of deposition, or lived recently and 

 became buried in surface sand, since indurated, is a matter that 

 requires some further attention. 



The silicification of all remains of organic origin may lead 

 to the discovery of remains of reptiles in Rhodesia such as are 

 found on the Karroo. 



During the middle Karroo period earth movements began. 



To the south-east of the coast monoclinal bends, which 

 Suess thought were great fault deplacements — especially the 

 scarp of the Drakensberg. which he considered as the fractured 

 edge of a tableland — have resulted in a considerable region of 

 the rocks that were formed in the Karroo basin being immersed 

 imder the ocean. In the Transvaal the Karroo strata east of the 

 great axis are bent in a flexure down from the plateau altitude 

 to nearly sea-level. West of this axis or on the continental side 

 the strata remain horizontal, and rest as a transgression on the 

 rocks of older formations. 



In construction, flora and fauna of the Karroo basin, and 

 in the breaking down of its southern edge, Suess saw an un- 

 deniable resemblance to the Indian peninsula, and thought that, 

 with Madagascar, they bear in common the stani]) of a once 

 continuous tableland. 



The movements of the crust in the south seem to have been 

 confined to folds along the margins of the axis — in Rhodesia 

 we have a series of great fault deplacements that have let down 

 areas of the Karroo into trenches in the older gneiss. The 

 Luangwa river runs in a canal-like cavity of some 500 miles long 

 by 30 miles broad, the floor being of Karroo rocks, which pro- 

 bably rested on the plateau to the south-east, but were turned 

 over its edge and cut oft' abruptly by the Machinga fault of 

 at least 5.000 feet and 500 miles long. A similar occurrence has 

 preserved the Lusenfwa valley. The Zambesi's middle course 

 runs along the axis of the extension of the Dekka fault. 



Nearer the backbone of Africa, and in Nyassaland, are many 

 small patches of Karroo rocks faulted or bent down into cavities 

 among the older rocks, by which means they have been preserved 

 from denudation. The narrow lakes Nyassa and Tanganyika lie 

 between parallel trough faults that have let down the included 

 portion. This feature is common in India, where a great 

 denudation took place after these movements. 



